Catastrophic Evaporation of Rocky Planets

TL;DR

This paper models the atmospheric escape of hot, rocky exoplanets, revealing that small planets can disintegrate rapidly due to thermal winds, and applies this to explain the observed properties of KIC 12557548b.

Contribution

It introduces a radiative-hydrodynamic model accounting for dust-gas energy exchange, predicting rapid disintegration of low-mass rocky planets under intense irradiation.

Findings

Planets with < 0.1 M_Earth disintegrate in < 10 Gyr.

KIC 12557548b likely has a mass < 0.02 M_Earth, possibly its iron core.

There are 10-100 progenitors of similar objects detectable by Kepler.

Abstract

Short-period exoplanets can have dayside surface temperatures surpassing 2000 K, hot enough to vaporize rock and drive a thermal wind. Small enough planets evaporate completely. We construct a radiative-hydrodynamic model of atmospheric escape from strongly irradiated, low-mass rocky planets, accounting for dust-gas energy exchange in the wind. Rocky planets with masses < 0.1 M_Earth (less than twice the mass of Mercury) and surface temperatures > 2000 K are found to disintegrate entirely in < 10 Gyr. When our model is applied to Kepler planet candidate KIC 12557548b --- which is believed to be a rocky body evaporating at a rate of dM/dt > 0.1 M_Earth/Gyr --- our model yields a present-day planet mass of < 0.02 M_Earth or less than about twice the mass of the Moon. Mass loss rates depend so strongly on planet mass that bodies can reside on close-in orbits for Gyrs with initial masses comparable to or less than that of Mercury, before entering a final short-lived phase of catastrophic mass loss (which KIC 12557548b has entered). Because this catastrophic stage lasts only up to a few percent of the planet's life, we estimate that for every object like KIC 12557548b, there should be 10--100 close-in quiescent progenitors with sub-day periods whose hard-surface transits may be detectable by Kepler --- if the progenitors are as large as their maximal, Mercury-like sizes (alternatively, the progenitors could be smaller and more numerous). According to our calculations, KIC 12557548b may have lost ~70% of its formation mass; today we may be observing its naked iron core.