A Chemical Modeling Roadmap Linking Protoplanetary Disks and Exoplanet Atmospheres

Abstract

Exoplanet atmospheres and protoplanetary disk chemistry are both active fields of research. Chronologically, they represent opposite ends of the process of planet formation: planets form in protoplanetary disks, and the composition of planets and their atmospheres should reflect the composition of the disk material they form from. The past decade of discoveries and insights by the ALMA observatory has provided a quantum leap in our understanding of the structures and chemical compositions of protoplanetary disks. Likewise, both ground- and space-based facilities have provided peeks into the compositions of exoplanet atmospheres. However, the near future, with, first and foremost, the James Webb Space Telescope, and later also ESO’s Extremely Large Telescope and ESA’s Ariel mission are expected to provided constraints on exoplanet atmospheric composition with unprecedented precision and, also, to shed new light on gas and ice compositions of protoplanetary disks. It is therefore timely to evaluate the current state of modeling efforts attempting to link protoplanetary disk chemistry and planet formation to exoplanet atmospheric compositions. The author has a background in astrochemistry related to planet formation. As such, this review discusses the differences between the simpler chemical approach of “iceline chemistry” and the more chemically rigorous approach of utilizing chemical kinetics. It is outlined which chemical effects may be at play in a planet-forming disk midplane, which effects are relevant under different conditions, and which tools are available for modeling chemical kinetics in a disk midplane. The review goes on to discuss some important efforts in the planet formation modeling community to treat chemical evolution and, vice versa, efforts in the chemical modeling community to implement more physical effects related to planet formation into the chemical modeling. The aim of this review is both to outline some concepts related to planet formation chemistry but also to encourage not just collaboration between the planet formation modeling community and the astrochemical community but also assistance and guidance from one community to the other. Guidance, regarding which effects, out of many, might be more relevant than others under certain planet formation conditions and regarding why certain included effects lead to certain important modeling outcomes. As the research fields of exoplanet atmospheres and protoplanetary disks near new frontiers in observational insights with upcoming facilities, developing appropriate modeling frameworks (including physical and chemical effects) is paramount to ultimately enable the linking of a chemically characterized exoplanet atmosphere to its formation history in its natal protoplanetary disk.