Recent observations identify a valley in the radius distribution of small exoplanets, with planets in the range 1.5-2.0 R® significantly less common than somewhat smaller or larger planets. This valley may suggest a bimodal population of rocky planets that are either engulfed by massive gas envelopes that significantly enlarge their radius, or do not have detectable atmospheres at all. One explanation of such a bimodal distribution is atmospheric erosion by high-energy stellar photons. We investigate an alternative mechanism: the luminosity of the cooling rocky core, which can completely erode light envelopes while preserving heavy ones, produces a deficit of intermediate sized planets. We evolve planetary populations that are derived from observations using a simple analytical prescription, accounting selfconsistently for envelope accretion, cooling and mass-loss, and demonstrate that core-powered mass-loss naturally reproduces the observed radius distribution, regardless of the high-energy incident flux. Observations of planets around different stellar types may distinguish between photoevaporation, which is powered by the high-energy tail of the stellar radiation, and corepowered mass-loss, which depends on the bolometric flux through the planet's equilibrium temperature that sets both its cooling and mass-loss rates.
Bibliographical noteFunding Information:
This research was partially supported by ISF (Israel Science Foundation) and iCore (Israeli Centers of Research Excellence) grants. HES gratefully acknowledges support from NASA grant NNX15AK23G. We thank Yoram Lithwick for discussions during the 2015/6 Jerusalem Winter School. We also thank Fei Dai, Christoph Mordasini, James Owen, Allona Vazan, and Josh Winn for discussions and comments on the paper’s draft. Finally, we thank Eugene Chiang for a careful review that significantly improved the paper.
© 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.
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