Volume 624, April 2019
|Number of page(s)||12|
|Section||Planets and planetary systems|
|Published online||19 April 2019|
Rocky super-Earths or waterworlds: the interplay of planet migration, pebble accretion, and disc evolution
Max-Planck-Institut für Astronomie,
2 Laboratoire d’Astrophysique de Bordeaux, CNRS and Université de Bordeaux, Allée Geoffroy St. Hilaire, 33165 Pessac, France
3 UNESP, Universidade Estadual Paulista – Grupo de Dinàmica Orbital Planetologia, Guaratinguetà, CEP 12.516-410, São Paulo, Brazil
Accepted: 6 March 2019
Recent observations have found a valley in the size distribution of close-in super-Earths that is interpreted as a signpost that close-in super-Earths are mostly rocky in composition. However, new models predict that planetesimals should first form at the water ice line such that close-in planets are expected to have a significant water ice component. Here we investigate the water contents of super-Earths by studying the interplay between pebble accretion, planet migration and disc evolution. Planets’ compositions are determined by their position relative to different condensation fronts (ice lines) throughout their growth. Migration plays a key role. Assuming that planetesimals start at or exterior to the water ice line (r > rH2O), inward migration causes planets to leave the source region of icy pebbles and therefore to have lower final water contents than in discs with either outward migration or no migration. The water ice line itself moves inward as the disc evolves, and delivers water as it sweeps across planets that formed dry. The relative speed and direction of planet migration and inward drift of the water ice line is thus central in determining planets’ water contents. If planet formation starts at the water ice line, this implies that hot close-in super-Earths (r < 0.3 AU) with water contents of a few percent are a signpost of inward planet migration during the early gas phase. Hot super-Earths with larger water ice contents on the other hand, experienced outward migration at the water ice line and only migrated inwards after their formation was complete either because they become too massive to be contained in the region of outward migration or in chains of resonant planets. Measuring the water ice content of hot super-Earths may thus constrain their migration history.
Key words: accretion, accretion disks / planets and satellites: formation / planets and satellites: composition / planet-disk interactions
© B. Bitsch et al. 2019
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Open Access funding provided by Max Planck Society.
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