Overestimating Eta-Earth: The Importance of Photo-Evaporation
Defining Habitability in the Universe
Dr Eric Lopez
University of Edinburgh
Life as we understand it likely requires a close to Earth-sized or smaller planet to survive. Sub-Neptune and larger planets must have large gaseous envelopes, which reach enormous temperatures and pressures inhospitable to life, in order to explain their sizes. This has lead to a massive effort to estimate Eta-Earth, the frequency of Earth-sized planets orbiting within the habitable zone. However, the current population of Earth-sized planets and planet candidates discovered by NASA's Kepler mission, is dominated by highly irradiated planets on orbits less than ~50 days. On such orbits planets are bombarded by high amounts of EUV and x-ray radiation, which can photo-evaporate away planets with gaseous envelopes. Using models of planet evolution and photo-evaporation we show that the known population of short-period planets has been significantly sculpted by evaporation. We further show that most of the current population of Earth-sized planet candidates could in fact be the striped remnants of gas-rich sub-Neptunes. This in turn means that current efforts to extrapolate the short-period Earth-sized population out to the habitable zone, may significantly overestimate the frequency of Earth-size planets.
13:30 - 15:00