The evaporation valley in the Kepler planets
File(s)
Accepted version
Author(s)
Owen, James E
Wu, Yanqin
Type
Journal Article
Abstract
A new piece of evidence supporting the photoevaporation-driven evolution model for low-mass, close-in exoplanets was recently presented by the California–Kepler Survey. The radius distribution of the Kepler planets is shown to be bimodal, with a "valley" separating two peaks at 1.3 and 2.6 R ⊕. Such an "evaporation valley" had been predicted by numerical models previously. Here, we develop a minimal model to demonstrate that this valley results from the following fact: the timescale for envelope erosion is the longest for those planets with hydrogen/helium-rich envelopes that, while only a few percent in weight, double its radius. The timescale falls for envelopes lighter than this because the planet's radius remains largely constant for tenuous envelopes. The timescale also drops for heavier envelopes because the planet swells up faster than the addition of envelope mass. Photoevaporation therefore herds planets into either bare cores (~1.3 R ⊕), or those with double the core's radius (~2.6 R ⊕). This process mostly occurs during the first 100 Myr when the stars' high-energy fluxes are high and nearly constant. The observed radius distribution further requires the Kepler planets to be clustered around 3 M ⊕ in mass, born with H/He envelopes more than a few percent in mass, and that their cores are similar to the Earth in composition. Such envelopes must have been accreted before the dispersal of the gas disks, while the core composition indicates formation inside the ice line. Lastly, the photoevaporation model fails to account for bare planets beyond ~30–60 days; if these planets are abundant, they may point to a significant second channel for planet formation, resembling the solar system terrestrial planets.
Date Issued
2017-09-20
Date Acceptance
2017-08-27
Citation
The Astrophysical Journal: an international review of astronomy and astronomical physics, 2017, 847 (1)
ISSN
0004-637X
Publisher
American Astronomical Society
Journal / Book Title
The Astrophysical Journal: an international review of astronomy and astronomical physics
Volume
847
Issue
1
Copyright Statement
© 2017 The American Astronomical Society. All rights reserved. This is an author-created, un-copyedited version of an article accepted for publication in The Astrophysical Journal. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The definitive publisher authenticated version is available online at https://dx.doi.org/10.3847/1538-4357/aa890a
Subjects
0201 Astronomical and Space Sciences
0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics
0306 Physical Chemistry (incl. Structural)
Astronomy & Astrophysics
Notes
journal_title: The Astrophysical Journal article_type: paper article_title: The Evaporation Valley in the Kepler Planets copyright_information: © 2017. The American Astronomical Society. All rights reserved. date_received: 2017-05-26 date_accepted: 2017-08-27 date_epub: 2017-09-20
Publication Status
Published
Article Number
29