Rocky super-Earths or waterworlds: the interplay of planet migration, pebble accretion, and disc evolution

Bertram Bitsch,Andre Izidoro,Sean N Raymond

doi:10.1051/0004-6361/201935007

Abstract

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.

Highlights

The most common type of planet around other stars are closein super-Earths with orbital periods of less than 100 days (Mayor et al 2011; Mulders et al 2018)
In this paper we present a disc evolution and planetary growth model that allows us to calculate the water ice fraction of formed planets
Our main findings are summarized as follows: 1) If the water ice line does not evolve in time and if planets do not migrate, the water ice fraction of a planet is solely determined by its starting position in respect to the water ice line (Fig. 2)

Summary

Introduction

The most common type of planet around other stars are closein super-Earths with orbital periods of less than 100 days (Mayor et al 2011; Mulders et al 2018) These planets have roughly radii of 1–4 Earth radii and weight up to 20 Earth masses. If the mass of a planet is known through RV detections and its radius through transit observations, the mean density of it can be calculated This gives important information about the planetary composition through interior structure models (Valencia et al 2007; Sotin et al 2007; Seager et al 2007; Fortney et al 2007; Selsis et al 2007; Adams et al 2008; Zeng & Sasselov 2013; Buchhave et al 2016). The observed planetary radius can be greatly influenced by its atmosphere

Methods

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Discussion

Conclusion