Tau Ceti

This star is located only about 11.9 light-years (ly) away. It lies in the south central part (01:44:04.08-15:56:14.93, ICRS 2000.0) of Constellation Cetus, the Whale -- southwest of Baten Kaitos (Zeta Ceti) and northeast of Deneb Kaitos or Diphida (Beta Ceti). While smaller and cooler than our own Sun, Sol, Tau Ceti is somewhat more like a sister star than nearby Epsilon Eridani. In Earth's night sky, it is clearly visible to the naked eye. In July 2004, astronomers announced that they had imaged a relatively large and dense disk of cold dust around this star (further discussion below -- RAS press release).

Medialab, ｩ ESA 2002



Larger illustration of the postponed,
infrared Darwin Mission.




Astronomers have identified Tau Ceti
as a prime target for the ESA's Darwin
Missions as well as NASA's now
postponed, optical SIM Mission.

JPL, CalTech, NASA



Larger illustration




Astronomers also identified Tau Ceti as a prime
target for NASA's Terrestrial Planet Finder (TPF),
now also indefinitely postponed.

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	Orbital Distance (a=AUえーゆーs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Earths)	Diameter (Earths)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
Tau Ceti	0.0	...	...	...	306,000	83	...	...	0.22-0.74
Planet b?	0.105	0.038	0.16	?	=>2.0	>1?	...	...	...
Planet c?	0.195	0.097	0.03	?	=>3.1	>1?	...	...	...
Planet d?	0.374	0.258	0.08	?	=>3.6	>1?	...	...	...
Inner H.Z. Edge?	~0.58	0.465	0	60-90?	...	...	...	...	...
Planet e?	0.552	0.460	0.05	?	=>4.3	>1?	...	...	...
Outer H.Z. Edge?	~1.16	1.305	0	60-90?	...	...	...	...	...
Planet f?	1.35	1.76	0.03	?	=>6.6	>1?	...	...	...
Inner Dust Disk Edge?	~10	~33	?	60-90?	...	...	...	...	...
Outer Dust Disk Edge?	~55	~428	?	60-90?	...	...	...	...

The Star

Tau Ceti is a main sequence, yellow-orange dwarf (G8 Vp). Smaller and dimmer than Sol, the star has around 81 to 92 percent of Sol's mass (SIM project; and NASA Star and Exoplanet Database, interpolated from a table by David F. Gray, 1992), although analysis of the star's oscillations of half Sol's amplitude leads to a mean density combined with its interferometric radius that suggests around only 0.783 +/-0.012 Solar-mass (Teixeira et al, 2009). The star may have around 75 to 77 percent of Sol's diameter (Pijpers et al, 2003; and NASA Star and Exoplanet Database, derived from a power law formula by Kenneth R. Lang, 1980), but only 59 percent of its visual(?) luminosity (Saumon et al, 1996, Table 1, page 1009) and 46.5 +/-0.3 percent to 53 percent of its bolometric luminosity (SIM project; and NASA Star and Exoplanet Database, derived from the exponential formula of Kenneth R. Lang, 1980). It appears to be elderly for a main sequence dwarf, perhaps around 10 billion years old (Di Folco et al, 2007), but another study suggests that it only somewhat older than Sol at 5.8 billion years (Mamajek and Hillebrand, 2008).

The star does not appear to be as enriched as Sol in elements heavier than hydrogen ("metals"), with only 22 to 89 percent of Sol's abundance of iron (Soubiran et al, 1998; and Cayrel de Strobel et al, 1991, page 6). It rotates more slowly than Sol with a period at 34 days (Baliunas et al, 1996) and appears to be further along in core hydrogen-fusion, with a relatively weak 11-year cycle of star spots and related chromospheric activity. Due to its slow rotational period, some astronomers proposed that the star is viewed pole-on from Earth (Gray and Baliunas, 1994), but others have speculated that Tau Ceti may be experiencing a phase like the Solar Maunder minimum (Judge et al, 2004). Given its its moderately high velocity through space, Tau Ceti may be a temporary visitor from the galaxy's thick disk (see more discussion from Professor Jim Kaler).

Dust has been detected around Tau Ceti, as has been found in the Solar System (Kuchner et al, 1998 -- in pdf -- more discussion below). The star appears to have a dim optical stellar companion, possibly a red dwarf of 13th magnitude that is seen in telescopes but is probably not actually bound by gravity to Tau Ceti itself. In addition, Tau Ceti does not appear to have a close stellar or substellar companion based on astrometric measurements (Lippincott and Worth, 1980), radial velocity variations (Campbell et al, 1988; and near-infrared interferometry (Di Folco et al, 2007). Some useful star catalogue numbers for Tau Ceti are: Tau Cet, 52 Cet, HR 509, Gl 71, Hip 8102, HD 10700, BD-16 295, SAO 147986, FK5 59, LHS 146, LTT 935, LPM 84, and LFT 159.

Habitable Zone

According to one type of model calculations performed for the NASA Star and Exoplanet Database, the inner edge of Tau Ceti's habitable zone is located relatively far from the star at around 0.582 AUs from the star, while the outer edge lies even farther out at around 1.157 AUs. According to the SIM project, the distance from Tau Ceti where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.74 AUえーゆー -- between the orbital distance of Venus and Earth in the Solar System. Assuming that Tau Ceti has 92 percent of Sol's mass, such a planet would have an orbital period under 240 days -- less than two-thirds of an Earth year -- at that distance from the star. (For an illustrated discussion, see Christoph Kulmann's web page on the potential habitable zone around Tau Ceti.)

Cometary Dust Disk

On July 6, 2004, a team of astronomers (including Jane Greaves, Mark Wyatt, Wayne Holland, and William Dent) using the Submillimetre Common-User Bolometer Array (SCUBA) of the James Clerk Maxwell Telescope at the Joint Astronomy Center on the Big Island of Hawaii announced that they had detected a large and relatively dense, cold dust disk around Tau Ceti (RAS press release; and (Greaves et al, 2004). This disk may be a ring that extends mostly between 10 around 55 AUs from the star and at an inclination of 60 to 90 degrees from Earth's line of sight, but the star appears to be viewed pole-on given that its low, observed rotation velocity (Saar and Osten, 1997). Given that Tau Ceti does not appear to be a young star, the ring of dusty debris is believed to be produced by collisions between larger comets and asteroids that break them down into smaller and smaller pieces, and Tau Ceti's disk is similar in size and shape to the disk of comets and asteroids that orbits the Sun, Sol. Given Tau Ceti's estimated age around 10 billion years, the estimated mass of its dust disk fits an expected decline over time compared to the disk mass of the even dustier, younger nearby star Epsilon Eridani, which may only be 500 million to one billion years old.

SCUBA, JCMT, JAC




Larger submillimeter image.





A relatively dense and large,
cold dust disk of cometary
and asteroid material has
been detected around Tau
Ceti (more).

Modelling of Tau Ceti's dust disk observations by the astronomers indicate, however, that the mass of the colliding bodies up to 10 kilometers (6.2 miles) in size may total around 1.2 Earth-masses, compared with 0.1 Earth-masses estimated to be in the Solar System's Edgeworth-Kuiper Belt (Greaves et al, 2004). Thus, Tau Ceti's dust disk may have around 10 times more cometary and asteroidal material than is currently found in the Solar System. Why the Tau Ceti System would have a more massive cometary disk than the Solar System is not fully understood. One theory is that Sol may have passed relatively close to another star at some point in its history and that the close encounter stripped off most of its comets and asteroids (Maggie McKee, New Scientist, July 7, 2004).

ｩ PPARC,
David A. Hardy



Larger illustration.



Any Earth-type
planet orbiting
Tau Ceti would
experience more
cometary and
asteroidal
impacts than in
the Solar System
today, as imagined
by Hardy (more).

Although no planets have been detected orbiting Tau Ceti as yet, it is likely that any planet found to orbit within the star's dust disk would experience relatively frequent bombardment from asteroids and comets of the size that is believed to have wiped out the dinosaurs and other types of multi-cellular life on Earth. As a result, some astronomers have speculated that it is likely that with so many large impacts, large and complex forms of Earth-type multi-cellular life may not have had the opportunity to evolve and persist on inner terrestrial planets orbiting this star. Others (such as Glenn Schneider of the University of Arizona and Scott Kenyon of the Smithsonian Astrophysical Observatory), however, argue that a giant planet in the system could gravitationally deflect comets and asteroids away from inner planets that may support life in the liquid water zone, in the same way that Jupiter protects Earth in the Solar System.

Hunt for Substellar Companions

Astronomers using the Hubble Space Telescope have failed to find a large substellar companion (large Jupiter or brown dwarf) around Tau Ceti thus far, including a Jupiter-mass companion in a circular orbit within five AUs (Wittenmyer et al, 2006; and Schroeder et al, 2000). On the other hand, the failure to find large substellar objects like brown dwarfs or a Jupiter- or Saturn-class planet in a "torch" orbit (closer than the Mercury to Sun distance) around Tau Ceti -- with even the highly effective radial-velocity technique of Geoffrey Marcy and Paul Butler -- bodes well for the possibility of Earth-type terrestrial planets around this star. Astronomers are hoping to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around Tau Ceti. Although both projects are now indefinitely delayed, the TPF would include two complementary observatory groups: a visible-light coronagraph; and a "formation-flying" infrared interferometer, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub.

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