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Recent observations have found a valley in the size distribution of close-insuper-Earths that is interpreted as a signpost that close-in super-Earths aremostly rocky in composition. However, new models predict that planetesimalsshould first form at the water ice line such that close-in planets are expectedto have a significant water ice component. Here we investigate the watercontents of super-Earths by studying the interplay between pebble accretion,planet migration and disc evolution. Planets' compositions are determined bytheir position relative to different condensation fronts (ice lines) throughouttheir growth. Migration plays a key role. Assuming that planetesimals start ator exterior to the water ice line ($r>r_{\rm H_2O}$), inward migration causesplanets to leave the source region of icy pebbles and therefore to have lowerfinal water contents than in discs with either outward migration or nomigration. The water ice line itself moves inward as the disc evolves, anddelivers water as it sweeps across planets that formed dry. The relative speedand direction of planet migration and inward drift of the water ice line isthus central in determining planets' water contents. If planet formation startsat 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 migrationduring the early gas phase. Hot super-Earths with larger water ice contents onthe other hand, experienced outward migration at the water ice line and onlymigrated inwards after their formation was complete either because they becometoo massive to be contained in the region of outward migration or in chains ofresonant planets. Measuring the water ice content of hot super-Earths may thusconstrain their migration history.

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