Where Planets Become Brown Dwarfs: Tracing a Hidden Boundary in the Metal Content of Stars
Giacalone et al. analyze companions between 1–50 au and find that host-star metallicities split into two groups at a transition mass of about 27 MJup. Lower-mass companions orbit metal-rich stars, consistent with bottom-up planet formation, while higher-mass companions orbit stars with near-solar metallicity, indicating star-like formation. Orbital eccentricities also differ, supporting two distinct formation pathways.
Exploring the Coldest Brown Dwarfs with Near-Infrared Colors
Leggett and collaborators use JWST data to study extremely cold Y dwarfs, comparing their near-infrared colors across JWST, Euclid, and Roman filters. They show that mid-infrared brightness at 4.6 microns reliably tracks temperature, while near-infrared colors vary with metallicity and gravity. The work highlights both the promise of upcoming surveys and the challenges of incomplete atmospheric models.
Shining Too Bright: Testing Brown Dwarf Models with HD 4747 B and HD 19467 B
Wood and collaborators studied two brown dwarfs, HD 4747 B and HD 19467 B, using precise age estimates from their host stars. They found that current substellar evolutionary models under-predict the brown dwarfs’ brightness and overestimate their masses. Including atmospheric effects like clouds improves the match but doesn’t fully resolve the discrepancy, suggesting missing physics such as metallicity effects.