Rapid Formation of Gas Giant Planets and Super-Earths Around M Dwarfs

Astronomy and Astrophysics – Astronomy

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Scientific paper

Extrasolar planet surveys have begun to detect planets in orbit around M dwarf stars. The frequency of gas giant planets around M dwarfs appears to be lower than that around G dwarfs, but it is not zero. Core accretion is too slow to form gas giant planets around M dwarfs. Disk instability is fast enough to form gas giant protoplanets around low mass stars well before the gaseous disk disappears. A new suite of three dimensional radiative, gravitational hydrodynamical models is presented that calculates the evolution of initially marginally gravitationally unstable disks with masses of 0.021 to 0.065 solar masses, orbiting stars with masses of 0.1 and 0.5 solar masses, respectively. The models show that gas giant planets are indeed likely to form by the disk instability mechanism in orbit around M dwarf stars, the opposite of the prediction for formation by core accretion. Recent microlensing discoveries of super-Earths orbiting two M dwarf stars have been interpreted as failed cores formed by core accretion. However, these super-Earths could also have formed by disk instability, coupled with photoevaporative loss of their gaseous envelopes by an external source of UV radiation, i.e., an O star. M dwarfs that form in regions of future high-mass star formation would then be expected to have super-Earths orbiting at distances of several AU and beyond while those that form in regions of low-mass star formation would be expected to have gas giants at those distances. Given that most stars are born in the former rather than in the latter regions, M dwarfs should have significantly more super-Earths than gas giants, as seems to be indicated by the microlensing surveys.

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