# Tau Ceti: Wikis

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Observation data Characteristics Epoch J2000      Equinox J2000 Tau Ceti (highlighted center) in the southern part of the constellation Cetus. Constellation Cetus Pronunciation /ˌtaʊ ˈsiːtaɪ/ Right ascension 01h 44m 04.0829s[1] Declination −15° 56′ 14.928″[1] Apparent magnitude (V) 3.50 ± 0.01[2] Spectral type G8 V[1] U-B color index +0.22[1] B-V color index +0.72[1] Variable type None Radial velocity (Rv) −16.4[1] km/s Proper motion (μ) RA: −1721.94[1] mas/yr Dec.: 854.17[1] mas/yr Parallax (π) 273.96 ± 0.17[2] mas Distance 11.905 ± 0.007 ly (3.65 ± 0.002 pc) Absolute magnitude (MV) 5.69 ± 0.01[2] Mass 0.783 ± 0.012[2] M☉ Radius 0.793 ± 0.004[2] R☉ Surface gravity (log g) 4.4[3] Luminosity 0.52 ± 0.03[4] L☉ Temperature 5,344 ± 50[5] K Metallicity 22–74%[3][6] Rotation 34 days[7] Age ~10[8] Gyr Durre Menthor,[9][10] 52 Ceti, HD 10700, HR 509, BD-16°295, GCTP 365.00, GJ 71, LHS 146, LTT 935, LFT 159, SAO 147986, LPM 84, FK5 59, HIP 8102.[1]

Tau Ceti (τ Cet / τ Ceti) is a star in the constellation Cetus that is similar to the Sun in mass and spectral type. At just under 12 light years' distance from the Solar System, it is a relatively close star. Tau Ceti is "metal-deficient" and so it is thought to be less likely to host rocky planets. Observations have detected more than 10 times as much dust surrounding Tau Ceti as is present in the Solar System. The star appears stable, with little stellar variation.

No companions have yet been detected around Tau Ceti through astrometric or radial velocity measurements, but given current search refinement this only excludes substellar companions such as large brown dwarfs. Because of the debris disk, any planet orbiting Tau Ceti would face far more impact events than the Earth. Despite this hurdle to habitability, its "solar analog" (Sun-like) characteristics have led to widespread interest in the star. It has been consistently listed as a target for Search for Extra-Terrestrial Intelligence (SETI) searches, given its stability and similarity to the Sun, and it appears in some science fiction literature.

Tau Ceti does not have a widely recognized traditional name, as do many other prominent stars.[nb 1] It can be seen with the unaided eye as a faint third-magnitude star.[nb 2] As seen from Tau Ceti, the Sun would be a third-magnitude star in the constellation Boötes.[nb 3]

## Motion

The proper motion of a star is its amount of movement across the celestial sphere, determined by comparing its position relative to more distant background objects. Tau Ceti is considered to be a high-proper-motion star, although it only has an annual traverse of just under two arc seconds.[nb 4] It will require several thousand years before the location of this star shifts by more than a degree. A high proper motion is an indicator of close proximity to the Sun.[11] Nearby stars can traverse an angle of arc across the sky more rapidly than the distant background stars and are good candidates for parallax studies. In the case of Tau Ceti, the parallax measurements indicate a distance of 11.9 light-years. This makes it one of the closest star systems to the Sun, and the next-closest spectral class-G star after Alpha Centauri A.[12]

The radial velocity of a star is its motion toward or away from the Sun. Unlike proper motion, a star's radial velocity can not be directly observed, but must be determined through measurement of the spectrum. Due to the Doppler shift, the absorption lines in the spectrum of a star will be shifted slightly toward the red (or longer wavelengths) if the star is moving away from the observer, or toward blue (or shorter wavelengths) when it moves toward the observer. In the case of Tau Ceti, the radial velocity is about −17 km/s, with the negative value indicating that it is moving toward the Sun.[13]

The distance to Tau Ceti, along with its proper motion and radial velocity, allow the motion of the star through space to be calculated. The space velocity relative to the Sun is about 37 km/s.[nb 5] This result can then be used to compute an orbital path of Tau Ceti through the Milky Way galaxy. It has a mean galacto-centric distance of 9.7 kiloparsecs (32,000 light-years) and an orbital eccentricity of 0.22.[14]

## Physical properties

The Sun (left) is both larger and somewhat hotter than the less active Tau Ceti (right).

The Tau Ceti system is believed to have only one stellar component. A dim optical companion has been observed, which is possibly gravitationally bound, but it is more than 10 arcseconds distant from the primary.[15] No astrometric or radial velocity perturbations have been deduced, suggesting a star that does not have a large companion in a close orbit, such as a "hot Jupiter." Any potential gas giants around Tau Ceti are likely to be located more at Jupiter-like distances.[16][17]

Most of what is known about the physical properties of Tau Ceti has been determined through spectroscopic measurements. By comparing the spectrum to computed models of stellar evolution, the age, mass, radius and luminosity of Tau Ceti can be estimated. However, using an astronomical interferometer, measurements of the radius of the star can be made directly to an accuracy of 0.5%.[2] It deploys a long baseline to measure angles much smaller than can be resolved with a conventional telescope. Through this means, the radius of Tau Ceti has been measured as 79.3 ± 0.4% of the solar radius.[2] This is about the size that is expected for a star with somewhat lower mass than the Sun.[8]

### Rotation

The rotation period for Tau Ceti was measured by periodic variations in the classic H and K absorption lines of singly ionized Calcium, or Ca II. These lines are closely associated with surface magnetic activity,[18] so the period of variation measures the time required for the activity sites to complete a full rotation about the star. By this means the rotation period for Tau Ceti is estimated to be 34 days.[7] Due to the Doppler effect, the rotation rate of a star affects the width of the absorption lines in the spectrum. (Light from the side of the star moving away from the observer will be shifted to a longer wavelength; light from the side moving towards the observer will be shifted toward a shorter wavelength.) So by analyzing the width of these lines, the rotational velocity of a star can be estimated. The projected rotation velocity for Tau Ceti is:

$\begin{smallmatrix} v_{eq} \cdot \sin i\ \approx\ 1\ \text{km/s} \end{smallmatrix}$.

where veq is the velocity at the equator and i is the inclination angle of the rotation axis to the line of sight. For a typical G8 star, the rotation velocity is about 2.5 km/s. The relatively low rotational velocity measurements may indicate that Tau Ceti is being viewed from nearly the direction of its pole.[19][20]

### Metallicity

The chemical composition of a star provides important clues to its evolutionary history, including the age at which it formed. The interstellar medium of dust and gas from which stars form is primarily composed of hydrogen and helium with trace amounts of heavier elements. As nearby stars continually evolve and die, they seed the interstellar medium with an increasing portion of heavier elements. Thus younger stars will tend to have a higher portion of heavy elements in their atmospheres than do the older stars. These heavy elements are termed metals by astronomers and the portion of heavy elements is the metallicity.[21] The amount of metallicity in a star is given in terms of the ratio of iron (Fe), an easily observed heavy element, to hydrogen. A logarithm of the relative iron abundance is compared to the Sun. In the case of Tau Ceti, the atmospheric metallicity is roughly:

$\begin{smallmatrix} \left [ \frac{Fe}{H} \right ] \; = \; -0.50 \end{smallmatrix}$

or about a third the solar abundance. Past measurements have varied from -0.13 to -0.60.[3][6]

This lower abundance of iron indicates that Tau Ceti is almost certainly older than the Sun: its estimated age is about 10 Gyr compared to 4.57 Gyr for the Sun. Ten billion years represents a substantial portion of the age of the visible universe. However, computed age estimates for Tau Ceti can range from 4.4–12 Gyr, depending on the model adopted.[8]

Besides rotation, another factor that can widen the absorption features in the spectrum of a star is pressure-broadening. (See spectral line.) The presence of nearby particles will affect the radiation emitted by an individual particle. So the line width is dependent on the surface pressure of the star, which in turn is determined by the temperature and surface gravity. This technique was used to determine the surface gravity of Tau Ceti. The log g, or logarithm of the star's surface gravity, is about 4.4—very close to the log g = 4.44 for the Sun.[3]

### Luminosity and variability

The luminosity of Tau Ceti is equal to only 55% of the Sun's luminosity.[14] A terrestrial planet would need to orbit this star at a distance of about 0.7 astronomical units (or AU, the average distance from the Earth to the Sun) in order to match the solar-insolation level of the Earth. This is approximately the same as the average distance between Venus and the Sun.

The chromosphere of Tau Ceti—the portion of a star's atmosphere just above the light-emitting photosphere—currently displays little or no magnetic activity, indicating a stable star.[22] One nine-year study of temperature, granulation, and the chromosphere showed no systematic variations; Ca II emissions around the H and K infrared bands show a possible 11-year cycle, but this is weak relative to the Sun.[19] Alternatively it has been suggested that the star could be in a low-activity state analogous to a Maunder minimum—a historical period, associated with the Little Ice Age in Europe, when sunspots became exceedingly rare on the Sun's surface.[23][24] Spectral line profiles of Tau Ceti are extremely narrow, indicating low turbulence and observed rotation.[25] The amplitude of the star's oscillations are about half those of the Sun, and have a lower mode lifetime.[2]

### Debris disk

In 2004, a team of UK astronomers led by Jane Greaves discovered that Tau Ceti has more than 10 times the amount of cometary and asteroidal material orbiting it than does our Sun. This was determined by measuring the disk of cold dust orbiting the star produced by collisions between such small bodies.[26] This result puts a damper on the possibility of complex life in the system, as any planets would suffer from large impact events roughly ten times more frequently than Earth. Greaves noted at the time of her research: "it is likely that [any planets] will experience constant bombardment from asteroids of the kind believed to have wiped out the dinosaurs."[27] Such bombardments would inhibit the development of biodiversity between impacts.[28] However, it is possible that a large Jupiter-sized gas giant could deflect comets and asteroids.[26][nb 6]

The debris disk was discovered by measuring the amount of radiation emitted by the system in the far infrared portion of the spectrum. The disk forms a symmetric feature that is centered on the star, and the outer radius averages 55 AU. The lack of infrared radiation from the warmer parts of the disk near Tau Ceti imply an inner cut-off at a radius of 10 AU. By comparison, the Solar System's Kuiper belt extends from 30 to 50 AU. To be maintained over a long period of time, this ring of dust must be constantly replenished through collisions by larger bodies.[26] The bulk of the disk appears to be orbiting Tau Ceti at a distance of 35–50 AU, well outside the orbit of the habitable zone. At this distance, the dust belt may be analogous to the Kuiper belt that lies outside the orbit of Neptune in the solar system.[26]

Tau Ceti shows that stars need not lose large disks as they age and such a thick belt may not be uncommon among Sun-like stars.[29] Tau Ceti's belt is only 1/20th as dense as the belt around its young neighbor, Epsilon Eridani.[26] The relative lack of debris around the Sun may be the unusual case: one research team member suggests the Sun may have passed close to another star early in its history and had most of its comets and asteroids stripped away.[27] Stars with large debris disks have altered astronomical thinking about planet formation; debris disk stars, where dust is continually generated by collisions, appear to form planets readily.[29]

## Life and planet searches

Principal factors driving research interest in Tau Ceti are its Sun-like characteristics and their implications for possible planets and life. Hall and Lockwood report that "the terms 'solarlike star,' 'solar analog,' and 'solar twin' [are] progressively restrictive descriptions."[30] Tau Ceti fits the second category, given its similar mass and low variability, but relative lack of metals.[nb 7] The similarities have inspired popular culture references for decades, as well as scientific examination.

Tau Ceti was a target of a few radial velocity planetary searches, which have failed to find any periodical variations attributable to planets.[17] The velocity precision reached so far is about 11 m/s measured over a five year time span.[31] This result excludes the presence of hot Jupiters, and probably excludes any planets with minimum mass greater than or equal to Jupiter’s mass and with orbital periods less than 15 years.[32] In addition, a survey of nearby stars by the Hubble Space Telescope's Wide Field and Planetary Camera was completed in 1999, including a search for faint companions to Tau Ceti; none were discovered to limits of the telescope's resolving power.[33]

These searches only excluded larger brown dwarf bodies and giant planets so a smaller, Earth-like planet in orbit around the star is not precluded.[33] If "hot Jupiters" did exist in close orbit they would likely disrupt the star's habitable zone; their exclusion is thus a positive for the possibility of Earth-like planets.[16][34] General research has shown a positive correlation between the presence of extrasolar planets and a relatively high metal parent star, suggesting that stars with lower metallicity such as Tau Ceti have a reduced chance of possessing planets.[35] The evidence of a thick debris disk increases the likelihood that one or more rocky planets orbit the star, however, even if it suggests a high bombardment scenario. If planets are found, subsequent searches, with telescopes of sufficient resolution, would look for atmospheric water and temperatures suitable for habitability. Primitive life might reveal itself through an atmospheric composition unlikely to be inorganic, just as oxygen on Earth is indicative of life.[36]

### SETI and HabCat

Tau Ceti may be a search target for the Terrestrial Planet Finder

The most optimistic search project to date was Project Ozma, which was intended to "search for extraterrestrial intelligence" (SETI) by examining selected stars for indications of artificial radio signals. It was run by the astronomer Frank Drake, who selected Tau Ceti and Epsilon Eridani as the initial targets. Both are located near the solar system and are physically similar to the Sun. No artificial signals were found despite 200 hours of observations.[37] Subsequent radio searches of this star system have also turned up negative.

This lack of results has not dampened interest in observing the Tau Ceti system for biosignatures. In 2002, astronomers Margaret Turnbull and Jill Tarter developed the Catalog of Nearby Habitable Systems (HabCat) under the auspices of Project Phoenix, another SETI endeavour. The list contained more than 17,000 theoretically habitable systems, approximately 10% of the original sample.[38] The next year, Turnbull would further refine the list to the 30 most promising systems out of 5,000 within one hundred light-years of the Sun, including Tau Ceti; this will form part of the basis of radio searches with the Allen Telescope Array.[39] She also chose Tau Ceti for a final shortlist of just five stars suitable for searches by the Terrestrial Planet Finder telescope system, commenting that "these are places I'd want to live if God were to put our planet around another star."[40] While NASA's TPF mission is indefinitely postponed beyond its original, planned launch date of 2014 due to congressional budget restrictions[41], the European Space Agency's Darwin Mission may target Tau Ceti among others.

## Notes

1. ^ Its name is a Bayer designation: 'Tau' is a Greek letter and 'Ceti' the possessive form of Cetus.
2. ^ It can not be observed above latitude 75°N, as that is 90° north of the declination, 15°S. In practice, atmospheric effects will reduce visibility of the object when it is near the horizon.
3. ^ From Tau Ceti the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=13h 44m 04s, Dec=15° 56′ 14″, which is located near Tau Boötis. The absolute magnitude of the Sun is 4.8, so, at a distance of 3.64 parsecs, the Sun would have an apparent magnitude:
$\begin{smallmatrix} m = M_v + 5\cdot((\log_{10} 3.64) - 1) = 2.6 \end{smallmatrix}$.
4. ^ The net proper motion is given by:
$\begin{smallmatrix} \mu = \sqrt{ {\mu_\delta}^2 + {\mu_\alpha}^2 \cdot \cos^2 \delta } = 1907.79\,\text{mas/y} \end{smallmatrix}$ where μα and μδ are the components of proper motion in the R.A. and Declination, respectively, and δ is the Declination. See:
Majewski, Steven R. (2006). "Stellar Motions". University of Virginia. Retrieved 2007-09-27.
5. ^ The space velocity components are: U = +18; V = +29, and W = +13. This yields a net space velocity of:
$\begin{smallmatrix} \sqrt{18^2 + 29^2 + 13^2} = 36.5\,\text{km/s.} \end{smallmatrix}$
6. ^ Whether Jupiter actually provides protection to the inner solar system is still unresolved. See, for instance:
"Jupiter: Friend Or Foe?". Science daily. 2007-08-25. Retrieved 2009-03-10.
7. ^ The star 18 Scorpii, arguably the truest Solar twin, presents a contrastive example to Tau Ceti: its metallicity is in keeping with Sol but its variability is significantly higher. See:
Hall, J. C.; Lockwood, G. W. (2000). "Evidence of a Pronounced Activity Cycle in the Solar Twin 18 Scorpii". The Astrophysical Journal 545 (2): L43–L45. doi:10.1086/317331.

## References

1. ^ a b c d e f g h i "LHS 146 – High proper-motion Star". Centre de Données astronomiques de Strasbourg. Retrieved 2009-01-14.
2. ^ a b c d e f g h Teixeira, T. C.; Kjeldsen, H.; Bedding, T. R.; Bouchy, F.; Christensen-Dalsgaard, J.; Cunha, M. S.; Dall, T.; Frandsen, S.; Karoff, C.; Monteiro, M. J. P. F. G.; Pijpers, F. P. (January 2009). "Solar-like oscillations in the G8 V star τ Ceti". Astronomy and Astrophysics (1): 237–242. doi:10.1051/0004-6361:200810746.
3. ^ a b c d de Strobel, G. Cayrel; Hauck, B.; Francois, P.; Thevenin, F.; Friel, E.; Mermilliod, M.; Borde, S. (1991). "A catalogue of Fe/H determinations - 1991 edition". Astronomy and Astrophysics Supplement Series 95 (2): 273–336. Retrieved 2007-08-14.
4. ^ Pijpers, F. P. (2003). "Selection criteria for targets of asteroseismic campaigns". Astronomy and Astrophysics 400: 241–248. doi:10.1051/0004-6361:20021839.
5. ^ Santos, N. C.; Israelian, G.; García López, R. J.; Mayor, M.; Rebolo, R.; Randich, S.; Ecuvillon, A.; Domínguez Cerdeña, C. (2004). "Are beryllium abundances anomalous in stars with giant planets?". Astronomy and Astrophysics 427: 1085–1096. doi:10.1051/0004-6361:20040509. Retrieved 2007-02-26.
6. ^ a b Flynn, C.; Morell, O. (1997). "Metallicities and kinematics of G and K dwarfs". Monthly Notices of the Royal Astronomical Society 286 (3): 617–625. Retrieved 2007-08-14.
7. ^ a b Baliunas, S.; Sokoloff, D.; Soon, W. (1996). "Magnetic Field and Rotation in Lower Main-Sequence Stars: an Empirical Time-dependent Magnetic Bode's Relation?". Astrophysical Journal Letters 457: L99. doi:10.1086/309891. Retrieved 2007-08-14.
8. ^ a b c Di Folco, E.; Thévenin, F.; Kervella, P.; Domiciano de Souza, A.; du Foresto, V. Coudé; Ségransan, D.; Morel, P. (2004). "VLTI near-IR interferometric observations of Vega-Like Stars". Astronomy and Astrophysics 426: 601–617. doi:10.1051/0004-6361:20047189. Retrieved 2007-08-14.
9. ^ Malin, David (June 8, 2008). "Cetus". David Malin Images. Retrieved 2009-06-24.
10. ^ Anonymous. "Cetus". Omnipelagos.com. Retrieved 2009-06-24.   < الدرر المنثور al durr' al-manthūur The Scattered Pearls (of the Broken Necklace).
11. ^ Reid, Neill (February 23, 2002). "Meeting the neighbours: NStars and 2MASS". Space Telescope Science Institute. Retrieved 2006-12-11.
12. ^ Henry, Todd J. (October 1, 2006). "The One Hundred Nearest Star Systems". Research Consortium on Nearby Stars. Retrieved 2006-12-11.
13. ^ Butler, R. P.; Marcy, G. W.; Williams, E.; McCarthy, C.; Dosanjh, P.; Vogt, S. S. (1996). "Attaining Doppler Precision of 3 M s-1". Publications of the Astronomical Society of the Pacific 108: 500. doi:10.1086/133755. Retrieved 2006-12-11.
14. ^ a b Porto de Mello, G. F.; del Peloso, E. F.; Ghezzi, L. (2006). "Astrobiologically interesting stars within 10 parsecs of the Sun". Astrobiology 6 (2): 308–331. doi:10.1089/ast.2006.6.308.
15. ^ Pijpers, F. P.; Teixeira, T. C.; Garcia, P. J.; Cunha, M. S.; Monteiro, M. J. P. F. G.; Christensen-Dalsgaard, J. (2003). "Interferometry and asteroseismology: The radius of τ Ceti". Astronomy & Astrophysics 401: L15–L18. doi:10.1051/0004-6361:20030837. Retrieved 2007-09-24.
16. ^ a b Campbell, Bruce; Walker, G. A. H. (August 1988). "A Search for Substellar Companions to Solar-Type Stars". Astrophysical Journal 331: 902–921. doi:10.1086/166608. Retrieved 2007-09-24.
17. ^ a b "Tables of Stars monitored by spectroscopy, with NO planet found". Extrasolar Planets Encyclopedia. Retrieved 2007-09-28.
18. ^ "H-K Project: Overview of Chromospheric Activity". Mount Wilson Observatory. Retrieved 2006-11-15.
19. ^ a b Gray, D. F.; Baliunas, S. L. (1994). "The activity cycle of tau Ceti". Astrophysical Journal 427 (2): 1042–1047. doi:10.1086/174210.
20. ^ Hall, J. C.; Lockwood, G. W.; Gibb, E. L. (1995). "Activity cycles in cool stars. 1: Observation and analysis methods and case studies of four well-observed examples". Astrophysical Journal 442 (2): 778–793. doi:10.1086/175483.
21. ^ Carraro, G.; Ng, Y. K.; Portinari, L. (1999). "Age Metallicity Relation and Star Formation History of the Galactic Disk". Monthly Notices of the Royal Astronomical Society 296 (4): 1045–1056. Retrieved 2007-08-14.
22. ^ Frick, P.; Baliunas, S. L.; Galyagin, D.; Sokoloff, D.; Soon, W. (1997). "Wavelet Analysis of Stellar Chromospheric Activity Variations". The Astrophysical Journal 483 (1): 426–434. doi:10.1086/304206.
23. ^ Judge, P. G.; Saar, S. H. (July 18, 1995). "The outer solar atmosphere during the Maunder Minimum: A stellar perspective". High Altitude Observatory. Retrieved 2007-08-14.
24. ^ Judge, Philip G.; Saar, Steven H.; Carlsson, Mats; Ayres, Thomas R. (2004). "A Comparison of the Outer Atmosphere of the "Flat Activity" Star τ Ceti (G8 V) with the Sun (G2 V) and α Centauri A (G2 V)". The Astrophysical Journal 609 (1): 392–406. doi:10.1086/421044.
25. ^ Smith, G.; Drake, J. J. (July 1987). "The wings of the calcium infrared triplet lines in solar-type stars". Astronomy and Astrophysics 181 (1): 103–111. Retrieved 2007-09-26.
26. ^ a b c d e J. S. Greaves, M. C. Wyatt, W. S. Holland, W. R. F. Dent (2004). "The debris disc around tau Ceti: a massive analogue to the Kuiper Belt". Monthly Notices of the Royal Astronomical Society 351 (3): L54–L58. doi:10.1111/j.1365-2966.2004.07957.x. Retrieved 2007-08-14.
27. ^ a b McKee, Maggie (July 7, 2004). "Life unlikely in asteroid-ridden star system". New Scientist. Retrieved 2007-09-25.
28. ^ Schirber, Michael (March 12, 2009). "Cometary Life Limit". NASA Astrobiology. Retrieved 2009-03-12.
29. ^ a b Greaves, Jane S. (January 2005). "Disks Around Stars and the Growth of Planetary Systems". Science 307 (5706): 68–71. doi:10.1126/science.1101979. PMID 15637266.
30. ^ Hall, J. C.; Lockwood, G. W. (2004). "The Chromospheric Activity and Variability of Cycling and Flat Activity Solar-Analog Stars". The Astrophysical Journal 614: 942–946. doi:10.1086/423926.
31. ^ Endl, M.; Kurster M.; Els S. (2002). "The planet search program at the ESO Coud´e Echelle spectrometer". Astronomy & Astrophysics 362: 585–594. doi:10.1051/0004-6361:20020937. Retrieved 2008-06-15.
32. ^ Walker, Gordon A. H.; Walker Andrew H.; Irwin W.Alan; et al. (1995). "A Search for Jupiter-Mass Companions to Nearby Stars". Icarus 116: 359–375. doi:10.1006/icar.1995.1130. Retrieved 2008-06-15.  —Note that this study does not exclude the possibility of a large planet with a mass greater than Jupiter's and an orbital plane that is nearly perpendicular to the line of sight.
33. ^ a b Schroeder, D. J.; Golimowski, D. A.; Brukardt, R. A. et al. (2000). "A Search for Faint Companions to Nearby Stars Using the Wide Field Planetary Camera 2". Astronomical Journal 119 (2): 906–922. doi:10.1086/301227. Retrieved 2007-08-14.
34. ^ "Tau Ceti". Sol Company. Retrieved 2007-09-25.
35. ^ Gonzalez, G. (March 17–21, 1997). "The Stellar Metallicity - Planet Connection". ASP Conference Series. Retrieved 2006-11-08.
36. ^ Woolf, Neville; Angel, J. Roger (September 1998). "Astronomical Searches for Earth-like Planets and Signs of Life". Annual Review of Astronomy and Astrophysics 36: 507–537. doi:10.1146/annurev.astro.36.1.507.
37. ^ Alexander, Amir (2006). "The Search for Extraterrestrial Intelligence, A Short History". The Planetary Society. Retrieved 2006-11-08.
38. ^ Turnbull, Margaret C.; Tarter, Jill (March 2003). "Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems". Astrophysical Journal Supplement Series 145 (1): 181–198. doi:10.1086/345779. Retrieved 2007-09-21.
39. ^ "Stars and Habitable Planets". Sol Company. Retrieved 2007-09-21.
40. ^
41. ^ "NASA budget statement". Planetary Society. 2006-02-06. Retrieved 2006-07-17.