Tau Ceti

e x t r a s o l a r &nbsp &nbsp p l a n e t s

The Sun’s back yard

Tau Ceti, also known as HD 10700, is a mature G8 star in the immediate Solar neighborhood, located 11.9 light years away in the constellation Cetus. As the nearest non-binary G-type star to our Sun, Tau Ceti has long been popular with outer space enthusiasts of all stripes. In 1960 it was one of the stars targeted by Project Ozma, an effort to intercept radio signals transmitted by hypothetical alien civilizations. Tau Ceti avoided radio detection then, as it avoids radial velocity detection now.

In contemporary astronomy, this star is notable as the host of a massive debris disk (Greaves et al. 2004) resembling a larger version of our own system’s Kuiper Belt (see Debris Disk Systems). Recent observations suggest that Tau Ceti also harbors an inner-system analog of our Asteroid Belt (Di Folco et al. 2007). Despite these parallels with our system, however, various search techniques have failed to detect any planets.

Tau Ceti is substantially less metallic than our Sun. Recent estimates of its metal content range from -0.59 to -0.31, with the high end of this range generally favored (Valdes et al. 2004, Di Folco et al. 2004). It is also less massive than our Sun, at 0.83 MSOL, and cooler by about 10%, with an effective temperature of 5375 K (Di Folco et al. 2004). Its radius has been measured at 0.79 Solar (Di Folco et al. 2007), and its luminosity is estimated at 0.59 Solar (Saumon et al. 1996).

The latter value implies a habitable zone centered at an astrocentric radius of 0.77 AU – very close to the semimajor axis of Venus (0.72 AU). At early times, the system’s ice line lay at a radius of about 2.25 AU, not very different from the Solar System value of 2.7 AU. Nevertheless, Tau Ceti is much older than our Sun. Recent estimates of its age range from 8.7 billion years (Rocha Pinto et al. 2004) to 10 billion years (Di Folco et al. 2004), as compared to the Solar System’s age of 4.6 billion years.

Tau Ceti’s complement of cool debris orbits between 10 and 55 AU, occupying a much larger orbital space than the Kuiper Belt. Collisions among its components – pebbles, boulders, comets, dwarf planets – give rise to an infrared excess indicating a total mass at least 10 times greater than the Kuiper Belt (Greaves et al. 2004). Theoretical modeling suggests that an outer disk of this magnitude will contain a significant population of ice dwarf planets in the mass range of Pluto and Eris (Kenyon & Bromley 2008).

A belt of much warmer debris has recently been detected within a few AU of the star, with an infrared excess suggesting a mass very similar to our own Asteroid Belt (Di Folco et al. 2007). Accordingly, we can expect a population of rocky objects at small astrocentric radii, some of them possibly comparable to the asteroids Ceres and Vesta.

Many investigators regard debris disks as “signposts of planetary system formation” (Trilling et al. 2007). In the case of Tau Ceti, the question is, “How far did formation go?” Long-term surveys have ruled out any planets of 0.67 MJUP or more on low-eccentricity orbits within 3 AU of the star, and of 0.90 MJUP or more on similar orbits within 5.2 AU (Wittenmyer et al. 2006). Nevertheless, these results leave open the possibility that Mars- to Neptune-mass planets populate the orbital space within 10 AU. Such planets would perturb the orbits of objects in the debris belts, causing collisions that would produce the dust detected by infrared observations.

Index of exoplanetary topics

One formidable argument against the presence of full-size planets around Tau Ceti derives from the star’s low metallicity. Both theory and observation demonstrate that planet formation becomes increasingly likely as the metal content of the host star rises. Yet Tau Ceti’s metallicity is markedly sub-Solar. On the other hand, as high-precision radial velocity search programs extend into their second decade of activity, the range of exosystem parameters is becoming increasingly diverse. The Extrasolar Planets Encyclopaedia currently (July 2008) lists 12 host stars with metallicities of -0.3 or lower, ranging down to HD 155358 at -0.68. This sample covers the range of metallicities estimated for Tau Ceti, suggesting that the star’s chemical composition in itself did not pose insurmountable obstacles to planet formation.

In fact, the nearby K2 star HD 40307 has recently been shown to host three short-period planets with masses in the Super Earth range. With a mass of 0.77 MSOL and a metallicity of -0.31, HD 40307 is a near-twin to Tau Ceti, lacking only a debris belt. Thus our next-door neighbor may yet be found to sustain an analogous retinue of low-mass rocky and icy planets.

By chance, we see Tau Ceti in an almost pole-on orientation (Gray & Baliunas 1994). This viewing angle is uniquely ill-suited to radial velocity measurements, a factor that may explain the null results of all searches to date, particularly if the system’s hypothetical planets are as small as those orbiting HD 40307.

Last updated July 2008

All text is copyright Raymond Harris 2006-2008