Speaker
Description
I make the case for q < 0.02 as a working definition for giant planets based on their formation via core accretion. Analyzing microlensing surveys sensitive to giant planets on 1–10 au orbits, my recent work (Zhang 2025; ApJL 995, L55) showed that the giant-planet mass-ratio distribution follows a power law that is sharply truncated at q = 0.017–0.02 (95% CI). Similar mass-ratio boundaries are also observed in direct imaging and Gaia+RV surveys, and together these point to the Brown Dwarf Desert as fundamentally a mass-ratio desert. The q~0.02–0.05 boundary also aligns with an emerging bimodal distribution of planet/host metallicities: companions below this threshold preferentially orbit metal-rich stars and are themselves metal-enriched, consistent with core accretion, while companions above it show no such trends, favoring gravitational instability or core fragmentation. Under this definition, brown dwarfs and planets are not mutually exclusive: a 20 Jupiter-mass companion around a 2 solar-mass host satisfies q < 0.02 and is thus a planet by formation, yet also fuses deuterium and qualifies as a brown dwarf.