Close-in Exoplanets of Less Than Ten Earth Mass may "Boil off" Their Atmospheres

Atmospheres of low-mass planets: the "boil-off"

Authors:

Owen et al

Abstract:

We show that, for a low-mass planet that orbits its host star within several tenths of an AU (like the majority of the Kepler planets), the atmosphere it was able to accumulate while embedded in the proto-planetary disk may not survive unscathed after the disk disperses. This gas envelope, if more massive than a few percent of the core (with a mass below 10M⊕), has a cooling time that is much longer than the time-scale on which the planet exits the disk. As such, it could not have contracted significantly from its original size, of order the Bondi radius. So a newly exposed proto-planet would be losing mass via a Parker wind that is energized by the stellar continuum radiation. The surface mass-loss induces a mass movement within the envelope that advects internal heat outward. As a result, the planet atmosphere rapidly cools down and contracts, until it has reached a radius of order 0.1 Bondi radius, at which time the mass-loss effectively shuts down. Within a million years after the disk disperses, we find a planet retains only about ten percent of its original envelope, and has a Kelvin-Helmholtz time that is much longer than its actual age. We suggest that this "boil-off" process may be responsible for the lack of planets above a radius of 2.5R⊕ in the Kepler data.