How drifting and evaporating pebbles shape giant planets III. The formation of WASP-77A b and τ Bootis b

Atmospheric abundances of exoplanets are thought to constrain the planet formation pathway because different species evaporate at different temperatures and therefor radii in the protoplanetary disk, leaving distinct signatures inside the accreted planetary atmosphere. In particular the planetary C/O ratio is thought to constrain the planet formation pathway because of the condensation sequence of H2O, CO2, CH4, and CO, resulting in an increase in the gas phase C/O ratio with increasing distance to the host star. Here we use a disk evolution model including pebble growth, drift, and evaporation coupled with a planet formation model that includes pebble and gas accretion as well as planet migration to compute the atmospheric compositions of giant planets. We compare our results to the recent observational constraints of the hot Jupiters WASP-77A b and tau Bootis b. WASP-77A b's atmosphere features subsolar C/H, O/H, and H2O/H with slightly super-solar C/O, while tau Bootis b's atmosphere features super-solar C/H, O/H, and C/O with subsolar H2O/H. Our simulations qualitatively reproduce these measurements and show that giants similar to WASP-77A b should start to form beyond the CO2 evaporation front, while giants similar to tau Bootis b should originate from beyond the water ice line. Our model allows for the formation of subsolar and super-solar atmospheric compositions within the same framework. On the other hand, simulations without pebble evaporation, as used in classical models, cannot reproduce the super-solar C/H and O/H ratios of tau Bootis b's atmosphere without the additional accretion of solids. Furthermore, we identify the a viscosity parameter of the disk as a key ingredient regarding planetary composition because the viscosity drives the inward motion of volatile enriched vapor, which is responsible for the accretion of gaseous carbon and oxygen. Depending on the planet's migration history through the disk across different evaporation fronts, order-of-magnitude differences in atmospheric carbon and oxygen abundances should be expected. Our simulations additionally predict super-solar N/H for tau Bootis b and solar N/H for WASP-77A b. We thus conclude that pebble evaporation is a key ingredient to explain the variety of exoplanet atmospheres because it can explain both subsolar and super-solar atmospheric abundances.

Automated summary

By studying the observations and simulations of hot Jupiters, researchers have found that the abundances of exoplanet atmospheres can reveal the planet formation pathway. Different species evaporate and condense during planet formation, leading to distinct atmospheric compositions and influencing the C/O ratio. The evaporation of pebbles and the viscosity parameter of the disk are identified as key factors in determining planetary composition.