Influence of the water content in protoplanetary discs on planet migration and formation

TL;DR

This study investigates how varying water content in protoplanetary discs influences planet migration and formation, revealing that water-rich discs allow larger planets to migrate outward and suggesting water-rich super-Earths may originate beyond the ice line.

Contribution

It demonstrates the impact of water and dust composition on planet migration limits and formation pathways, highlighting the role of disc chemistry in planetary system development.

Findings

Discs with low water content restrict outward migration to planets up to 4 Earth masses.

Discs with high water content allow outward migration for planets up to 8 Earth masses.

Water-rich super-Earths likely formed beyond the ice line and orbit stars with low oxygen abundance.

Abstract

The temperature and density profiles of protoplanetary discs depend crucially on the mass fraction of micrometre-sized dust grains and on their chemical composition. A larger abundance of micrometre-sized grains leads to an overall heating of the disc, so that the water ice line moves further away from the star. An increase in the water fraction inside the disc, maintaining a fixed dust abundance, increases the temperature in the icy regions of the disc and lowers the temperature in the inner regions. Discs with a larger silicate fraction have the opposite effect. Here we explore the consequence of the dust composition and abundance for the formation and migration of planets. We find that discs with low water content can only sustain outwards migration for planets up to 4 Earth masses, while outwards migration in discs with a larger water content persists up to 8 Earth masses in the late stages of the disc evolution. Icy planetary cores that do not reach run-away gas accretion can thus migrate to orbits close to the host star if the water abundance is low. Our results imply that hot and warm super-Earths found in exoplanet surveys could have formed beyond the ice line and thus contain a significant fraction in water. These water-rich super-Earths should orbit primarily around stars with a low oxygen abundance, where a low oxygen abundance is caused by either a low water-to-silicate ratio or by overall low metallicity.