Wednesday, December 31, 2014

The Implications of SuperEarths and Mini Neptunes Atmospheric Envelopes

The Formation of Super-Earths and Mini-Neptunes with Giant Impacts


Inamdar et al


The majority of discovered exoplanetary systems harbor a new class of planets, bodies typically several times more massive than Earth but orbiting their host stars well inside the orbit of Mercury. The origin of these close-in super-Earths and mini-Neptunes is a major unanswered question in planet formation. Unlike Earth, whose atmosphere contains less than 10−6 of its total mass, a large fraction of close-in planets have significant gaseous envelopes, containing 1% to 10% or more of their total mass. It has been proposed that these close-in planets formed in situ either by delivery of 50−100M of rocky material to inner regions of the protoplanetary disc or in a disc enhanced relative to the MMSN. In both cases, final assembly of the planets occurs by giant impacts (GIs). Here we test the viability of these scenarios. We show that atmospheres accreted by isolation masses are small (typically 10−3−10−2 the core mass) and that atmospheric mass loss during GIs is significant, resulting in typical post-GI atmospheres that are 10−4−10−3 the core mass. In the most optimistic scenario where there is no core luminosity from GIs and/or planetesimal accretion, we find post-GI envelope accretion from a depleted gas disc can yield envelope masses several percent the core mass, but still smaller than observed for many close-in planets. If the gravitational potential energy resulting from the last mass doubling of the planet is released over the disc dissipation timescale as core luminosity, then accreted envelope masses are reduced by about an order of magnitude. Finally we show that even in the absence of Type I migration, radial drift timescales due to gas drag for many isolation masses are shorter than typical disc lifetimes. Given these challenges, we conclude that most observed close-in planets with large envelopes likely formed at larger separations from their host stars.

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