The evaporation valley in the Kepler planets

James E. Owen (IAS), Yanqin Wu (Toronto)
(Submitted on 30 May 2017)

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 Kepler planets inwards of 100 days is shown to be clearly 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 use a simple analytical model to reproduce the earlier numerical results and to demonstrate that this valley results from the following fact: the timescale for envelope erosion is the longest for those planets with He/He-rich envelopes that, while only a few percent in weight, swell their sizes by a factor of two. The timescale falls for envelopes lighter than this because, as the envelope becomes more tenuous, the planet’s radius remains largely constant and so does the photo-evaporating flux it receives. 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 radius (∼2.6 R⊕). This fact explains both the observed valley and the steep fall-off of planets above 2.6 R⊕. The observed radius distribution further requires that the Kepler planets are clustered around 3 M⊕ in mass, are endowed at birth with H/He envelopes more than a few percent in mass, and that their cores are uniformly similar to the Earth in composition. Such envelopes must have been accreted before the dispersal of the gas disks, as opposed to being outgassed, while the core composition indicates planet assembly inward of the ice-line. Lastly, photoevaporation cannot produce bare planets beyond ∼50 days. If such planets are present, they could indicate a second channel for planet formation.

Comments:    14 pages, submitted to AAS journals
Subjects:    Earth and Planetary Astrophysics (astro-ph.EP)
Cite as:    arXiv:1705.10810 [astro-ph.EP] (or arXiv:1705.10810v1 [astro-ph.EP] for this version)
Submission history
From: James Owen
[v1] Tue, 30 May 2017 18:13:47 GMT (2500kb,D)
https://arxiv.org/abs/1705.10810