Formation of Close in Super-Earths and Mini-Neptunes

Recent observations by the Kepler space telescope have led to the discovery of more than 4000 exoplanet candidates consisting of many systems with Earth- to Neptune-sized objects that reside well inside the orbit of Mercury, around their respective host stars.

How and where these close-in planets formed is one of the major unanswered questions in planet formation. Here we calculate the required disk masses for {\it in situ} formation of the {\it Kepler} planets. We find that, if close-in planets formed as {\it isolation masses}, then standard gas-to-dust ratios yield corresponding gas disks that are gravitationally unstable for a significant fraction of systems, ruling out such a scenario. We show that the maximum width of a planet's accretion region in the absence of any migration is 2 v esc /Ω , where v esc is the escape velocity of the planet and Ω the Keplerian frequency and use it to calculate the required disk masses for {\it in situ} formation with giant impacts. Even with giant impacts, formation without migration requires disk surface densities in solids at semi-major axes less than 0.1~AU of 10 3 − 10 5  g c m −2 implying typical enhancements above the minimum-mass solar nebular (MMSN) by at least a factor of 20. Corresponding gas disks are below, but not far from, the gravitational stability limit. In contrast, formation beyond a few AU is consistent with MMSN disk masses. This suggests that migration of either solids or fully assembled planets is likely to have played a major role in the formation of close-in super-Earths and mini-Neptunes.

Formation of Close in Super-Earths and Mini-Neptunes: Required Disk Masses and Their Implications

Hilke E. Schlichting (MIT) (Submitted on 4 Oct 2014)

Comments: Accepted for publication in ApJL

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:1410.1060 [astro-ph.EP] (or arXiv:1410.1060v1 [astro-ph.EP] for this version)

Submission history From: Hilke Schlichting [v1] Sat, 4 Oct 2014 16:45:16 GMT (871kb)

http://lanl.arxiv.org/abs/1410.1060

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