Authors:Coleman et alAbstract:We present a study of 4 different formation scenarios that may explain the origin of the recently announced planet `Proxima b' orbiting the star Proxima Centauri. The aim is to examine how the formation scenarios differ in their predictions for the multiplicity of the Proxima planetary system, the water/volatile content of Proxima b and its eccentricity, so that these can be tested by future observations. A scenario of in situ formation via giant impacts from a locally enhanced disc of planetary embryos and planetesimals, predicts that Proxima b will be a member of a multiplanet system with a measurably finite value of orbital eccentricity. Assuming that the local solid enhancement needed to form a Proxima b analogue with a minimum mass of 1.3 Earth masses arises because of the inwards drift of solids in the form of small planetesimals/boulders, this scenario also likely results in Proxima b analogues that are moderately endowed with water/volatiles, arising from the dynamical diffusion of icy planetesimals from beyond the snowline during planetary assembly. A scenario in which multiple embryos form, migrate and mutually collide within a gaseous protoplanetary disc also results in Proxima b being a member of a multiple system, but where its members are Ocean planets due to accretion occurring mainly outside of the snowline, possibly within mean motion resonances. A scenario in which a single accreting embryo forms at large distance from the star, and migrates inwards while accreting either planetesimals/pebbles results in Proxima b being an isolated Ocean planet on a circular orbit. A scenario in which Proxima b formed via pebble accretion interior to the snowline produces a dry planet on a circular orbit. Future observations that characterise the physical/orbital properties of Proxima b, and the multiplicity of the system, will provide valuable insight into its formation history.
Wednesday, September 7, 2016
Exploring plausible formation scenarios for the planet candidate orbiting Proxima Centauri
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