Formation of rocky super-earths from a narrow ring of planetesimals

Abstract

The formation of super-Earths, the most abundant planets in the Galaxy, remains elusive. These planets have masses that typically exceed that of the Earth by a factor of a few, appear to be predominantly rocky, although often surrounded by H/He atmospheres, and frequently occur in multiples. Moreover, planets that encircle the same star tend to have similar masses and radii, whereas those belonging to different systems exhibit remarkable overall diversity. Here we advance a theoretical picture for rocky planet formation that satisfies the aforementioned constraints: building upon recent work, which has demonstrated that planetesimals can form rapidly at discrete locations in the disk, we propose that super-Earths originate inside rings of silicate-rich planetesimals at approximately ~1 au. Within the context of this picture, we show that planets grow primarily through pairwise collisions among rocky planetesimals until they achieve terminal masses that are regulated by isolation and orbital migration. We quantify our model with numerical simulations and demonstrate that our synthetic planetary systems bear a close resemblance to compact, multi-resonant progenitors of the observed population of short-period extrasolar planets. A planetary origin model that forms exoplanets from a narrow ring of silicate material at a stellocentric distance of 1 au is able to explain the physical properties of super-Earths and reproduce the ‘peas in a pod’ pattern of uniformity within planetary architecture.