The Evaporation Valley in the Kepler Planets

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 Myrs when the stars' high energy flux are high and nearly constant. The observed radius distribution further requires that the Kepler planets are clustered around 3 M ⊕ in mass, are 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.

275 Candidates and 149 Validated Planets Orbiting Bright Stars in K2 Campaigns 0–10

“…They argued that the underlying astrophysical effect could be photoevaporation, whereby stellar incident flux strips a planet's H/He atmosphere if the atmosphere is not thick enough, leaving a population of stripped, rocky planets and an untouched population of larger, gaseous mini-Neptunes (Owen & Wu 2013;Lopez & Rice 2016;Owen & Wu 2017;Van Eylen et al 2017). As an alternative hypothesis, they also suggested that gas accretion could be delayed during planet formation until the protoplanetary disk is already gas poor, creating a population of small, rocky planets (Lee et al 2014;Lee & Chiang 2016).…”

The California-Kepler Survey. IV. Metal-rich Stars Host a Greater Diversity of Planets

“…Owen & Wu (2017) performed a population synthesis starting with 3-10M Å cores and a period distribution of df P d P log 1.9 µ , which is similar to the power law index presented in this work. They then gave the simulated planets a range of envelope fractions from 1% to 30% and tracked the radii of these planets as they were subjected to photo-evaporation.…”

“…They then gave the simulated planets a range of envelope fractions from 1% to 30% and tracked the radii of these planets as they were subjected to photo-evaporation. The Owen & Wu (2017) simulations produced very few sub-Neptunes with P 10 days < . In the Lee & Chiang (2017) model, hot super-Earth/ sub-Neptune occurrence rates are set by stellar rotation on the pre-main sequence.…”

Young planets under extreme UV irradiation

“…During this short lived phase, the planet rapidly cools and contracts, losing a substantial fraction of its gaseous envelope before the planetary atmosphere settles into a more stable configuration and continues to evolve on slower timescales (e.g. Jin et al 2014;Fossati et al 2017;Jin & Mordasini 2017;Owen & Wu 2017). For planets close enough to the host star, the atmosphere may then go through a long phase of efficient hydrodynamic escape, or even "blow-off", which, depending on the the planet's atmosphere mass, may or may not significantly affect the planet (e.g.…”