James Kasting (Professor of Geosciences & Meteorology - Pennsylvania State University)
NASA’s Kepler Space Telescope has been in operation for more than 4 years, and its dataset on transiting exoplanets is becoming more and more complete. It is now becoming feasible to make estimates for Eta_Earth—the fraction of Sun-like stars that have at least one rocky planet within their habitable zone. The habitable zone, as conventionally defined, is the region around a star where liquid water can exist on a planet’s surface. Published estimates for Eta_Earth for FGK stars, based on much earlier versions of the Kepler dataset, range from 0.03 to 0.3. More recent published estimates of Eta_Earth for M stars range from 0.15 to 0.6. Why do these estimates differ by so much, and are any of them believable? One reason for disagreement has to do with how the habitable zone boundaries are defined. Theoretical limits derived from 1-D climate modeling tend to be pessimistic, whereas empirical limits based on Venus and Mars are more optimistic. I will talk about what our group has been doing on this problem recently and what we hope to accomplish in the near future.