# Epsilon Eridani: Wikis

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# Encyclopedia

Observation data Characteristics Epoch J2000.0      Equinox J2000.0 The position of ε Eridani Constellation Eridanus Right ascension 03h 32m 55.8442s[1] Declination -09° 27′ 29.744 ″[1] Apparent magnitude (V) 3.73[1] Spectral type K2V[1] Apparent magnitude (B) ~4.61[1] Apparent magnitude (J) 2.228 ±0.298[1] Apparent magnitude (H) 1.880 ±0.276[1] Apparent magnitude (K) 1.776 ±0.286[1] U-B color index +0.58[2] B-V color index +0.88[2] V-R color index +0.50 R-I color index +0.42 Variable type BY[1][3] Radial velocity (Rv) +15.5±0.9[1] km/s Proper motion (μ) RA: -976.36[1] mas/yr Dec.: 17.98[1] mas/yr Parallax (π) 310.74 ± 0.85[1] mas Distance 10.5 ± 0.03 ly (3.218 ± 0.009 pc) Absolute magnitude (MV) 6.19[4] Mass 0.85[4] M☉ Radius 0.84[5] R☉ Surface gravity (log g) 4.57[6] Luminosity 0.28 L☉ Temperature 5084 ± 5.9 [7] K Metallicity [Fe/H]=-0.13±0.04[8] Rotation 11.1 days Age (0.5–1.0) × 109[9] years SIMBAD data Extrasolar Planets Encyclopaedia data 18 Eridani, BD -09°697, GCTP 742.00, GJ 144, HD 22049, HIP 16537, HR 1084, LHS 1557, SAO 130564, WDS 03330-0928[1]

Epsilon Eridani (ε Eri / ε Eridani) is a main-sequence star of spectral class K2. Only 10.5 light years away, it is the closest star in the constellation Eridanus, as well as the third closest star visible to the naked eye. Its age is estimated at less than a billion years. Because of its relative youth, Epsilon Eridani has a higher level of magnetic activity than the Sun, with a stellar wind 30 times as strong. Its rotation period is a relatively rapid 11.1 days, although this varies by latitude. Epsilon Eridani is both smaller and less massive than the Sun, with a lower metallicity (enrichment in elements heavier than helium).[9]

Radial velocity observations over the past 20 years yield evidence of a gas giant planet orbiting Epsilon Eridani, making it the nearest extrasolar system with a candidate exoplanet.[10] This unconfirmed object, conventionally known as Epsilon Eridani b, was formally announced in 2000 by a team of astronomers led by Artie Hatzes.[10] However, the radial velocity data contain a high level of background noise due to the star's magnetic activity, and the proposed planet is not universally accepted.[11] Current data indicate that this planet orbits in a period of about 7 years at a mean separation of 3.4 astronomical units (AU), corresponding to 505 million kilometers.[12]

The system also includes two debris belts composed of rocky asteroids, one at about 3 AU and the second at about 20 AU, whose structure may be maintained by a hypothetical second planet.[13] In addition, Epsilon Eridani harbors an extensive outer debris disk corresponding to the Solar System's Kuiper belt.[11] The density of orbiting material, which is considerably more than that around the Sun, corroborates the star's youth.

As one of the nearest Sun-like stars, Epsilon Eridani regularly appears in science fiction. Its closest neighbor is the M dwarf binary system Luyten 726-8, at a distance of about 5 light years.[14]

## Observation

This star is located in the northern part of the constellation Eridanus, about 3° east of the slightly brighter star Delta Eridani. With a declination of −9.45°, Epsilon Eridani can be viewed from much of the Earth's surface. Only to the north of latitude 80° N is it permanently hidden below the horizon.[15] The apparent magnitude of 3.73 can make this star difficult to observe from an urban area with the unaided eye, as the night skies over cities are compromised by light pollution.[16]

The Bayer designation for this star was established in 1603 as part of the Uranometria, a star catalog produced by Johann Bayer. Epsilon is the fifth letter in the Greek alphabet, and it was assigned to the fifth brightest star in the constellation of Eridanus.[17] The preliminary version of the star catalog by John Flamsteed, published in 1712, gave this star the Flamsteed designation 18 Eridani.[1] In 1918 this star appeared in the Henry Draper Catalogue with the designation HD 22049 and a preliminary spectral classification of K0.[18]

Based on observations between 1800 and 1880, Epsilon Eridani was found to have a large proper motion, which at the time was estimated at three arcseconds annually. This implied a relatively close proximity to the Sun,[19] making it a star of interest for the purpose of trigonometric parallax measurements.[20] From 1881–3, William L. Elkin made a series of heliometric measurements from the Royal Observatory at the Cape of Good Hope, South Africa. As a result of these observations, a preliminary parallax of 0.14 ± 0.02 arcseconds was computed for Epsilon Eridani.[21][22] By 1917, observers had refined their parallax estimate to 0.317 arcseconds,[23] which is quite close to the modern value of 0.3107 arcseconds.[1] This parallax is equivalent to a distance of about 10.5 light years, making Epsilon Eridani the 13th nearest known star (and ninth nearest stellar system) to the Sun.[4]

In 1960, Project Ozma, headed by Dr. Frank Drake, used a radio telescope at Green Bank, West Virginia, to search for signals from putative extraterrestrial intelligences. Its target stars were Epsilon Eridani and Tau Ceti. However, no signals of extraterrestrial origin were detected.[24]

In 1983, NASA's orbiting telescope IRAS detected excess infrared emissions originating from Epsilon Eridani, indicating the presence of an orbiting disk of fine-grained dust.[25][26] This so-called debris disk has been extensively studied since that time.

In 1995, based on its location within 7.2 pc, Epsilon Eridani was among the target stars of Project Phoenix, a microwave survey for signals from extraterrestrial intelligence.[27] By 2004 Project Phoenix had checked about 800 stars, but had not yet detected an unimpeachable signal.[28]

Based on perturbations in the position of Epsilon Eridani between 1938 and 1972, it was suspected that the star had an unseen companion with an orbital period of 25 years.[29] However, this claim was refuted in 1993. Radial velocity observations between 1980 and 2000 then provided convincing evidence of a planet orbiting the star with a period of about seven years.[10] The evidence for a planetary system was further strengthened in 1998 by the discovery of asymmetries in the dust ring around star. These clumps of dust could be explained by interaction with a planet orbiting just inside the dust ring.[30]

## Properties

The relative size of Epsilon Eridani (left) compared to the Sun (right).

Epsilon Eridani has an estimated 85% of the Sun's mass[4] and 84% of the Sun's radius,[5] but only 28% of its luminosity. It is the second-nearest spectral class K star, after Alpha Centauri B.[4] Its metallicity or enrichment in elements heavier than helium is slightly lower than the Sun's, with its chromospheric abundance of iron estimated at 74% Solar.[8]

The chromosphere of Epsilon Eridani is more magnetically active than the Sun's. Approximately 9% of the deep photosphere is found to have a magnetic field with a strength about 0.14 teslas.[31] The overall magnetic activity level of this star is irregular, but it may vary with a five-year period. Assuming that the radius of the star does not change over this interval, then the variation in activity level appears to produce a temperature variation of 15 K, which corresponds to a magnitude variation of 0.014.[32]

Rotational modulation of the magnetic activity suggests that the equator of the star rotates with a period of 11.10 ± 0.03 days, which is less than half of the rotation period of the Sun. Stars that vary in magnitude because of magnetic activity coupled with rotation are classified as BY Draconis variables.[3] Observations have show this star to vary as much as 0.050 in magnitude due to starspots and other short-term magnetic activity.[33] Photometry has also shown that the surface of Epsilon Eridani, like the Sun, is undergoing differential rotation. That is, the rotation period at the surface varies by latitude, with the measured periods ranging from 10.8 to 12.3 days.[32] The axial tilt of this star remains uncertain, with estimates ranging from a low of 24° up to 72°.[34]

The high level of chromospheric activity, strong magnetic field, and relatively fast rotation rate indicate that this is a young star.[35] Computer models give an estimated age of 700–850 million years, although the actual age may be a low as 500 million or as high as a billion years.[9] However, the somewhat low abundance of heavy elements is characteristic of a much older star. This anomaly might be caused by a diffusion process that has transported some of the helium and heavier elements out of the photosphere to a region below the star's outer convection zone.[36] As such the age of Epsilon Eridani remains subject to much debate.

Relative to the Sun, the outer atmosphere of Epsilon Eridani appears both larger and hotter. This is caused by a 30-fold higher mass loss rate from the star's stellar wind. The wind is generating an astrosphere that spans about 8,000 AU and a bow shock that lies 1,600 AU from the star. At the star's estimated distance from the Earth, this astrosphere would span an angle of 42 arcminutes, which is wider than the appearance of a full Moon.[37]

The space velocity components of Epsilon Eridani are U = −3 km/s, V = +7 km/s and W = −20 km/s. It is orbiting within the Milky Way at a mean galactocentric distance of 8.8 kpc and an orbital eccentricity of 0.09.[38] During the past million years, three stars are believed to have come within two parsecs of Epsilon Eridani. The most recent encounter was with Kapteyn's Star, which approached within about a parsec about 12,500 years ago. None of these encounters are thought to have affected the circumstellar disk.[39] Epsilon Eridani made its closest approach to the Solar System about 105,000 years ago, when the two stars were separated by seven light years.[40]

## Planetary system

The Epsilon Eridani system
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
Eccentricity
Asteroid belt 3 AU
b (unconfirmed) 1.55 ± 0.24 MJ 3.39 ± 0.36 2502 ± 10 0.702 ± 0.039?
Asteroid belt 20 AU
c (unconfirmed) 0.1 MJ 40? 102200? 0.3
Dust disk 35 — 100 AU

### Planets

As one of the nearest Sun-like stars, Epsilon Eridani has been the target of many attempts to search for planetary companions.[10][41] However, the star's chromospheric activity and variability means that finding planets with the radial velocity method is difficult, because stellar activity may create signals that mimic the presence of planets.[42] Attempts at direct photographic imaging of potential exoplanets have proved unsuccessful to date.[41]

The proposed planet Epsilon Eridani b is shown orbiting within a zone that has been cleared of dust.

Nevertheless, long-term radial velocity observations led to the announcement in 2000 of a gas giant planet with a relatively long-period orbit around this star.[10] Conventionally known as Epsilon Eridani b, this planet has remained controversial. A comprehensive study in 2008 called the detection "tentative" and described the proposed planet as "long suspected but still unconfirmed."[11] Published sources remain in disagreement as to the unconfirmed planet's basic parameters. Values for its orbital period range from 6.85 to 7.2 years;[43][44] values for its semimajor axis range from 3.38 AU to 3.50 AU;[45][44] values for its orbital eccentricity range from 0.25±0.23 to 0.702±0.039;[45][43] and values for its m sin i or minimum mass range from 0.60 Jupiter masses to 1.06 Jupiter masses.[44][45]

Note that the frequently cited value of 1.55 Jupiter masses for the unconfirmed planet's mass is an estimate of true mass, derived by using 0.78 for m sin i and 30° for i (inclination).[43] Unfortunately, this estimate is only as reliable as the input data. Out of all these uncertainties, the value for orbital eccentricity seems to be the most significant. The frequently cited value of 0.7 for Epsilon Eridani b's eccentricity is inconsistent with the presence of the proposed asteroid belt at a distance of 3 AU from the star. If the eccentricity was actually this high, the planet would pass through the asteroid belt and rapidly clear it out.[46]

A possible 280 year-period low-mass planet Epsilon Eridani c orbits at 40 AU in a less eccentric orbit — 0.3. No bodies of 3 or more Jupiter masses exist in this system.[41]

A second planet may orbit the star, but this has not been confirmed.

The presence of an outer planet in orbit around Epsilon Eridani would have a perturbing effect on cometary bodies within the dust ring. Some of these bodies would fall toward the inner part of the system, and could cross any planetary orbits within 1 AU of the star. Thus, a terrestrial planet would be subject to bombardment similar to what happened to the Earth during its first 600 million years with the Late Heavy Bombardment.[47]

In the 1964 RAND Corporation study, Habitable Planets for Man by Stephen H. Dole, the odds of a habitable planet in orbit around Epsilon Eridani were estimated as 3.3%. Among the known stars within 22 light years, it was listed with the 14 stars that were thought most likely to have a habitable planet.[48] The maximum habitable zone for Epsilon Eridani currently stretches from about 0.5–1.0 astronomical units (AU), where an astronomical unit is equal to the average distance between Earth and the Sun. As the star ages over a period of 20 billion years, this zone will slowly expand outward to about 0.6–1.4 AU.[49] However, the presence of a large planet with a highly elliptical orbit in proximity to the habitable zone of the star reduces the likelihood of a terrestrial planet having a stable orbit within the habitable zone.[50]

Epsilon Eridani is a target for planet finding programs because it has the properties to allow an Earthlike planet to form. Although this system was not chosen as a primary candidate for the now-cancelled Terrestrial Planet Finder, it is a target star for NASA's proposed Space Interferometry Mission that will search for Earth-sized planets.[51]

### Dust disk

Two asteroid belts surrounding Epsilon Eridani.

Observations with the James Clerk Maxwell Telescope showed an extended flux of radiation with sub-millimetre wavelengths at a radius of 35 arcseconds around the star. The peak emission occurs at an angular radius of 18 arcseconds, which at the distance of the star corresponds to a distance of about 60 AU. A lower level of emission is also seen at 30 AU. This emission was interpreted as coming from an analog of the Kuiper Belt in the Solar System; a compact dusty disk structure surrounding the star. The belt is being viewed at an inclination of roughly 25° to the line of sight.[52]

The asymmetrical structure of the dust belt may be explained as the gravitational perturbation by a planet. The clumps in the dust occur at orbits that have an integer resonance with the orbit of the suspected planet. Thus, for example, the region of the disk that completes two orbits for every three orbits of a planet are in 3:2 resonance.[53] With computer simulations, the ring morphology can be reproduced by the capture of dust particles in 5:3 and 3:2 orbital resonances with a planet that has an orbital eccentricity of about 0.3.[54]

The dust disk contains an estimated mass equal to a sixth of mass of the Moon. This dust is being generated by the collision of comets, which range up to 10 to 30 km in diameter and have a combined mass of 5 to 9 times the mass of the Earth. This is similar to the estimated 10 Earth masses in the Kuiper Belt.[47]

Within 35 AU of the star the dust is depleted, which may mean that the system has formed planets which have cleared out the dust in this region. This is consistent with currently accepted models of the inner solar system, and so there may be terrestrial planets around the star.

On October 27, 2008, NASA's Spitzer Space Telescope revealed observations indicating that Epsilon Eridani actually has two asteroid belts, and a cloud of exozodiacal dust. One belt sits at approximately the same position as the one in our solar system. The second, denser belt, most likely also populated by asteroids, lies between the first belt and the comet ring. The presence of the asteroid belts implies additional planets in the Epsilon Eridani system.[46]

## Footnotes and references

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