Volatile-rich Earth-Mass Planets in the Habitable Zone

A small planet is not necessarily a terrestrial planet. Planets that form beyond the snow line with too little mass to seed rapid gas accretion (≲10 M ⊕ ) should be rich in volatile ices like H 2 O and NH 3 . Some of these planets should migrate inward by interacting with a circumstellar disk or with other planets. Such objects can retain their volatiles for billions of years or longer at ~1 AU as their thick steam atmospheres undergo slow hydrodynamic escape. These objects could appear in future surveys for extrasolar Earth analogs.