Where Do Substellar Companions Come From? Combining Retrievals and Host-Star Abundances

Dr. Caprice Phillips

Abstract: Brown dwarfs and giant planets occupy a shared, poorly understood regime. They overlap in mass, temperature, and atmospheric physics, yet we still do not know where one formation channel ends and the other begins. Does a given companion assemble by core accretion in a protoplanetary disk, collapse like a star from a fragmenting cloud or disk? The answer matters well beyond classification. It sets how we interpret directly imaged planets, calibrates atmospheric and evolutionary models, and tells us how efficiently disks build massive companions, which in turn shapes the architectures of the planetary systems that form alongside them. In this talk, I will give an overview of the two measurements that most directly address this problem, atmospheric retrievals and host star abundances. Atmospheric retrievals of substellar companions, now transformed by JWST, record carbon-to-oxygen ratios, metallicities, and cloud properties. The photospheres of their FGK host stars preserve the composition of the natal disk and provide the baseline against which a companion’s atmosphere can be judged. I will review what retrievals can and cannot tell us about formation and cloud composition, why host-star abundances are the missing context, and how elemental ratios such as C/O, C/Si, O/Si, and Mg/Si connect disk chemistry to companion atmospheres and to silicate cloud composition. I also present abundance measurements for hosts of giant planets and brown dwarfs, from HERMES and PEPSI/LBT spectra analyzed with BACCHUS, including a sample of wide-orbit brown dwarf hosts, and show how they test whether companions inherit the composition of their disks. I will discuss how results show implications for systems hosting both brown dwarfs and smaller planets, and with prioritizing targets for JWST and the upcoming GPI 2.0 instrument where host-star context can sharpen the interpretation of every companion spectrum.