Atmospheres of low-mass planets: the "boil-off"
File(s)
Published version
OA Location
Author(s)
Owen, James E
Wu, Yanqin
Type
Journal Article
Abstract
We show that, for a low-mass planet that orbits its host star within a few tenths of an AU (like the majority of the Kepler planets), the atmosphere it was able to accumulate while embedded in the protoplanetary 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 $10{M}_{\oplus }$), has a cooling time that is much longer than the timescale on which the planet exits the disk. As such, it could not have contracted significantly from its original size, of the order of the Bondi radius. So a newly exposed protoplanet would be losing mass via a Parker wind that is catalyzed by the stellar continuum radiation. This represents an intermediate stage of mass-loss, occurring soon after the disk has dispersed, but before the EUV/X-ray driven photoevaporation becomes relevant. 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 the order of 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 that has only about 10% of its original envelope, and a Kelvin–Helmholtz time that is much longer than its actual age. We suggest that this "boil-off" process may be partially responsible for the lack of planets above a radius of $2.5{R}_{\oplus }$ in the Kepler data, provided planet formation results in initial envelope masses of tens of percent.
Date Issued
2016-02-01
Date Acceptance
2015-12-11
Citation
Astrophysical Journal, 2016, 817 (2)
ISSN
0004-637X
Publisher
American Astronomical Society
Journal / Book Title
Astrophysical Journal
Volume
817
Issue
2
Copyright Statement
© 2016. The American Astronomical Society. All rights reserved.
Subjects
Astronomy & Astrophysics
0201 Astronomical and Space Sciences
0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics
0306 Physical Chemistry (incl. Structural)
Publication Status
Published
Article Number
107
Date Publish Online
2016-01-26