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Top row: Uranus, Neptune; Centre row: Earth, white dwarf star Sirius B, Venus (to scale) (see inset below for Mars and Mercury)
Top row: Mars and Mercury; bottom row: the Moon, dwarf planets Pluto and Haumea (to scale)
A planet (from Greek πλανήτης, alternative form of πλάνης "wanderer") is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.[a][1][2]
The term planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. .In 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ They see pictures of planets of placemats, on lunch boxes, on walls at school, but none get the scale of the solar system even remotely correct.

^ Last week I wrote about the International Astronomical Union (#IAU) General Assembly taking place in Rio de Janeiro, to which I was headed.

.This definition has been both praised and criticized, and remains disputed by some scientists.^ Thankfully I have a backup line on both the neighbour’s network and on the cellphone so I’m not entirely cut out, but it definitely was some trouble even just for the moment it happened.

The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. .Though the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei.^ I started thinking about where 2009 YE7 is in the sky, what telescopes I could use to point at it, how to time the observations.

^ A weekly column from astronomer Mike Brown on space and science, planets (full and dwarf), the sun and the moon and the stars, and the joys and frustrations of search, discovery, and life.

^ The real confirmation, though, would come from an infrared spectrum that shows evidence of deep water ice absorption features, but that requires a pretty big telescope.

By careful analysis of the observation data, Johannes Kepler found the planets' orbits to be not circular, but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some share such features as ice-caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.
.Since 1992, through the discovery of hundreds of planets around other stars, called extrasolar planets, scientists are beginning to understand that planets throughout the Milky Way Galaxy share characteristics with those in our Solar System.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ The spectacular winter skies are caused by the fact that we are looking straight in to the Milky Way galaxy, instead of out of it as we do in the spring and fall.

^ But, really, for most of my life, I’ve been just as guilty when it comes to those other things that occupy our night skies: the satellites.

.As of March 2010, there are 430 known extrasolar planets, ranging from the size of gas giants to that of terrestrial planets.^ Rather, we would talk of the great difference between giant planets and terrestrial planets, we would talk of the band of asteroids, and we would talk of the ever-increasing number of tiny icy objects out there on the very edge of the solar system.

^ There is no choice except to dispense with trying to depict both the distances between planets and the sizes of planets on the same scale.

[3]
Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. .Under IAU definitions, there are eight planets in the Solar System.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ They see pictures of planets of placemats, on lunch boxes, on walls at school, but none get the scale of the solar system even remotely correct.

^ There is an entire weeklong program called “Icy Bodies in the Solar System” with talks about the Kuiper belt, comets, icy satellites, dwarf planets, and even one talk about Pluto.

.In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune.^ Make Jupiter and Saturn significantly smaller, make the tiny tiny terrestrial planets significantly bigger.

^ If you take Jupiter to be the right size and scale everything from there, Mercury, Venus, Earth, and Mars should be 6,4,4, and 5 times smaller, respectively.

.Apart from the planets, the Solar System also contains at least five dwarf planets[4] and hundreds of thousands of small solar system bodies.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ They see pictures of planets of placemats, on lunch boxes, on walls at school, but none get the scale of the solar system even remotely correct.

^ There is an entire weeklong program called “Icy Bodies in the Solar System” with talks about the Kuiper belt, comets, icy satellites, dwarf planets, and even one talk about Pluto.

.With the exception of Mercury and Venus, all planets are orbited by one or more natural satellites.^ Shoving the planets together a bit more allows them all to be somewhat bigger.

^ Here is what I hoped to see: that night the funny oblong fast spinning dwarf planet Haumea was passing directly in front of one of its satellites (Namaka is its name).

^ It keeps the relative sizes of planets correct and keeps their ordering correct, but, like all of the ones above, it has to dispense with the relative spacing between planets.

Contents

History

Printed rendition of a geocentric cosmological model from Cosmographia, Antwerp, 1539
The idea of planets has evolved over its history, from the divine wandering stars of antiquity to the earthly objects of the scientific age. .The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems.^ But I knew that it was my only chance of finding something like Sedna, something else strange in the outer part of the solar system.

^ Titan is the only place in the solar system other than the earth that appears to have large quantities of liquid sitting on the surface.

^ Include links to knowledge resources, other concepts, and external resources, so people can find more things.

The ambiguities inherent in defining planets have led to much scientific controversy.
In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights "πλάνητες ἀστέρες" (planetes asteres: wandering stars) or simply "πλανήτοι" (planētoi: wanderers),[5] from which today's word "planet" was derived.[6][7] .In ancient Greece, China, Babylon and indeed all pre-modern civilisations,[8][9] it was almost universally believed that Earth was in the centre of the Universe and that all the "planets" circled the Earth.^ In April 2005 I still believed it possible that they were all 3 larger than Pluto and that they would eventually be called the 10 th , 11 th , and 12 th planets.

.The reasons for this perception were that stars and planets appeared to revolve around the Earth each day,[10] and the apparently common sense perception that the Earth was solid and stable, and that it is not moving but at rest.^ Five years ago I was sitting at work in that quiet week between Christmas and New Year’s Day desperately looking for the 10 th planet.

^ In my backyard I see this: each night as the moon moves further and further in its circle around the earth and we see more and more of the illuminated half, the moon is getting just a little brighter.

Babylon

.The first civilisation known to possess a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th century BC copy of a list of observations of the motions of the planet Venus that probably dates as early as the second millennium BC.[11] The Babylonian astrologers also laid the foundations of what would eventually become Western astrology.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ In April 2005 I still believed it possible that they were all 3 larger than Pluto and that they would eventually be called the 10 th , 11 th , and 12 th planets.

^ So from that moment on, I expected that G.Scholar would eventually become no longer supported in Metalib.

[12] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,[13] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.[14]
.The Sumerians, predecessors of the Babylonians who are considered as one of the first civilizations and are credited with the invention of writing, had identified at least Venus by 1500 BC.[15] Shortly afterwards, the other inner planet Mercury and the outer planets Mars, Jupiter and Saturn were all identified by Babylonian astronomers.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ If you take Jupiter to be the right size and scale everything from there, Mercury, Venus, Earth, and Mars should be 6,4,4, and 5 times smaller, respectively.

^ The trick, though, is to pile the planets on top of each other, and to not even show all of the monster Jupiter.

.These would remain the only known planets until the invention of the telescope in early modern times.^ I just needed some telescopes, some computers, and some time, and everything would fall into place.

[16]

Hellenic world

Ptolemy's "planetary spheres"
Modern Moon Mercury Venus the Sun Mars Jupiter Saturn
Medieval Europe [17] ☾ LVNA ☿ MERCVRIVS ♀VENVS ☉ SOL ♂ MARS ♃ IVPITER ♄ SATVRNVS
The ancient Greek cosmological system was taken from that of the Babylonians,[15] from whom they began to acquire astronomical learning from around 600 BC, including the constellations and the zodiac.[18] In the 6th century BC, the Babylonians' astronomical knowledge at the time was far in advance of the Greeks. The earliest known Greek sources, such as the Iliad and the Odyssey, do not mention the planets.[12]
By the first century BC, during the Hellenistic period, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. .These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye.^ Above, when I say it is “almost certainly” a fragment, the assessment is a judgment based on experience, rather than a scientific fact.

^ In April 2005 I still believed it possible that they were all 3 larger than Pluto and that they would eventually be called the 10 th , 11 th , and 12 th planets.

^ It was, perhaps, the most amazing pictures I had ever seen a telescope make before, and it was just over our heads, rather than in the remote depths of space.

.These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century AD. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.^ These communities would not be bounded by geography; a top selector would reach patrons around the world, just as Enrico Caruso's voice did.

^ I had no good ideas about what Haumea would be like; I had no theories I was testing, no hypothesis to work out, no predictions to boldly claim.

^ As an antidote to crankiness about the job of astronomy or about the bureaucrats of the IAU, I’m keeping my program from Rio with the names of all of the talks and all of the posters from everywhere around the world.

[11][19]
To the Greeks and Romans there were seven known planets, each presumed to be circling the Earth according to the complex laws laid out by Ptolemy. .They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.^ If you take Jupiter to be the right size and scale everything from there, Mercury, Venus, Earth, and Mars should be 6,4,4, and 5 times smaller, respectively.

[7][19][20]

Ancient China

In contrast to the Aristotelian-Ptolemaic system of Greek-Hellenistic astronomy which insisted on the planets being tied to concentric crystalline celestial spheres and moving in uniform circular motion, the Chinese Xuan Ye cosmology viewed the planets as floating about in an infinite heaven, "with the speed of the luminaries depending on their individual natures" [21]

Ancient India

.In 499, the Indian astronomer Aryabhata propounded a planetary model where the motion of planets was treated to be elliptical rather than circular.^ Santa, which now goes by the official name of Haumea, we now know to be only about ½ the size of Pluto, and we call it – and Pluto – a dwarf planet rather than a planet.

Aryabhata's model also explicitly incorporated the Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars.[22] .This model was widely accepted by many Indian astronomers who came after him.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ I feel that many of the astronomers pushing and pushing and pushing to get the Shuttle to fly to the Space Telescope never once thought about the risks, never drove around a town with schools memorializing astronauts who never came home.

Ayrabhata's followers were particularly strong in South India, where his principles of the diurnal rotation of the earth, among others, were followed and a number of secondary works were based on them.[23]
In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model.[24] .In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits the Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century.^ If you take Jupiter to be the right size and scale everything from there, Mercury, Venus, Earth, and Mars should be 6,4,4, and 5 times smaller, respectively.

.Most astronomers of the Kerala school who followed him accepted his planetary model.^ My credibility as a young astronomer (I had just started graduate school that year) was seriously diminished amongst the friends who had seen me frightened of the moon.

[24][25]

Islamic world

In the 11th century, the Andalusian astronomer Arzachel hypothesized that the orbits of the planets are elliptic orbits and not circular orbits.[26] .In the 11th century, the transit of Venus was observed by Avicenna, who established that Venus was, at least sometimes, below the Sun.^ Who would not be at least a little afraid at this time every year that the sun would somehow not decide to stop and then come back?

[27] In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun," which was later identified as the transit of Mercury and Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.[28]
Fakhr al-Din al-Razi (1149-1209), in dealing with his conception of physics and the physical world, rejected the Aristotelian and Avicennian view of a single world, but instead proposed that that there are "a thousand thousand worlds (alfa alfi 'awalim) beyond this world such that each one of those worlds be bigger and more massive than this world as well as having the like of what this world has." His rejection arose from his affirmation of atomism, as advocated by the Ash'ari school of Islamic theology.[29] .Ibn Qayyim Al-Jawziyya (1292-1350), in his criticism of astrology, recognized that the stars are much larger than the planets, and that Mercury is the smallest planet known to him.^ In April 2005 I still believed it possible that they were all 3 larger than Pluto and that they would eventually be called the 10 th , 11 th , and 12 th planets.

^ That was the end, too, of Eris (the slightly-larger-than-Pluto iceball that I discovered that had precipitated the mess) as a planet.

[30]

European Renaissance

Renaissance planets
Mercury Venus Earth Mars Jupiter Saturn
The five classical planets, being visible to the naked eye, have been known since ancient times, and have had a significant impact on mythology, religious cosmology, and ancient astronomy. As scientific knowledge progressed, however, understanding of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the heliocentric model of Copernicus, Galileo and Kepler gained sway.
.Thus the Earth became included in the list of planets,[31] while the Sun and Moon were excluded.^ A weekly column from astronomer Mike Brown on space and science, planets (full and dwarf), the sun and the moon and the stars, and the joys and frustrations of search, discovery, and life.

.At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century.^ Make Jupiter and Saturn significantly smaller, make the tiny tiny terrestrial planets significantly bigger.

[32] Until the mid-19th century, the number of "planets" rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.

19th Century

Planets in early 1800s
Mercury Venus Earth Mars Vesta Juno Ceres Pallas Jupiter Saturn Uranus
.In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas, and Vesta) were very different from the traditional ones.^ One reason for describing all of this poorly was that the real process was actually quite different from the way I attempted to describe it in the paper.

^ So it’s not that I am an astronomer and thus know the future, no, I think that the one qualification maybe I have for talking about the future really is my 11 month old daughter.

^ And you might think that I am uniquely qualified to talk about the future since I’m an astronomer and all, but, um, really, being an astronomer has very little to do with predicting the future.

These bodies shared the same region of space between Mars and Jupiter (the Asteroid belt), and had a much smaller mass; as a result they were reclassified as "asteroids." In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. .Since there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.^ Five years ago I was sitting at work in that quiet week between Christmas and New Year’s Day desperately looking for the 10 th planet.

^ Since the launch of Google Wave , there seems to be confusion about how best to use it.

^ Rather, we would talk of the great difference between giant planets and terrestrial planets, we would talk of the band of asteroids, and we would talk of the ever-increasing number of tiny icy objects out there on the very edge of the solar system.

[33]

20th Century

Planets from late 1800s to 1930
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
However, in the 20th century, Pluto was discovered. .After initial observations led to the belief it was larger than Earth,[34] the object was immediately accepted as the ninth planet.^ In April 2005 I still believed it possible that they were all 3 larger than Pluto and that they would eventually be called the 10 th , 11 th , and 12 th planets.

^ That was the end, too, of Eris (the slightly-larger-than-Pluto iceball that I discovered that had precipitated the mess) as a planet.

Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton suggested that Pluto may be an escaped satellite of Neptune,[35] and Fred Whipple suggested in 1964 that Pluto may be a comet.[36] However, as it was still larger than all known asteroids and seemingly did not exist within a larger population,[37] it kept its status until 2006.
Planets from 1930 to 2006
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
.In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.^ A weekly column from astronomer Mike Brown on space and science, planets (full and dwarf), the sun and the moon and the stars, and the joys and frustrations of search, discovery, and life.

[38] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[39]
.The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star.^ A weekly column from astronomer Mike Brown on space and science, planets (full and dwarf), the sun and the moon and the stars, and the joys and frustrations of search, discovery, and life.

.Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as "brown dwarfs".[40] Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen.^ Many of the fabulous finds about dwarf planets over the past decade have been made by or aided by the Hubble Space Telescope.

^ And you can even see many of the dwarf planets out in the Kuiper belt.

^ A weekly column from astronomer Mike Brown on space and science, planets (full and dwarf), the sun and the moon and the stars, and the joys and frustrations of search, discovery, and life.

.While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium.^ For some time after I first moved in I tried to remember to bring a flashlight with me to light my way, but more often than not I forgot.

^ It’s not that I don’t see them all the time when I am looking at the sky, but I never think of them as anything more than spots of light moving across the heavens.

.However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.^ With the nights so long and the sun moving further and further south, who would not want to try to do their part to make up for the absence of the light and the heat?

^ It was, perhaps, the most amazing pictures I had ever seen a telescope make before, and it was just over our heads, rather than in the remote depths of space.

^ A weekly column from astronomer Mike Brown on space and science, planets (full and dwarf), the sun and the moon and the stars, and the joys and frustrations of search, discovery, and life.

[41]

21st Century

Planets from 2006 to present
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
.With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ Titan is the only place in the solar system other than the earth that appears to have large quantities of liquid sitting on the surface.

^ They see pictures of planets of placemats, on lunch boxes, on walls at school, but none get the scale of the solar system even remotely correct.

.There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium.^ A new large Kuiper belt object found from a telescope Chile, by David Rabinowitz!

^ The best example of this change comes, I think, from the discovery of a large Kuiper belt object that was announced just a few days ago.

^ I checked my email and found out that there was a large Kuiper belt object that someone else had discovered.

.A growing number of astronomers argued for Pluto to be declassified as a planet, since many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt) during the 1990s and early 2000s.^ (Is it possible that in the early solar system things from the Kuiper belt got mixed out to the asteroid belt?

^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ And you can even see many of the dwarf planets out in the Kuiper belt.

.Pluto was found to be just one small body in a population of thousands.^ I had just found something bigger than Pluto.

Some of them including Quaoar, Sedna, and Eris were heralded in the popular press as the tenth planet, failing however to receive widespread scientific recognition. .The discovery of Eris, an object 27 percent more massive than Pluto, brought things to a head.^ With her fourth birthday now more than a month behind her it seems to the natural thing to start contemplating.

^ Sure, finding things bigger than Pluto is cool, but it doesn’t fundamentally change the science of what is out there.

^ Soon after the discovery of Haumea, we tripled the jackpot by first discovering Eris – the one we now know to be larger than Pluto – just two weeks later, and then Makemake – the one we now know to be just a bit smaller than Pluto – a few months later.

.Acknowledging the problem, the IAU set about creating the definition of planet, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).^ Many of the fabulous finds about dwarf planets over the past decade have been made by or aided by the Hubble Space Telescope.

^ If you take Jupiter to be the right size and scale everything from there, Mercury, Venus, Earth, and Mars should be 6,4,4, and 5 times smaller, respectively.

^ In April 2005 I still believed it possible that they were all 3 larger than Pluto and that they would eventually be called the 10 th , 11 th , and 12 th planets.

[42]

Extrasolar planet definition

The Earth Dysnomia Eris Charon Pluto Makemake Haumea Sedna Orcus 2007 OR10 Quaoar File:EightTNOs.png
Comparison of Eris, Pluto, Makemake, Haumea, Sedna, Orcus, 2007 OR10, Quaoar, and Earth (all to scale).
In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated the following working definition, mostly focused upon the boundary between planets and brown dwarves:[2]
.
  1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun[43]) that orbit stars or stellar remnants are "planets" (no matter how they formed).^ A weekly column from astronomer Mike Brown on space and science, planets (full and dwarf), the sun and the moon and the stars, and the joys and frustrations of search, discovery, and life.

    ^ But still, it is hard not to see the similarity between the lights strung in the town below trying to dispel the night and call back the sun, and the lights above, also seemingly strung for the same reason.

    ^ OK, no, but really most of the thoughts that I have when I think about what the world will be like in my daughter’s future involve things that few of us – no matter how influential Time magazine says we are – can do much about.

    The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
  2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.
  3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
This definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journals.[44] Although temporary, it remains an effective working definition until a more permanent one is formally adopted. .However, it does not address the dispute over the lower mass limit,[45] and so it steered clear of the controversy regarding objects within the Solar System.^ Why are they so attached to the 18 th largest object in the solar system when they probably can’t even name all of the 17 larger things?

.This definition also makes no comment on the status of planets orbiting brown dwarfs such as 2M1207b.^ A weekly column from astronomer Mike Brown on space and science, planets (full and dwarf), the sun and the moon and the stars, and the joys and frustrations of search, discovery, and life.

^ No, really, I’ve got no advice to give, unless, of course, you wanna run off and find planets, then I’m definitely your guy.

Dwarf planets from 2006 to present
Ceres Pluto Makemake Haumea Eris

2006 definition

.The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly.^ Last week I wrote about the International Astronomical Union (#IAU) General Assembly taking place in Rio de Janeiro, to which I was headed.

^ Most people, if they even ever heard of the IAU only know it for its role in the demotion of Pluto at the last General Assembly three years ago.

After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System as:[1]
A celestial body that is (a) in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
.Under this definition, the Solar System is considered to have eight planets.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ They see pictures of planets of placemats, on lunch boxes, on walls at school, but none get the scale of the solar system even remotely correct.

^ In an overwhelming vote, astronomers agreed to tighten the definition of the word “planet” to mean, essentially, the large dominant things in the solar system.

.Bodies which fulfill the first two conditions but not the third (such as Pluto, Makemake and Eris) are classified as dwarf planets, provided they are not also natural satellites of other planets.^ In April 2005 I still believed it possible that they were all 3 larger than Pluto and that they would eventually be called the 10 th , 11 th , and 12 th planets.

^ Santa, which now goes by the official name of Haumea, we now know to be only about ½ the size of Pluto, and we call it – and Pluto – a dwarf planet rather than a planet.

^ We found that Makemake is covered in thick layers of frozen methane, that Eris is bigger and heavier than Pluto, and, most importantly, that things were beginning to make sense.

.Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion.^ In April 2005 I still believed it possible that they were all 3 larger than Pluto and that they would eventually be called the 10 th , 11 th , and 12 th planets.

^ If you had grown up with a picture of the real solar system on your placemat, you would be forgiven for thinking the number of planets was precisely zero.

[46] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.[47]
.This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects.^ Based on its brightness it might well be a perfect size to test one of my new theories about medium-sized Kuiper belt objects.

As described by astronomer Steven Soter:[48]
The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. .Asteroids and comets, including KBOs [Kuiper belt objects], differ from planets in that they can collide with each other and with planets.^ They wrote lots of interesting articles, including ones filled with math (pdf) and others fit to be read by librarians and publishers .

.In the aftermath of the IAU's 2006 vote, there has been controversy and debate about the definition,[49][50] and many astronomers have stated that they will not use it.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ (Facet-oriented displays let you do all this, for instance, and are useful even when they provide no information about concepts other than their names.

^ Since the launch of Google Wave , there seems to be confusion about how best to use it.

[51] .Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition.^ And now I must in the end admit that one of those is actually true.

^ Santa, which now goes by the official name of Haumea, we now know to be only about ½ the size of Pluto, and we call it – and Pluto – a dwarf planet rather than a planet.

The next IAU conference is not until 2009, when modifications could be made to the IAU definition, also possibly including extrasolar planets.
.Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status since its discovery in 1930. The discovery of Eris was widely reported in the media as the tenth planet and therefore the reclassification of all three objects as dwarf planets has attracted a lot of media and public attention as well.^ Well somehow I missed that email (as I am a member of the group – but get way way way too many emails in general).

^ I want to emphasize that I'm not saying that public libraries don't generate economic value, just that libraries generate a lot of value in the print economy that won't be generated in the digital economy.

[52]

Former classifications

The table below lists Solar System bodies formerly considered to be planets:
Body (current classification) Notes
Star Dwarf planet Asteroid Moon
Sun The Moon Classified as planets in antiquity, in accordance with the definition then used.
Io, Europa, Ganymede, and Callisto The four largest moons of Jupiter, known as the Galilean moons after their discoverer Galileo Galilei. He referred to them as the "Medicean Planets" in honor of his patron, the Medici family.
Titan,[b] Iapetus,[c] Rhea,[c] Tethys,[d] and Dione[d] Five of Saturn's larger moons, discovered by Christiaan Huygens and Giovanni Domenico Cassini.
Ceres[e] Pallas, Juno, and Vesta The first known asteroids, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.[53]
Ceres has subsequently been classified as a dwarf planet in 2006.
Astrea, Hebe, Iris, Flora, Metis, Hygeia, Parthenope, Victoria, Egeria, Irene, Eunomia More asteroids, discovered between 1845 and 1851. The rapidly expanding list of planets prompted their reclassification as asteroids by astronomers, and this was widely accepted by 1854.[54]
Pluto[f] The first known Trans-Neptunian object (i.e. minor planet with a semi-major axis beyond Neptune). In 2006, Pluto was reclassified as a dwarf planet.

Mythology and naming

The gods of Olympus, after whom the Solar System's planets are named
The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene; the farthest planet was called Phainon, the shiner; followed by Phaethon, "bright"; the red planet was known as Pyroeis, the "fiery"; the brightest was known as Phosphoros, the light bringer; and the fleeting final planet was called Stilbon, the gleamer. The Greeks also made each planet sacred to one of their pantheon of gods, the Olympians: Helios and Selene were the names of both planets and gods; Phainon was sacred to Kronos, the Titan who fathered the Olympians; Phaethon was sacred to Zeus, Kronos's son who deposed him as king; Pyroeis was given to Ares, son of Zeus and god of war; Phosphorus was ruled by Aphrodite, the goddess of love; and Hermes, messenger of the gods and god of learning and wit, ruled over Stilbon.[11]
The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphorus after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon after their chief god, Marduk.[55] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[11] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.[56]
Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. .While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (or Latin) names rather than the Greek ones.^ They claim that they will also add a Verizon network friendly version – but still no version that will work on other networks such as AT&T. To me as an end user, it means nothing if a phone is unlocked if it will only work on one network because of hardware limitations.

^ (Facet-oriented displays let you do all this, for instance, and are useful even when they provide no information about concepts other than their names.

^ Update : using the latest (1.0b2) version of Prism , which installs as a Firefox extension rather than standalone, Prism apps have access to everything Firefox does!

.The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods.^ Official Evergreen Documentation produced by the Documentation Interest Group should be licensed under the Creative Commons Attribution Share-Alike 3.0 License (http://creativecommons.org/licenses/by-sa/3.0/).

^ Of course, like many others who feel a special bond with books and our cultural heritage, I wish that the Google Books project was not under the control of a private entity.

^ Licensed under Creative Commons Attribution-Share Alike 2.5 Australia.

During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.[57] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Kronos). .When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptūnus (Poseidon).^ You probably can't name any musical performers from the 18th or 19th century.

Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).[58] .Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus.^ January 1st was Public Domain Day [and when I started on writing some of what became this post].

Since each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved many modern languages.[59] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively[60].
.Since Earth was only generally accepted as a planet in the 17th century,[31] there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets).^ (Facet-oriented displays let you do all this, for instance, and are useful even when they provide no information about concepts other than their names.

^ There is no mechanism for permanently reserving a domain name.

^ But local libraries lose their advantage in collective acquisition when books become digital because there is no longer a necessity for users to be geographically close to books.

.The name originates from the 8th century Anglo-Saxon word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300.[61][62] It is the only planet whose name in English is not derived from Greco-Roman mythology.^ Library managers see the current difficulties as being an opportunity to rethink what ‘library’ means in the twenty first century.

Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" (as opposed to "sea").[63] However, the non-Romance languages use their own respective native words. The Greeks retain their original name, Γή (Ge or Yi); the Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground,"[62] as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde.
.Non-European cultures use other planetary naming systems.^ (Facet-oriented displays let you do all this, for instance, and are useful even when they provide no information about concepts other than their names.

India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, and Budha, Shukra, Mangala, Bṛhaspati and Shani for the traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China and the countries of eastern Asia influenced by that country historically (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).[59]

Formation

It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets.[64] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[65]
An artist's impression of protoplanetary disk
When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting-Robertson drag and other effects.[66][67] .Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.^ If you’ve explored the links from my post giving examples of such catalogs , you may have noticed that there’s no single look and feel across the catalogs.

[68] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small Solar System bodies.[citation needed]
The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.[69] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.[70] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)
.With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account.^ The other possibility is that punk rock stars could take jobs now and then as librarians.

^ (Facet-oriented displays let you do all this, for instance, and are useful even when they provide no information about concepts other than their names.

The level of metallicity – an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium) – is now believed to determine the likelihood that a star will have planets.[71] .Hence it is thought less likely that a metal-poor, population II star will possess a more substantial planetary system than a metal-rich population I star.^ She facilitated integration of more than 20 million documents and applications into this system utilizing federated search connectors to improve their content findability.

Solar System

Planets and dwarf planets of the Solar System. (Sizes to scale, distances not to scale)
The terrestrial planets: Mercury, Venus, Earth, Mars (Sizes to scale, distances not to scale)
The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale, distances not to scale)
According to the IAU's current definitions, there are eight planets and five dwarf planets in the Solar System. In increasing distance from the Sun, the planets are:
  1. ☿ Mercury
  2. ♀ Venus
  3. ⊕ Earth
  4. ♂ Mars
  5. ♃ Jupiter
  6. ♄ Saturn
  7. ♅ Uranus
  8. ♆ Neptune
Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.
The planets of the Solar System can be divided into categories based on their composition:
  • Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars. .At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System, while Earth is the largest terrestrial planet.
  • Gas giants (Jovians): Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune.^ She facilitated integration of more than 20 million documents and applications into this system utilizing federated search connectors to improve their content findability.

    Jupiter, at 318 Earth masses, is the largest planet in the Solar System, while Saturn is one third as big, at 95 Earth masses.
  • Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass (only 14 and 17 Earth masses), and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
  • Dwarf planets: Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently five dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Haumea, Makemake and Eris. Several other objects in both the Asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. .There may be as many as 200 that could be discovered once the Kuiper belt has been fully explored.^ If you’ve explored the links from my post giving examples of such catalogs , you may have noticed that there’s no single look and feel across the catalogs.

    .Dwarf planets share many of the same characteristics as planets, although notable differences remain – namely that they are not dominant in their orbits.^ They beat the Longhorns pretty easily although without the injury to Colt McCoy the game would have been much different.

    By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto, Haumea, and Makemake are members of the Kuiper belt and Eris is a member of the scattered disc. .Scientists such as Mike Brown believe that there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.^ If you’ve explored the links from my post giving examples of such catalogs , you may have noticed that there’s no single look and feel across the catalogs.

    [72]
Planetary attributes
Name Equatorial
diameter[a]
Mass[a] Orbital
radius (AU)
Orbital period
(years)
Inclination
to Sun's equator
(°)
Orbital
eccent-ricity
Rotation period
(days)
Named
moons[c]
Rings Atmosphere
Terrestrials Mercury 0.382 0.06 0.39 0.24 3.38 0.206 58.64 no minimal
Venus 0.949 0.82 0.72 0.62 3.86 0.007 -243.02 no CO2, N2
Earth[b] 1.00 1.00 1.00 1.00 7.25 0.017 1.00 1 no N2, O2
Mars 0.532 0.11 1.52 1.88 5.65 0.093 1.03 2 no CO2, N2
Gas giants Jupiter 11.209 317.8 5.20 11.86 6.09 0.048 0.41 49 yes H2, He
Saturn 9.449 95.2 9.54 29.46 5.51 0.054 0.43 52 yes H2, He
Uranus 4.007 14.6 19.22 84.01 6.48 0.047 -0.72 27 yes H2, He
Neptune 3.883 17.2 30.06 164.8 6.43 0.009 0.67 13 yes H2, He
Dwarf planets
Ceres 0.08 0.000 2 2.5–3.0 4.60 10.59 0.080 0.38 0 no none
Pluto 0.19 0.002 2 29.7–49.3 248.09 17.14 0.249 -6.39 3 no temporary
Haumea 0.37×0.16 0.000 7 35.2–51.5 282.76 28.19 0.189 0.16 2
Makemake ~0.12 0.000 7 38.5–53.1 309.88 28.96 0.159  ? 0  ?  ? [d]
Eris 0.19 0.002 5 37.8–97.6 ~557 44.19 0.442 ~0.3 1  ?  ? [d]
a  Measured relative to the Earth.
b  See Earth article for absolute values.
c  Jupiter has the most secured satellites (63) in the solar system.[73]
d  Like Pluto, when near perihelion, a temporary atmosphere is suspected.

Extrasolar planets

Exoplanets, by year of discovery, through 2010-01-27.
The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. Of the 415 extrasolar planets discovered by December 2009, most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed.[74] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.[75] There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,[74] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436.[76] These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths).[77] Another new category are the so-called "super-Earths", possibly terrestrial planets far larger than Earth but smaller than Neptune or Uranus. .To date, six possible super-Earths have been found: Gliese 876 d, which is roughly six times Earth's mass,[78] OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing,[79][80] COROT-Exo-7b, a planet with a diameter estimated at around 1.7 times that of Earth, (making it the smallest super-Earth yet measured), but with an orbital distance of only 0.02 AU, which means it probably has a molten surface at a temperature of 1000–1500 °C,[81] and two planets orbiting the nearby red dwarf Gliese 581.^ In my spare time (ha ha) I train librarians around the world how to use various technologies to make their lives easier, their work more efficient and their budgets less scary.

Gliese 581 d is roughly 7.7 times Earth's mass,[82] while Gliese 581 c is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone.[83] .However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient greenhouse gases.^ I'd say that we are rapidly approaching the point where much of the earth has been photographed in detail.

^ This is just a quick overview, we'll blog each of these in much more depth in a few days, stay tuned for more details and screenshots.

[84]
Size comparison of HR 8799 c (gray) with Jupiter. Most exoplanets discovered thus far are larger than Jupiter, though discoveries of smaller planets are expected in the near future.
.It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets.^ They beat the Longhorns pretty easily although without the injury to Colt McCoy the game would have been much different.

In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. .They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all.^ That is partly because they are lawyered up, and thus hamstrung from responding to some questions academics have, or from instituting more liberal policies and features.

^ Few of them have enough money to access all the types of information they need, and they're too small a group to attract the attention of a publisher-sponsored "ebook-club".

^ So public libraries are inherently beneficial if it costs more than $4.09 for publishers to make, sell, and deliver an extra book.

There may also exist a class of hot Jupiters, called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.[85]
.More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes.^ More announcements coming soon, including a few new LibraryThing for Libraries features, and an update on the party we're hosting Saturday evening.

Currently the COROT spacecraft is searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's Kepler Mission, Terrestrial Planet Finder, and Space Interferometry Mission programs, the ESA's Darwin, and the CNES' PEGASE.[86] The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b and HD 189733 b).[87][88] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.[89]

Interstellar "planets"

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[90] .Some scientists have argued that such objects found roaming in deep space should be classed as "planets," although others have suggested that they could be low-mass stars.^ The other possibility is that punk rock stars could take jobs now and then as librarians.

^ Going the other way, they should also make it easy for external resources to stably link to a particular concept (or concept set) in the catalog.

^ They leech money out of library budgets every year that could be going to other, more productive uses.

[91][92] In 2005, astronomers announced the discovery of Cha 110913-773444, the smallest brown dwarf found to date, at only seven times Jupiter's mass. Since it was not found in orbit around a fusing star, it is a sub-brown dwarf according to the IAU's working definition.[91][92] For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos", or "planetary mass objects". However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarfs.[93][94][95]

Attributes

Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also common to extrasolar planets.

Dynamic characteristics

Orbit

The orbit of the planet Neptune compared to that of Pluto. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination).
According to current definitions, all planets must revolve around stars; thus, any potential "rogue planets" are excluded. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates (counter-clockwise as seen from above the Sun's north pole). At least one extrasolar planet, WASP-17b, has been found to orbit in the opposite direction to its star's rotation.[96] The period of one revolution of a planet's orbit is known as its sidereal period or year.[97] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), while its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory.[98]
Each planet's orbit is delineated by a set of elements:
  • The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits.[97] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.[99][100]
  • Illustration of the semi-major axis
    The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not the same as its apastron, as no planet's orbit has its star at its exact centre.[97]
  • The inclination of a planet tells how far above or below an established reference plane its orbit lies. In the Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. For extrasolar planets, the plane, known as the sky plane or plane of the sky, is the plane of the observer's line of sight from Earth.[101] The eight planets of the Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it.[102] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.[97] The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. The argument of periapsis (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.[97]

Axial tilt

Earth's axial tilt is about 23°.
Planets also have varying degrees of axial tilt; they lie at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses seasons; changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.[103] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.[104]

Rotation

The planets also rotate around invisible axes through their centres. A planet's rotation period is known as its day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the sun's north pole, the exceptions being Venus[105] and Uranus[106] which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.[107] However regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit. There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours.[108] The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are tidally locked (their orbits are in sync with their rotations). This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.[109]

Orbital clearing

The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU's official definition of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.[1] Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.[110]

Physical characteristics

Mass

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.[111]
Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood is roughly 13 times Jupiter's mass, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System; however a number of extrasolar planets lie at that threshold. The Extrasolar Planets Encyclopedia lists several planets that are close to this limit: HD 38529c, AB Pictorisb, HD 162020b, and HD 13189b. A number of objects of higher mass are also listed, but since they lie above the fusion threshold, they would be better described as brown dwarfs.[74]
The smallest known planet, excluding dwarf planets and satellites, is PSR B1257+12 a, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.[74]

Internal differentiation

Illustration of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen
Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle which either is or was a fluid. The terrestrial planets are sealed within hard crusts,[112] but in the gas giants the mantle simply dissolves into the upper cloud layers. The terrestrial planets possess cores of magnetic elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to possess cores of rock and metal surrounded by mantles of metallic hydrogen.[113] Uranus and Neptune, which are smaller, possess rocky cores surrounded by mantles of water, ammonia, methane and other ices.[114] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.[112]

Atmosphere

Earth's atmosphere
All of the Solar System planets have atmospheres as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases hydrogen and helium close by, while the smaller planets lose these gases into space.[115] The composition of the Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen.[116] The only solar planet without a substantial atmosphere is Mercury which had it mostly, although not entirely, blasted away by the solar wind.[117]
Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars), an Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune).[103] At least one extrasolar planet, HD 189733 b, has been claimed to possess such a weather system, similar to the Great Red Spot but twice as large.[118]
Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.[119][120] These planets may have vast differences in temperature between their day and night sides which produce supersonic winds,[121] although the day and night sides of HD 189733b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.[118]

Magnetosphere

Schematic of Earth's magnetosphere
One important characteristic of the planets is their intrinsic magnetic moments which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet.[122]
Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.[122] In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System (so strong in fact that it poses a serious health risk to future manned missions to its moons). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.[122]
In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949, which appeared to be creating a sunspot on the surface of its parent star. The team hypothesised that the planet's magnetosphere was transferring energy onto the star's surface, increasing its already high 14,000 degree temperature by an additional 750 degrees.[123]

Secondary characteristics

Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus have natural satellites, often called "moons." Earth has one, Mars has two, and the gas giants have numerous moons in complex planetary-type systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa).[124][125][126]
The four gas giants are also orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny 'moonlets' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's Roche limit and were torn apart by tidal forces.[127][128]
No secondary characteristics have been observed around extrasolar planets. However the sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc.[91]

Related terms

See also

Notes

  1. ^ This definition is drawn from two separate IAU declarations; a formal definition agreed by the Union in 2006, and an informal working definition established by the Union in 2003. The 2006 definition, while official, applies only to the Solar System, while the 2003 definition applies to planets around other stars. The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference.
  2. ^ Referred to by Huygens as a Planetes novus ("new planet") in his Systema Saturnium
  3. ^ Both labelled nouvelles planètes (new planets) by Cassini in his Découverte de deux nouvelles planetes autour de Saturne
  4. ^ Both once referred to as "planets" by Cassini in his An Extract of the Journal Des Scavans.... The term "satellite", however, had already begun to be used to distinguish such bodies from those around which they orbited ("primary planets").
  5. ^ Recently reclassified as a dwarf planet in 2006.
  6. ^ Regarded as a planet from its discovery in 1930 until redesignated as a trans-Neptunian dwarf planet in August 2006.

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External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

PLANET (Gr. irXavi rns, a wanderer), in the ancient .astronomy, one of seven heavenly bodies characterized by being in motion relative to the fixed stars, which last appeared immovable upon the celestial sphere.^ The retrograde motion of a planet shows up if you look at its motion relative to the stars in the sky.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.As thus defined the planets were the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn.^ Mercury and Venus have no moons .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If P moon is the sidereal period of such a moon (in its orbit around Jupiter), and P J is the sidereal period of Jupiter (in its orbit around the Sun), then the synodical period P syn of the moon is equal to .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ I think that a planet has a greater chance of having moons if the planet has more mass (i.e., is larger) and if the planet is further away from the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

In modern astronomy since Copernicus, the term is applied to any opaque body moving around the sun. .Taken in its widest sense it applies to the satellites which are sometimes termed secondary planets. Each of these moves around a planet larger than itself, which it accompanies in its revolution round the sun.^ These same licensing terms should be applied to the Documentation Wiki.

A planet not revolving round another is termed a primary planet.
The primary planets are classified as major and minor. The former are eight in number and, with the sun, form the principal members of the solar system, under which head their arrangement is described. The earth on which we live is the third in the order of the major planets from the sun. .With respect to the positions of their orbits relative to the earth, the other planets are distinguished as inferior and superior. The former, only two in number, comprise Mercury and Venus, which revolve between the earth and the sun.^ Alabama, the other undeafeated, is number two.

The superior planets are those whose orbits are outside that of the earth. The synodic revolution of an inferior planet is the time in which it performs a revolution relative to the line joining the earth and the sun. This is greater than its actual time of revolution. The phases or appearances presented by such a planet depend upon its configuration with respect to the earth and sun, and therefore go through their complete periods in a synodic revolution. .At superior conjunction the illuminated hemisphere of the planet is presented to the earth so that it presents the form of a full moon.^ A weekly column from astronomer Mike Brown on space and science, planets (full and dwarf), the sun and the moon and the stars, and the joys and frustrations of search, discovery, and life.

.As it moves towards inferior conjunction, the lines from the planet to the sun and to the earth, or the angle sun-earth as seen from the planet, on which the phase depends, continually make a greater angle.^ With the nights so long and the sun moving further and further south, who would not want to try to do their part to make up for the absence of the light and the heat?

.At the time of greatest elongation this angle is 90 0, and the planet appears one half illuminated, like the moon at first or last quarter.^ At this time each late afternoon I like to get out the binoculars that I keep next to the back door, and I step outside to watch the last seconds of the sun setting and to find the spot where the last glimmer of light for the day appears.

^ This one works like filter() and will also return a list, whose contents are the items of another list (second argument) for which the result of calling the given function (first argument) is True .

^ The last time I tried a build the wifi on my Mini just worked, so it looks like those problems are a thing of the past.

.Then, as it approaches inferior conjunction, the visible portion of the disk assumes the crescent form, and while the circle bounding the disk continually increases owing to the approach of the planet to the earth, the crescent becomes thinner and thinner until, near inferior conjunction, the planet is no longer visible.^ That is, these approaches are attractive only until it is time to run on a multicore system, at which point their poor scalability will become painfully apparent.
  • Kernel Planet 28 January 2010 0:39 UTC planet.kernel.org [Source type: FILTERED WITH BAYES]

^ Since that time , Pluto is no longer a planet, but a dwarf planet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The moon is nearing full tonight, but it's no longer my nemesis.

After conjunction the phases occur in the reverse order. .The brilliancy of the planet, as measured by the total amount of light we receive from it, goes through a similar cycle of change.^ The amount of sunlight that a planet receives per unit area depends on the distance of the planet from the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ As seen from the planet , the Sun goes through the sky in one planet day.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The next table shows the average distance r from the Sun (in AU ), the amount L of sunlight that a planet receives per unit area, compared to the amount at a similar location on Earth , and the visual magnitude V of the Sun as seen from just outside that planet's atmosphere .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The point of greatest brilliancy is between inferior conjunction and greatest elongation.^ As seen from Earth , Mercury repeats its phenomena such as conjunctions and greatest elongations after 115.9 days.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.In the case of Venus this phase occurs about three or four weeks before and after inferior conjunction.^ She apparently have been watching it three or four times a day for the last few days (weeks?
  • Kernel Planet 28 January 2010 0:39 UTC planet.kernel.org [Source type: FILTERED WITH BAYES]

^ In addition, the Sun must then be in the same direction as Venus, as seen from Earth , so Venus must then be near an inferior conjunction with the Sun, and the Earth must then be near the same node of the orbit of Venus as Venus itself.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Missing image
Planet-1.jpg
.The dots round the orbits show the position of the planets at internals of a thousand days II The dots round the orbits show the position of the planets at intervals of ten days The symbols I' indicate the greatest distance of an orbit north and south of the Plane of the Ecliptic The arrow-head Bach orbit shows the direction of recolution,also the place of each planet en Jan.^ What are the least and greatest distances of the planets from the Sun?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The VSOP model of the motion of the planets (from which the positions of the planets can be calculated quite accurately for thousands of years ) indicates that the distance between the Sun and the Earth can get up to about 2400 km (about 2/5 of the diameter of the Earth) larger or smaller because of the influence of Jupiter.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If a spaceship were to leave Earth and keep going to the north or south, would it reach any planets?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

lst.1910 at noon
8 i rJ FIG. 2.
The Sun FIG. I.
040 The Earth 'Venus ' i'osilion of Axis unknown The Moon Mercury Scale about one hundred times that Orbits Scale ten thousand times that of Orbits Mars (The Earth,Mars,and the Sun, are shown as seen from the direction of the pole of the ecliptic in their true axial position) FIG. 3.
Jupiter
Neptune
.Moon 0 Venus Earth Mars 0 Mercury Scale Planets 20,000 times that of Orbits FIG. 4 .^ If you take Jupiter to be the right size and scale everything from there, Mercury, Venus, Earth, and Mars should be 6,4,4, and 5 times smaller, respectively.

Saturn's
System
of Satellites
.(Jupiter and Saturn are shown in their true axial position, Uranus and Neptune in the axial positions inferred from the motions of their Satellites).^ The other planets ( Jupiter , Saturn , Uranus , and Neptune ) are giant gas planets of which we can only see the thick layers of gas on the outside, and you can't build volcanoes out of gas.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Figure 2 shows the orbits of (from the inside out) Mars , Jupiter , Saturn , Uranus , Neptune , and Pluto , in the same way as the previous picture.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ In our Solar System, Jupiter, Saturn , Uranus , and Neptune are Jovian planets .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Scale about 4000 times that of the Orbits System W Neptune's Satellite Jupiter's of Satellites
s Uranus's Satellites FIG. 5.
.In the figures given above are shown the relative orbits of the planets, the orbits of Mars, the Earth, Venus and Mercury (fig.^ Figure 1 shows the orbits of the planets (from the inside out) Mercury, Venus, the Earth, and Mars .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The associated periods can be measured relative to the stars ( sidereal ) or relative to the Sun , as seen from the planet or from Earth ( synodical ).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The orbits of Mars and the Earth are not perfect circles but rather like circles that are a little squashed, and the orbits are also shifted a little so that the Sun is not quite in the center of the orbits.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.1) being drawn to a scale twenty times that of the outer ones - Neptune, Uranus, Saturn, Jupiter (fig.^ Uranus and Neptune are 1.2 and 1.5 times too big, respectively.

^ The other planets ( Jupiter , Saturn , Uranus , and Neptune ) are giant gas planets of which we can only see the thick layers of gas on the outside, and you can't build volcanoes out of gas.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ After on average one such synodical period, Jupiter and the moon are in about the same relative positions, so the time between successive transits or occultations of the moon will be close to multiples of the synodical period.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

2). .The positions of the planets at ten-day intervals; their actual position on the 1st of January 1910 at noon, of their nodes and nearer apses, and the points when they are farthest distant north and south of the ecliptic, are also given.^ Planets rotate around their axis, which generates Coriolis forces that make air traveling north or south deviate from its straight course.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Subversion setup and Gorg (January 05, 2010, 18:30 UTC) Hello, first actual post in linux planets .

^ More than two planets never return to exactly the same relative positions that they had before.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The relative sizes of the planets are also given, orientated in their true axial position with regard to the ecliptic.^ That period is called the synodical period of both planets , and after that period both planets are again in the same relative position.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ What is the inclination of the orbit of Saturn and the other planets, relative to the ecliptic?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If you take ever more accurate approximations for the orbital periods of the planets, then the common period after which the planets return to the same relative positions gets on the whole longer and longer.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The nearer planets (and also the Moon) are separately compared (fig.^ What is the orientation of the planes of the orbits of the moons of Jupiter, compared to the axis of rotation of the planet?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.3); and then shown (on a smaller scale) in comparison with the more distant ones (fig.^ Figure 3 is the same as Figure 2 , but shown at a smaller scale so the orbit of Pluto is shown completely.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

4). .Finally scale diagrams of the distances of the orbits of the satellite-systems of Saturn, Uranus, Jupiter and Neptune are given (fig.^ The other planets ( Jupiter , Saturn , Uranus , and Neptune ) are giant gas planets of which we can only see the thick layers of gas on the outside, and you can't build volcanoes out of gas.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Figure 2 shows the orbits of (from the inside out) Mars , Jupiter , Saturn , Uranus , Neptune , and Pluto , in the same way as the previous picture.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Pluto is merely a relatively large example of a large class of celestial bodies in the Kuiper Belt beyond the orbit of Neptune , that were left over from the formation of the Solar System.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

5).
.The phases of a superior planet are less strongly marked, because the lines from the planet to the earth and sun never increase to a right angle.^ The VSOP model of the motion of the planets (from which the positions of the planets can be calculated quite accurately for thousands of years ) indicates that the distance between the Sun and the Earth can get up to about 2400 km (about 2/5 of the diameter of the Earth) larger or smaller because of the influence of Jupiter.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The associated periods can be measured relative to the stars ( sidereal ) or relative to the Sun , as seen from the planet or from Earth ( synodical ).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The planets and the Earth all orbit around the Sun , each at its own speed, so the distance of a planet from the Earth is not always the same.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The result is that although the apparent disk of Mars is sometimes gibbous in a very marked degree, it is always more than half illuminated.^ The Sun attracts a planet just as hard as the planet attracts the Sun, but the Sun is very much more massive than the planets so it is much harder to move, and that's why the planets have wide orbits while the Sun hardly moves at all.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ No human has ever gone beyond the Moon , and Mars and the other planets are always more than 100 times further away from Earth than the Moon is, so nobody has visited any other planet yet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Mars is now very cold (on average −48 degrees Centigrade) but there has been flowing water on it in the past.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.In the case of the other superior planets, from Jupiter outward, no variation in phase is perceptible even to telescopic vision.^ It was a very productive meeting, even if most of it was getting across the features and requirements of our various projects to the other projects involved.

^ The trick, though, is to pile the planets on top of each other, and to not even show all of the monster Jupiter.

^ The other planets ( Jupiter , Saturn , Uranus , and Neptune ) are giant gas planets of which we can only see the thick layers of gas on the outside, and you can't build volcanoes out of gas.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The entire disk always seems fully illuminated.
.The most favourable time for viewing an inferior planet is near that of greatest brilliancy.^ If tying the viewing direction to a planet or star is not possible with your planetarium program, then you can change the time in steps of 23 hours and 56 minutes .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.As it recedes further from the earth, although a continually increasing proportion of its disk is illuminated by the sun, this advantage is neutralized by the diminution in its size produced by the increasing distance.^ Uranus is on average about 19 times further from the Sun than the Earth is, and takes about 84 years to complete an orbit around the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ In my backyard I see this: each night as the moon moves further and further in its circle around the earth and we see more and more of the illuminated half, the moon is getting just a little brighter.

^ A minor difference in the size of the semimajor axis (roughly equal to the average distance from the Sun) of the orbits of the Earth and the anti-Earth would already be sufficient.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.When a superior planet is in opposition to the sun it rises at sunset and is visible all night.^ Does the Sun shine on all nine planets?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ For the inferior planets , this is approximately when they are in their inferior conjunction , and for the superior planets this is approximately when they are in opposition .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The planets and the Earth all orbit around the Sun , each at its own speed, so the distance of a planet from the Earth is not always the same.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.This is also the time when nearest the earth, and therefore when the circumstances are most favourable for observation.^ In ancient times, most people thought that the Earth is fixed and the stars rotate around the Earth.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ A long time ago, most people thought that the Earth stood still and that the starry sky rotated around the Earth.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The greater the distance of a planet from the sun the less is the speed with which it moves in its orbit.^ What are the least and greatest distances of the planets from the Sun?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Planets orbit around the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The average speed is the speed that the planets have when their distance from the Sun is equal to the length of the semimajor axis of their orbit.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The orbit being larger, the time of its revolution is greater in a yet larger degree. .An approximation to the general laws of speed in different planets is that the linear speed is inversely proportional to the square root of the mean distance.^ There is a pattern to the distances between the Sun and some of the planets , which is called the Law of Titius-Bode.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Even though most of the planets seem to follow the Law of Titius-Bode quite well, there is probably quite a lot of chance to the distances between the planets and the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The planets and the Earth all orbit around the Sun , each at its own speed, so the distance of a planet from the Earth is not always the same.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

From this follows Kepler's third law, that the squares of the times of revolution are proportional to the cubes of the mean distances.
Notes on the Plate showing Planetary Spectra. - .Only those lines and bands are mentioned which are peculiar to the planets; the Fraunhofer lines are therefore omitted.^ Even if you could ignite all of those other elements with a burning match (which you can't), even then only a very small fraction of the gas planet would burn.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The symbols for the other planets were of course only invented after those planets were discovered.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ A gas planet also contains other elements, but those make up only a tiny proportion (less than 1 percent).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Wave
length.
Remarks.
4600
Neptune.
4800
F hydrogen, H 1 3 strong.
Neptune, Uranus, Saturn (?)
5090
Neptune, Uranus.
5190
Broad.
Neptune, Uranus.
537 0
Neptune, Uranus.
543 0
Broad, unsymmetrical,
strong.
Neptune, Uranus, Saturn,
Jupiter.
5570 w
Neptune, Uranus (?).
57 00
Broad, unsymmetrical,
strong.
Neptune, Uranus, Saturn (?)
Jupiter (?).
59 80
Strong.
Neptune, Uranus.
6090
Neptune, Uranus.
6190
Very strong.
Neptune, Uranus, Saturn,
Jupiter.
6400
Broad (?).
Neptune, Uranus.
6500
Neptune, Uranus, Jupiter,
Saturn (?).
6560
C hydrogen, Ha.
Neptune, Uranus.
6670 t
Broad band.
Neptune, Uranus, Saturn,
Jupiter.
[6780
Bright region due to ab-
sence of selective ab-
sorption which is strong
both above and below.
Neptune, Uranus.
6820
Strong, narrow, near
above B.
Neptune, Uranus, Saturn,
Jupiter.
7020
Strong, broad.
Neptune, Uranus, Saturn,
Jupiter.
[7140
Bright, unabsorbed region
similar to that at 6780.
Neptune, Uranus.
7260
Strongest band present.
Saturn, Jupiter.
75 00
Band (?).
Saturn.
.It was once supposed that the planets were surrounded by comparatively dense atmospheres.^ Then it may even get warm enough, and the atmosphere dense enough, for water to flow on Mars once again.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Once in a while, reports start circulating about special conjunctions of planets that are supposed to influence the Earth or to herald special events.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The question whether such is the case, and, if so, what is the physical constitution of the atmospheres, is a difficult one, on which of little light is thrown except by the spectroscope.^ With that definition, all planets except Mercury and (probably) Pluto have an atmosphere, and the moon Titan of Saturn has one, too.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The atmospheres of the other planets that have one contain far too little oxygen or none at all.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.the Planets. If any of these bodies is surrounded by a transparent atmosphere like that of the earth, the light which reaches us from it will have passed twice through this atmosphere.^ If a spaceship were to leave Earth and keep going to the north or south, would it reach any planets?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ What would life be like on an Earth-like planet that was ten times as big?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Though many planets have an atmosphere , there is only one that has an atmosphere in which we can live, and that is the Earth .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.If the latter were materially different in its constitution from that of the earth, that fact would be made known by the spectrum showing absorption lines or bands different from those found in the solar spectrum as we observe it.^ Pictures sent back to Earth by NASA's "Spirit" and "Opportunity" rovers show tracks that were made by the rovers or by their landing craft.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ I have used xnec2c to model 4 different antennas for the 160m band and found some interesting results.
  • Kernel Planet 28 January 2010 0:39 UTC planet.kernel.org [Source type: FILTERED WITH BAYES]

^ If one of the planets of the Solar System (other than Earth) were removed, then the other planets would not notice this very much, and their orbits woud remain practically the same.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.If, however, the planetary atmosphere had the same composition as ours we should see only an intensification of the atmospheric lines, which might be imperceptible were the atmosphere rare.^ As a rule of thumb, given only the commit message, another developer should be able to implement the same patch in a reasonable amount of time.

^ Note: that the above might contain wrong line breaks, almost all lines should start with option, and not with a file path.

^ I might implement zlib compression for network traffic in my engine, which should only be a few dozen lines...

.Actual observation has thus far shown no well marked deviation in the spectra of any of the inner group of planets, Mercury, Venus and Mars, from the solar spectrum as we see it.^ Mercury and Venus have no moons .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Figure 1 shows the orbits of the planets (from the inside out) Mercury, Venus, the Earth, and Mars .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ How many and what kind of moons a planet gets depends a lot on chance, so we cannot say for sure why Venus has no Moons .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

It
follows that any atmospheres these planets may have must, if transparent, be rare. .The evidence in the cases of Venus and Mars is given in the articles on these planets.^ I haven’t been happy with the amount of engagement I’ve given many of these books (or articles) after I have finished reading them, in a long time.

^ Figure 1 shows the orbits of the planets (from the inside out) Mercury, Venus, the Earth, and Mars .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If the Earth overtakes Mars in a stretch where their orbits are closer together, then the smallest distance between the planets will be smaller than average, as was the case in 2003.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Taking the outer group of planets, it is found that the spectrum of Jupiter shows one or more very faint shaded bands not found in that of the sun.^ If you take ever more accurate approximations for the orbital periods of the planets, then the common period after which the planets return to the same relative positions gets on the whole longer and longer.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The trick, though, is to pile the planets on top of each other, and to not even show all of the monster Jupiter.

^ The Sun attracts a planet just as hard as the planet attracts the Sun, but the Sun is very much more massive than the planets so it is much harder to move, and that's why the planets have wide orbits while the Sun hardly moves at all.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.In Saturn these bands become more marked, and in Uranus and Neptune many more are seen.^ The other planets ( Jupiter , Saturn , Uranus , and Neptune ) are giant gas planets of which we can only see the thick layers of gas on the outside, and you can't build volcanoes out of gas.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ When seen in that light it becomes clear that the years I have ahead are certain to be more productive than the ones behind.

^ Mark Turner : Highlights of 2009: Becoming more media savvy .

The spectra in question have been observed both optically and photographically by several observers, among whom Huggins, Vogel and Lowell have been most successful. .It may be said, in a general way, that seven or eight well marked dark bands, as well as some fainter ones are observable in the spectra of the two outer planets.^ At the same time, the original developer may have moved on and no-one knows why the code is that way.

^ Large gas planets may continue to settle, shrink ("collapse") very slowly, for billions of years and generate some heat in that way.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ This way one can inject some fresh air into an existing Debian setup if making a full system upgrading is not an option for you.

The general conclusion from this is that these planets are surrounded by deep and dense atmospheres, semi-transparent, of a constitution which is probably very different from that of the earth's atmosphere. .But it has not, up to the present time, been found practicable to determine the chemical constitution of these appendages, except that hydrogen seems to be an important constituent.^ Yes, it has taken me too long to put these up given that we’re more than a week in to 2010, but maybe getting my resolutions up on time will be a resolution for 2011 .

^ Each time my presentation seems to improve and the kids seem more engaged.

(See Plate.) Intimately associated with this subject is the question of the conditions necessary to the permanence of an atmosphere round a planet. Dr Johnstone Stoney investigated these of conditions, taking as the basis of his work the kinetic theory of gases (Trans. Roy. Dubl. Soc. vi.
305). .On this theory every molecule of a gaseous sp mass is completely disconnected from every other and is in rapid motion, its velocity, which may amount to one or more thousand feet per second, depending on the temperature and on the atomic weight of the gas.^ We may see some more match conditions before 1.8 is released, but for now the ones above seem to suffice for the general cases.

^ The average temperature on a planet depends on its distance fromm its star (further away means colder), on the brightness of the star (brighter means hotter), and on the presence of a greenhouse effect (more means hotter).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Others may have to take one of those bug fixes and apply it to a stable branch but not the other one.

At any temperature the velocities of individual molecules may now and then increase without any well-defined limit. .If at the boundary of an atmosphere the velocity should exceed a certain limit fixed by the mass and force of gravity of the planet, molecules might fly away through space as independent bodies.^ I think that a planet has a greater chance of having moons if the planet has more mass (i.e., is larger) and if the planet is further away from the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ How fast do the planets travel through space?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The force of gravity keeps the atmosphere and other things as close as possible to the ground, and friction with the Earth causes the atmosphere to rotate with the Earth on average, so the atmosphere on average stands still relative to the ground.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The absence of hydrogen from the atmosphere of the earth, and of an atmosphere from the moon, may be thus explained.^ The apparent motion of the Sun , Moon , planets , and stars in the sky can be explained in two ways: (1) the Sun , Moon , planets , and stars orbit around the Earth once a day, or (2) the Earth rotates around its axis once a day.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.If the fundamental hypotheses of Dr Stoney's investigations are correct and complete, it would follow that neither the satellites and minor planets of the solar system nor Mercury can have any atmosphere.^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ So, from 1930 until 2006 there were 9 planets in our Solar System, but since 2006 there are only 8.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If you measure the temperature at the bottom of the atmosphere (if the planet has an atmosphere), then Venus is certainly hotter than Mercury (we've measured this).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.If the separate molecules thus flying away moved according to the laws which would govern an ordinary body, they would, after leaving their respective planets, move round the sun in independent orbits.^ Planets orbit around the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ When things move across a rotating planet , then they notice the rotation in the form of Coriolis forces.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ I think that a planet has a greater chance of having moons if the planet has more mass (i.e., is larger) and if the planet is further away from the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The possibility is thus suggested that the matter producing the zodiacal light may be an agglomeration of gaseous molecules moving round the sun; but several questions respecting the intimate constitution of matter will have to be settled before any definite conclusions on this point can be reached.^ With the nights so long and the sun moving further and further south, who would not want to try to do their part to make up for the absence of the light and the heat?

.It is not to be assumed that a molecule would move through the ether without resistance as the minutest known body does, and there is probably a radical difference between the minutest particle of meteoric matter and the molecule of a gas.^ I had almost memorized the trail, but, every 28 days, I was reminded that, really, there is quite a big difference between memorization and almost-memorization.

^ Even though most of the planets seem to follow the Law of Titius-Bode quite well, there is probably quite a lot of chance to the distances between the planets and the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ There are a few different KDE4 eclasses available in the tree, and I had to read through 'em all to guess which one was most appropriate.

.The relations of identity or difference between such finely-divided matter as smoke and atmospheric haze and a true gas have yet to be fully established, and until this is done a definite and satisfactory theory of the subject does not seem possible.^ In theory the big difference between a process and a thread is that the first owns a memory space and the second share it, so the context switching among process should be more expensive that in threads.

^ A planet is a clump of matter in space, but there are very many clumps of matter in space that we yet do not call planets , such as moons and asteroids and stars .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If there is no difference between summer and winter, then the Sun does not ever get high enough above the horizon at the poles to thaw the ice.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Since the radiation of heat by a planet is, with our present instruments, scarcely capable of detection and measurement, the temperature of these bodies can be estimated only from general physical laws.^ If there is enough radioactive material present in a planet, then this may contribute a significant amount of heat.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Large gas planets may continue to settle, shrink ("collapse") very slowly, for billions of years and generate some heat in that way.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ So, from 1930 until 2006 there were 9 planets in our Solar System, but since 2006 there are only 8.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The laws govern- of ing the radiation of heat have been so developed during recent years that it is now possible to state at least the general principle on which a conclusion as to the temperature of a planet may be reached.^ In other words, if there is some guiding principle that makes most planets follow a Titius-Bode-like law, then astronomers haven't found it yet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The strength of the summer on Mars changes slowly (over hundreds of thousands of years ) and can get at least twice as strong as it is now.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The proposed [agreed now] process is for ADFI staff to prepare at least one scoped and costed software specification, with an agreed development methodology for a: .

.At the same time our knowledge of the conditions which prevail on other planets is so limited, especially as regards their atmospheres, that only more or less probable estimates of the temperature of their surfaces can even now be made.^ And while this is likely to be a sensible strategy, it is at the same time rather limiting.

^ This prevents the planet from cooling down below the average surface temperature that fits with the amount of sunlight that is absorbed by the surface.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ With the greenhouse effect, it can be a lot warmer at the surface, even when the amount of sunlight remains the same.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Summarily stated, some of the physical principles are these: i. A neutrally coloured body - understanding by that term
PLANETARY SPECTRA, PHOTOGRAPHED AT LOWELL OBSERVATORY, FLAGSTAFF, ARIZONA, BY V. M. Slipher.
C B a A Moon F b E D 4 600 430 0 g000 5200 J Upiter.
Satu Rn.
Missing image
Planetaryspectraphotographedatlowellobservatoryflagstaffarizona-1.jpg
.Comparison spectrograms of the Moon and Mars, showing absorption bands in that of the latter, which denote the presence of water vapour in the Martian atmosphere (see Mars).^ Then it may even get warm enough, and the atmosphere dense enough, for water to flow on Mars once again.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The real confirmation, though, would come from an infrared spectrum that shows evidence of deep water ice absorption features, but that requires a pretty big telescope.

^ There appears to have been flowing water on Mars in the past, because some pictures of the surface show branching flow channels.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

6000
Missing image
Planetaryspectraphotographedatlowellobservatoryflagstaffarizona-2.jpg
Missing image
Planetaryspectraphotographedatlowellobservatoryflagstaffarizona-3.jpg
Missing image
Planetaryspectraphotographedatlowellobservatoryflagstaffarizona-4.jpg
5600 I FIG. 2. i:Igel. - Typical of the heiiunr star spectrum; contains, in addition to the helium series and so-called Orion lines, dark lines of hvdroeen.
1 ,.;. Sirius. - .The hydrogen type spectrum; contains dark hydrogen lines at their greatest strength and many fine metallic lines.^ Gas planets also contain helium (only about a tenth as many helium atoms than hydrogen atoms), but helium is a noble gas and does not spontaneously react chemically with anything.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

FIG. 4. P Rocyon. - Transition or hydrogen-to-solar type spectrum. The hydrogen lines are weaker, the metallic lines stroneer than in Siri.,s.
Missing image
Planetaryspectraphotographedatlowellobservatoryflagstaffarizona-5.jpg
FIG. 5. SUN. - Hydrogen absorption fainter, metallic lines stronger and me, Hy - Mg He Fe HI Neb Mg He He HeHe Mg He H.
Ter He Ter He Ter Ter FIG. i. Orion Nebula. - .Typical of the spectrum of the gaseous nebulae; contains bright lines of nebulum, hydrogen and helium.^ Gas planets also contain helium (only about a tenth as many helium atoms than hydrogen atoms), but helium is a noble gas and does not spontaneously react chemically with anything.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

(The helium lines are too faint to show in this reproduction.) FIG. 6. Arcturus. - Advanced solar type spectrum.
FIG. 7. Betelgeuse. - Typical of the spectrum partaking of the nature of that of the sun and that of the long riod variable star g p,, 7500 A FIG. 8. Mira Ceti. - A spectrum typical of the long period variable, containing bright hydrogen lines and Clark bands or flutings which are sharply defined on the more refrangible edge. [Plate used in photographing this spectrum was not sensitive below X 7200, hence no-light action below that point.
Missing image
Planetaryspectraphotographedatlowellobservatoryflagstaffarizona-6.jpg
Missing image
Planetaryspectraphotographedatlowellobservatoryflagstaffarizona-7.jpg
4 0 I 55 D 60 65 ? a ui Fig. 8.
one which absorbs the same fraction of the thermal radiation falling upon it whatever the .wave length of this radiationexposed to the sun's radiation in void space tends to assume a definite temperature, called the normal temperature, the degree of which depends upon the distance of the body from the sun.^ What is the distance between the Earth and the Sun called?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If the Sun is infinitely far away, then that angular distance is 90 degrees .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The average speed is the speed that the planets have when their distance from the Sun is equal to the length of the semimajor axis of their orbit.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

This is a result of Kirchhoff's laws of radiation.
.2. An atmosphere surrounding such a body, if at rest, will tend to assume a state of thermal equilibrium, in which the temperature will be the same at all heights.^ A small thing like a pebble quickly loses heat until it has the same temperature as its surroundings, but a big thing like a planet takes billions of years to cool down.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

3. If the atmosphere is kept in constant motion by an interchange between its higher and lower portions, the tendency is towards adiabatic equilibrium, in which the temperature diminishes at a constant rate with the height, until it may approach the absolute zero. The rate of diminution depends upon the intensity of gravity and the physical constants of the gases composing the atmosphere.
.4. In the actual case of a planet surrounded by an atmosphere and exposed to the sun's radiation, the actual rate of diminution of temperature with height above the surface of the planet lies between the extreme limits just defined, the rate varying widely with the conditions.^ This prevents the planet from cooling down below the average surface temperature that fits with the amount of sunlight that is absorbed by the surface.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ As seen from the planet , the Sun returns to (about) the same place between the stars after this much time .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If you measure the temperature at the bottom of the atmosphere (if the planet has an atmosphere), then Venus is certainly hotter than Mercury (we've measured this).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The general tendency will be towards a condition in which the temperature at the base of the atmosphere is higher than the normal, while in the upper regions it is lower.^ If you measure the temperature at the bottom of the atmosphere (if the planet has an atmosphere), then Venus is certainly hotter than Mercury (we've measured this).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Only near the equator does the temperature sometimes get above freezing, but the atmosphere is so dry there than any open water would evaporate very quickly.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ It is likely that the atmospheres of Venus and the Earth started out similar, but today the atmosphere of Venus is very different from that of Earth: It has a humongous greenhouse effect , surface temperatures of about 450 ℃ or 800 ℉ higher than on Earth and atmospheric pressure at the surface that is about 90 times as great as on Earth.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The temperature of the surface of the planet on which the atmosphere rests is determined partly by the sun's radiation and partly by the temperature of the air.^ The below table shows the pressure at the bottom of the atmosphere of all planets and of the moons Titan (of Saturn ) and Triton (of Neptune ), measured in units of 1 bar (which is roughly equal to the air pressure at the surface of the Earth ).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ This prevents the planet from cooling down below the average surface temperature that fits with the amount of sunlight that is absorbed by the surface.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If you measure the temperature at the bottom of the atmosphere (if the planet has an atmosphere), then Venus is certainly hotter than Mercury (we've measured this).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.What we should generally expect in the absence of any selective absorption by the air is that the temperature of the lower air would be higher than that of the material surface on which it rests.^ This means that even without any greenhouse effect the average surface temperature on Venus would be about 50 ℃ or 90 ℉ higher than on Earth (if all other things were equal), which means that any water on Venus would likely have evaporated quickly, which is one way of getting a greenhouse effect.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If you stir a drink quickly, then the surface of the fluid becomes curved (lower in the middle, and higher near the edges where the speed is greatest).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Only near the equator does the temperature sometimes get above freezing, but the atmosphere is so dry there than any open water would evaporate very quickly.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

But this condition might be reversed by the effect of such absorption in either the air or the material of the planet.
ment. .Something of this sort has been suspected in the case of Jupiter, which has several points of resemblance to the sun.^ This is the case, for example, with the so-called Trojan and Greek asteroids that go around the Sun in the same orbit as Jupiter .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The planets Uranus and Neptune which, but for their atmospheres, would approximate to the absolute zero in temperature, may be prevented from doing so by the dense atmosphere which the spectroscope shows around them.^ An atmosphere is a layer of gases around a planet .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The below table shows the pressure at the bottom of the atmosphere of all planets and of the moons Titan (of Saturn ) and Triton (of Neptune ), measured in units of 1 bar (which is roughly equal to the air pressure at the surface of the Earth ).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Then it may even get warm enough, and the atmosphere dense enough, for water to flow on Mars once again.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.A very elaborate investigation of the probable mean temperatures of the surfaces of the several planets has been made by J. H. Poynting, Phil.^ This prevents the planet from cooling down below the average surface temperature that fits with the amount of sunlight that is absorbed by the surface.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ They may also have a rocky core far below the layers of gas, and that core is then probably made of similar elements as the earth-like planets .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The average temperature on a planet depends on its distance fromm its star (further away means colder), on the brightness of the star (brighter means hotter), and on the presence of a greenhouse effect (more means hotter).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Trans.
(vol. 202A, 1904).
Tables of Planetary Elements and Constants. Table I. gives the elements determining the motions of each major planet, and Table II. certain numbers pertaining to its physical condition. For explanation of terms used see Orbit. The elements are given for the epoch 1900, Jan. o, Greenwich mean time, except the mean longitudes, which are for 1910, Jan. o.
In interpreting or using the numbers it must be remembered that only the mean distances and mean daily motions can be regarded as well determined and invariable quantities. The other elements are subject to a. secular variation, and all vary more or less from the action of the planets. .In Table II. the reciprocal of the mass is given, the mass of the sun being unity.^ The sizes, mass, and density of the Sun and the planets are listed in the following table.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Some of these and other quantities are extremely uncertain. .This is especially the case with the mass of Mercury, which the astronomical tables put at I/6,000,000 that of the sun, while G. W. Hill has computed from an estimate of the probable density of the planet that it is probably less than i/Ii,000,000. In the table we assume the round number i/io,000,000. The volumes are derived from micrometric measures of the diameters, which are more or less uncertain.^ To put it another way, libraries can be economically justified if the cost to lend a book is less than the cost to produce and sell a book.

^ For purposes of comparison, an iterative C-language implementation computes fib(47) in less than 100 nanoseconds , more than 8 orders of magnitude faster.
  • Kernel Planet 28 January 2010 0:39 UTC planet.kernel.org [Source type: FILTERED WITH BAYES]

.From these and the mass follows the density of each planet.^ The sizes, mass, and density of the Sun and the planets are listed in the following table.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Table 10: Planets : Size, Mass, Density .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Size, Mass, and Density of Planets ...
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

From this again is derived the intensity of gravity at the surface; this is also frequently uncertain. .Finally the normal temperature is that which a black or neutrally coloured body would assume when every part of it is equally exposed to the sun's rays by a rapid revolution.^ If, for example, the anti-Earth were one kilometer (one part in a hundred fifty million) closer to the Sun than the Earth, then the orbital period (the year ) of the anti-Earth would be 0.3 seconds less than that of Earth.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ A minor difference in the size of the semimajor axis (roughly equal to the average distance from the Sun) of the orbits of the Earth and the anti-Earth would already be sufficient.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Who would not be at least a little afraid at this time every year that the sun would somehow not decide to stop and then come back?

.As has already been intimated, the actual temperature may also depend upon the interior heat of the planet, which is an unknown quantity.^ The average temperature on a planet depends on its distance fromm its star (further away means colder), on the brightness of the star (brighter means hotter), and on the presence of a greenhouse effect (more means hotter).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ A small thing like a pebble quickly loses heat until it has the same temperature as its surroundings, but a big thing like a planet takes billions of years to cool down.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Which planet is the hottest depends on where you measure the temperature .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.(S. N.) TABLE I.- Elements of the Orbits of the Eight Major Planets.^ Table 2: Planet Orbit Shapes .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The eccentricity e and flattening 1 − b / a of the orbits of the planets is shown in the following table.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Planet.
.Mean Distance from Sun.^ The Astronomical Unit is almost exactly equal to the mean average distance between the Sun and the Earth .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Eccentricity
of Orbit.
Longitude
g
of pen-
helion.
Longitude
of Node.
Inclina-
tion.
Period of
Revolution.
Mean Daily
Motion.
Mean Lon g-
itude 1910,
Jan. o.
Astronomical
Units.
Thousands
of Miles.
Days.
Mercury .
.
0.3870987
36,000
0.205614
75° 54'
47° 9'
7° 0'
87.969256
4°' 0927
3° 32'
Venus .
.
0.7 2 333 1 5
67,269
0 o06821
130° 10'
75° 47'
3° 24'
224.700798
1 0 6021
73° 53'
Earth .
.
i 0000000
92,998
0.016751
101° 13'
-
-
365.256360
0 0 9856
99° 17'
Mars. .
.
I. 523688
141,701
0.093309
334° 1 3'
48° 47'
I° 51'
686.979702
o ° 5240 3
47° 39'
Jupiter .
.
5.202804
483,853
0.048254
12° 3 6 '
99° 37'
1° 1 9'
4332.5879
00.083091
180 43'
Saturn .
9.538844
887,098
0.056061
900 49'
113° 3'
2° 30'
10759.2010
00.033460
28° 56'
Uranus .
.
19.19096
1,784,732
0.047044
169° 3'
73° 29'
0° 46'
30586.29
0° O11770
286° 42'
Neptune .
.
30.07067
2,796,528
0 008533
43° 45'
130° 41'
I° 47'
60187.65
o° 006020
107° I'
TABLE II.- Physical Constants pertaining to the Major Planets.
.
Planet.
Angular Semidiameter.
At
Dist.
Diameter
in Miles.
Reciprocal of
Mass.
(0's mass = I)
Density.
Gravity at
Surface.
= 1)
Orbital
Velocity.
Miles per sec.
Normal
Temperature.
Centigrade.
Equatorial.
ial.
Polar.
(Water = I)
Mercury .
3.30"
3.30"
I
2,976
10,000,000
3.5
633
0.24
29.76
195°
Venus. .
8.46"
8.46"
I
7,629
408,000
5.05
.913
0.880
21 77
70°
Earth. .
8.79"
8.76"
1
7,917
333,430
5.53
1.000
1.00
18.52
19°
Mars. .
4.80"
4'76"
I
4,316
3,093,500
3.68
666
0.363
15.00
- 36°
Jupiter .
18.75"
17.65"
5.203
86,259
1,047.35
I.363
247
2.68
8.12
-144°
Saturn .
8.75"
7.88"
9'539
7 2 ,77 2
3,500
0.678
123
1.13
6 00
-177°
Uranus .
I.90"
1.90"
19.19
32,879
22,869
1 13
204
0.85
4.24
-205°
Neptune .
1.10"
1 10"
30.07
29,827
19,314
1 79
322
I 22
3.40
-218°
.It would follow from these laws that the temperature of the superior planets diminishes rapidly with distance from the sun, and must therefore be far below that of the earth, unless they are surrounded by atmospheres of such height and density as to be practically opaque to the rays of heat, or unless they have no solid crust.^ What does the rotation of a planet such as the Earth do to its atmosphere?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ What are the least and greatest distances of the planets from the Sun?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ What is the distance between the Earth and the Sun called?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The resemblance of the spectra of Mars, Jupiter and Saturn to that of the sun leads to the conclusion that the atmospheres of these planets are transparent down to the reflecting surface of the body.^ With that definition, all planets except Mercury and (probably) Pluto have an atmosphere, and the moon Titan of Saturn has one, too.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The below table shows the pressure at the bottom of the atmosphere of all planets and of the moons Titan (of Saturn ) and Triton (of Neptune ), measured in units of 1 bar (which is roughly equal to the air pressure at the surface of the Earth ).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ It seems likely that the rotation of Mercury was slowed down (in part) because of tidal forces from the Sun , which is close to that planet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The temperature of these surfaces must therefore be determined by Kirchhoff's law, unless they resemble the sun in being entirely liquid or gaseous, or in having only solid nuclei surrounded by liquid matter in a condition of continual move


Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

German

Planet

Pronunciation

Noun

Planet m. (genitive Planeten, plural Planeten)
  1. planet (each of the seven major bodies which move relative to the fixed stars in the night sky)

Simple English

Image of the Solar System planets. Top image shows the Terrestrial planets and the bottom image shows the Gas giants.

A planet is a large object such as Earth or Jupiter that orbits a star. It is smaller than a star, and it does not make light. Planets are ball-shaped (spheres). Objects that orbit planets are called moons. There are eight planets in the Solar System. Pluto used to be known as a planet, but in August 2006, the International Astronomical Union decided it was a dwarf planet instead. There are four more known dwarf planets, Ceres, Makemake, Eris and Haumea.

The name "planet" is from the Greek word πλανήτης (planetes), meaning "wanderers", or "things that move". Until the 1990s, people only knew of those in the Solar System. As of January 2007, we know of 209 other planets. All of the newly found planets are orbiting other stars: they are extrasolar planets. Sometimes people call them "exoplanets".

Contents

In the Solar System

[[File:|thumb|right|300px|Planets from the solar system.]] The planets in the Solar System have names of Greek or Roman gods, apart from Earth, because people did not think Earth was a planet in old times. However, Earth is occasionally referred by the name of a Roman god: Terra. Other languages, for example Chinese, use different names. Moons also have names of gods and people from classical mythology, or from the plays of Shakespeare.

Planets

Here is a list of planets in the Solar System. They are ordered by how close they are to the Sun, nearest first.

Planet Symbol
Mercury File:Mercury
Venus File:Venus
Earth File:Earth
Mars File:Mars
Jupiter File:Jupiter
Saturn File:Saturn
Uranus File:Uranus symbol.
Neptune File:Neptune

Types of planets

Astronomers speak about major (or true) planets, and minor planets, which are smaller objects that go around the Sun. Some examples of "minor planets" are asteroids, comets, and trans-Neptunian objects.

Planets in the Solar System are of three sorts:

  • Terrestrial or rocky: Planets that are similar to Earth — in them is mostly rock: Mercury, Venus, Earth, Mars
  • Jovian or gas giant: These planets are mostly made of gas: Jupiter, Saturn, Uranus, Neptune. Uranian planets are a special sort of gas giants, they have less hydrogen and helium.
  • Icy: Sometimes people also have a third sort, for bodies such as Pluto (though Pluto is no longer called a planet by everyone). These planets are mostly made of ice.

Many objects in the Solar System that are not planets are also "icy". Examples are the icy moons of the outer planets of the Solar System (like Triton).

Other pages

frr:Planeeterue:Планета


Citable sentences

Up to date as of December 19, 2010

Here are sentences from other pages on Planet, which are similar to those in the above article.








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