The Impact of Stripped Cores on the Frequency of Earth-size Planets in the Habitable Zone

The frequency of Earth-size planets in the habitable zone (HZ) of Sun-like stars, hereafter eta(circle plus), is a key parameter to evaluate the yield of nearby Earth analogs that can be detected and characterized by future missions. Yet, this value is poorly constrained as there are no reliable exoplanet candidates in the HZ of Sun-like stars in the Kepler field. Here, we show that extrapolations relying on the population of small (<1.8 R-circle plus), short-period (<25 days) planets bias eta(circle plus) to large values. As the radius distribution at short orbital periods is strongly affected by atmospheric loss, we reevaluate eta(circle plus) using exoplanets at larger separations. We find that eta(circle plus) drops considerably, to values of only similar to 5%-10%. Observations of young (<100 Myr) clusters can probe short-period sub-Neptunes that still retain most of their envelope mass. As such, they can be used to quantify the contamination of sub-Neptunes to the population of Kepler short-period small planets and aid in more reliable estimates of eta(circle plus).