Tau Ceti

JPL, CalTech, NASA


Larger illustration


Astronomers have identified Tau
Ceti as a prime target for the
Terrestrial Planet Finder (TPF),
now planned for launch between
2014 and 2020.

The Star

Tau Ceti is a main sequence, yellow-orange dwarf (G8 Vp) that may be as much as 10 billion years old. It has about 81 to 82 percent of Sol's mass, around 77 percent its diameter (Pijpers et al, 2003), but only 59 percent of its luminosity (Saumon et al, 1996, page 17). The star does not appear to be as enriched as Sol in elements heavier than hydrogen ("metals") because it has only 22 to 74 percent of Sol's abundance of iron (Cayrel de Strobel et al, 1991, page 6).

Dust has been detected around Tau Ceti, as has been found in the Solar System (Kuchner et al, 1998 -- in pdf). There also may be an optical companion star seen in telescopes that is not actually bound by gravity to Tau Ceti, and the star does not appear to have a dim stellar or substellar companion based on astrometric measurements (Lippincott and Worth, 1980) or radial velocity variations (Campbell et al, 1988). 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.

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). Extending to around 55 AUs from the star, this dust 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 of 10 billion years, the estimated mass of its dust disk fits its expected decline with time compared to the disk mass of the 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).

© 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 (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. Indeed, the distance from Tau Ceti where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.68 AU -- at about the orbital distance of Venus in the Solar System. (For an illustrated discussion, see Christoph Kulmann's web page on the potential habitable zone around Tau Ceti.) At that distance from the star, such a planet would have an orbital period of about 228 days -- less than two-thirds of an Earth year.

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. As currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph to launch around 2014; and a "formation-flying" infrared interferometer to launch before 2020, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub beginning in 2015.