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.
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Data availability
Ascii output files summarizing the time series of our reference simulation (with an output interval of 1,000 years, totalling 1,010 files) are provided at https://www.konstantinbatygin.com/setimeseries.
Code availability
This work utilizes the publicly available mercury6 code (https://www.arm.ac.uk/~jec/). The subroutine detailing user-defined forces is available on request from the corresponding author (K.B.).
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Acknowledgements
K.B. is grateful to Caltech, Observatoire de la Côte d’Azur, the David and Lucile Packard Foundation and the National Science Foundation (grant number: AST 2109276) for their generous support. A.M. is grateful for support from the ERC advanced grant HolyEarth N. 101019380.
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K.B. and A.M. jointly conceived the project and collaborated on the interpretation of the results. K.B. carried out the N-body simulations and led the writing of the paper. A.M. ran particle-in-a-box simulations and contributed to writing the manuscript.
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Extended data
Extended Data Fig. 1 Mass budget of solids within a ringed protoplanetary disk.
Output of the disk evolution model delineated in ref. 3. Owing to a hydrodynamic balance between the radial outflow and drag exerted on dust by gas, solids concentrate in the vicinity of their respective sublimation lines, forming planetesimals within discrete rocky and icy rings. Depending on the minimal level of turbulent viscosity that the disk can attain, \({\alpha }_{\min }\), the mass entrained within the r ~ 1 AU annulus of silicate-rich material varies drastically. Importantly, for relatively quiescent nebulae, the rock ring can reach masses on the order of tens of M⊕, readily serving as the birthplace of super-Earths.
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Batygin, K., Morbidelli, A. Formation of rocky super-earths from a narrow ring of planetesimals. Nat Astron 7, 330–338 (2023). https://doi.org/10.1038/s41550-022-01850-5
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DOI: https://doi.org/10.1038/s41550-022-01850-5
