The Mass from Molecules

VHS 1256 b is a directly imaged companion at the low end of the brown dwarf mass range. Its mass matters — it determines whether this object burns deuterium (brown dwarf) or doesn’t (giant planet). But measuring the mass of a directly imaged companion is difficult: there’s no radial velocity signal, no transit timing, no dynamical constraint from the orbit on observable timescales.

Mâlin and colleagues use JWST’s mid-infrared spectrograph to detect CO at signal-to-noise 25 and H₂O at 76, with tentative detections of NH₃ and CH₄. The molecular abundances — particularly the carbon isotope ratio and ammonia abundance — constrain the atmospheric chemistry tightly enough to determine the temperature-gravity combination, which in turn constrains the mass.

The result: VHS 1256 b sits above the deuterium-burning limit. A brown dwarf, not a planet.

The inversion is elegant. Normally, you know the mass (from dynamics) and infer the atmospheric composition (from models). Here, the composition is measured directly through cross-correlation spectroscopy at high signal-to-noise, and the mass is the inference. The atmospheric chemistry determines the mass because different masses produce different atmospheric conditions, different equilibrium abundances, different observable molecular signatures.

The same molecules exist at every mass. But their relative abundances are a fingerprint. Read the fingerprint precisely enough and you weigh the object from its air.