An international group of scientist led by Mikko Toumi, from the University of Hertfordshire, announced the potential discovery of five planets around the nearby star Tau Ceti (aka HD 10700 or Gliese 71). This is the 19th closest stellar system to Earth at 12 light years away. After Alpha Centauri A and B it is the next closest Sun-like star and historically of astrobiological interest, including SETI. Out of the five possible planets, the two farthest away, Tau Ceti e and f, are potentially habitable.
Tau Ceti e and f have minimum masses of 4.3 and 6.6 Earth masses, respectively. They are both superterrans (aka super-Earths) orbiting within the habitable zone of their parent star. There is no information about their size so it is not possible to tell if they are rocky, water, or even gas worlds. However, if we assume that they are composed of a water-rock mix, their radius might be closer to 1.8 and 2.3 Earth radii, respectively.
Habitability of Tau Ceti e
The planet Tau Ceti e orbits close to the inner edge of the habitable zone. It receives about 60% more light than Earth from the Sun making it a hot planet probably only habitable to simple thermophilic (heat-loving) life. Its mean global surface temperature should be near 70°C assuming a similar terrestrial atmosphere. However, it is likely that superterran planets have much denser and heat trapping atmospheres and Tau Ceti e might be instead dominated by a strong greenhouse effect making it more likely a super-Venus than a super-Earth. Without any knowledge of its atmosphere we are not able to tell if it is a mildly hot planet tolerable for simply life forms or a very hot non habitable Venus-like world. Tau Ceti e has an Earth Similarity Index of 0.77 assuming a more terrestrial-like atmosphere.
Habitability of Tau Ceti f
The planet Tau Ceti f orbits close to the outer edge of the habitable zone. It only receives about 27% the light of Earth from the Sun making it a cold planet probably only habitable to simple psychrophilic (cold-loving) life. Its mean global surface temperature should be near -40°C assuming a similar terrestrial atmosphere. However, it is likely that as Tau Ceti e, it is also dominated by a strong greenhouse effect making it even acceptable for complex life, which requires temperatures from 0°C to 50°C. Without any knowledge of its atmosphere we are not able to tell if it is a frozen Mars-like planet tolerable for simply life forms or even an Earth-like world. Tau Ceti e has an Earth Similarity Index of 0.71 assuming a more terrestrial-like atmosphere.
Both Tau Ceti e and f are very interesting planet candidates for astrobiology that need to be confirmed by more observations. Neither fit yet the Earth-like distinction based on the little information we have from them. It is hard to tell which is better for life at this point without more information about their bulk composition and atmospheric structure. There is one thing we can be certain now. Tau Ceti is now the closest and most similar stellar system to our Solar System with potential habitable exoplanet candidates, displacing the notorious Gliese 581 stellar system.
Credit: The Planetary Habitability Laboratory @ UPR Arecibo (phl.upra.edu)