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Project Progress

The CO-Is outlines a simple approach to evaluate habitability of terrestrial planets by assuming different types of planetary atmospheres and using corresponding model calculations (Kaltenegger&Sasselov 2011). This approach can be applied for current and future candidates provided by the Kepler mission and other searches. Candidates with radii below 2 Earth’s radii are consistent with models of potentially rocky planets. To first order the HZ depends on the effective stellar flux distribution in wavelength and time, the planet albedo, and greenhouse gas effects. The Co-Is provided a simple set of parameters which can be used for evaluating future planet candidates from transit searches.

The NASA Kepler mission (Borucki et al. 2011) recently announced 1235 planetary candidates, among them 54 in and close to the HZ of their host stars. The CO-Is used atmospheric models to explore the potential for habitability of Kepler planetary candidates.

Fig. 1 shows the extent of the HZ for the Kepler planetary candidates that could potentially be habitable. The three main points of the paper are: (1) to provide a simple approximation for habitability of Earth-like planets from the data provided by transit surveys like Kepler to assess their potential as a habitat, (2) to demonstrate that one needs to consider the individual stellar parameters to determine the limits of the HZ, and (3) to explore the change in the sample located in the HZ due to individual factors and associated errors.

Applying the analysis to the whole Kepler planetary sample of 1235 transiting planetary candidates, assuming the maximum Earth-like Bond albedo for rocky planet atmospheres (see Fig.1), results in 12, 27, 67 planetary candidates with T_eq smaller than the water loss limit (T_surf = 373K) for 0%, 50% and 100% clouds respectively, and 18, 43, 76 planetary candidates with temperatures lower than the runaway greenhouse limit respectively. Among those are 2, 3, 6 as well as 3, 4, 6 planets respectively, that have radii below 2 Earth radii consistent with rocky planets (KOI1026.01, 854.01, 701.03, 268.01, 326.01, 70.03).

CO-I Kaltenegger led two separate papers on the modeling for one of the two confirmed planets in the Habitable Zone with mass below 10 Earth masses, consistent with rocky planet models (see Fig.2).

Gl581d has a minimum mass of 7 M_Earth , HD 85512b are the first two detected potentially habitable rocky Super-Earth. Our models confirm that a habitable atmosphere can exist on both Gl 581d and HD 85512b. We derive spectroscopic features for atmospheres with and without, assuming an Earth-like composition for Gl 581 d. We find that a minimum CO₂ partial pressure of about 7 bar, in an atmosphere with a total surface pressure of 7.6 bar, are needed to maintain a mean surface temperature above freezing on Gl 581d. Future observations of atmospheric features can be used to examine if our concept of habitability and its dependence on the carbonate-silicate cycle is correct, and assess whether Gl 581d is indeed a habitable super-Earth.