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.Comment: 15 pages, 12 figures, accepted by A&
