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…