Chemical composition of giant exoplanets due to pebble, planetesimal and gas accretion: volatiles and refractories in the atmospheres


Recommended citation: Schneider & Bitsch (in prep)

Recent studies of extrasolar gas giants suggest high C/O ratios in planetary atmospheres compared to their host stars. Interior models of observed extrasolar planets additionally suggest high heavy element contents for these giant planets. Previous research adopted either pebble- or planetesimal accretion for the growth of these planets to gain insights into this question.

We investigate whether the combination of pebble- and planetesimal accretion will help to explain the enrichment of heavy elements in exoplanets.

To achieve this goal, we perform semi analytical 1D models of protoplanetary disks including the treatment of viscous evolution and heating, pebble drift and simple chemistry to simulate the growth of planets from planetary embryos to Jupiter mass objects by accretion of pebbles, planetesimals and gas while they migrate through the disk.

The treatment of pebble- and planetesimal accretion simultaneously unveils that measured compositions mainly depend on the amount of heavy elements polluted into the gas by the evaporation of pebbles at evaporation fronts and consequently accreted by the giant planets. This process allows the giant planets to harbour heavy element contents in agreement with previous claims. In addition, our model reveals that giant planets originating further away from the central star have a higher C/O ratios on average due to evaporation of methane rich pebbles.

We conclude that the inclusion of pebble evaporation at evaporation lines is a key ingredient to determine the heavy element content of giant planets and their composition.