Constraining the formation history of the HAT-P-11 system using atmospheric abundances

Abstract

The chemical fingerprint of a planet can reveal information about its formation history regarding when and where the planet formed. In particular, the water content of a planet can help to constrain its formation pathway: If the planet formed in the outer regions of the disk and migrated inward, it would be water-rich due to the accretion of water-ice-rich solids. Conversely, formation in the inner disk region, where water-ice is not available, would result in a smaller atmospheric water content due to the limited accretion of water vapor. However, this process becomes complex with the presence of gap-opening giant planets. A gas giant exerts a pressure bump exterior to its orbit, preventing further influx of pebbles into the inner system, resulting in a water-poor environment and eventually leading to water-poor inner planets. These different formation scenarios can help to constrain the formation of the HAT-P-11 system, which contains an inner sub-Neptune with a mass of 23.4 M⊕ and substellar water abundances (XH2O ≈ 0.11, as well as an outer giant planet orbiting exterior to the water-ice line. Our planet formation model encompasses planetary growth through pebble and gas accretion, along with a pebble drift and evaporation module that enables us to track the chemical composition of the disk and the planet over time. We find that the presence of the gas giant is necessary to block water-ice-rich material, resulting in a substellar water content for the inner sub-Neptune, HAT-P-11b. On the other hand, if the giant planet forms too early, not enough solid material can enter the inner disk regions, preventing efficient growth of the inner planet. This highlights the importance of the timing of giant planet formation in explaining the inner system structure, including the formation of Jupiter in our Solar System. Furthermore, our simulations predict a roughly stellar C/O ratio with superstellar C/H and O/H ratios for HAT-P-11b, providing constraints for future observations of this system, which are essential for gaining a more detailed understanding of its formation.