Volatile-Rich Earth-Mass Planets in the Habitable Zone

Abstract

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 Earth masses) should be rich in volatile ices like water and ammonia. 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 atmospheres undergo slow hydrodynamic escape. These objects could appear in future surveys for extrasolar Earth analogs.

12 pages, including 1 figure. To appear in ApJ letters October 10, 2003