Daniel Perez-Becker, Eugene Chiang
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 ~ 1 M_Earth/Gyr --- our model yields a present-day planet mass of ~0.01 M_Earth or about 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 similar to 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 ~1% of the planet's life, we estimate that for every object like KIC 12557548b, there should be a few dozen close-in Mercury-sized progenitors whose direct transits might be detectable by Kepler with enough period folding. According to our calculations, KIC 12557548b has lost ~80% of its formation mass; today we may be observing its naked iron core.
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http://arxiv.org/abs/1302.2147
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