We combine Keck/HIRES radial velocities , imaging with HiCIAO/Subaru and the Hubble Space Telescope , and absolute astrometry from Hipparcos and Gaia to measure a dynamical mass of 70 \pm 5 M _ { Jup } for the brown dwarf companion to Gl 229 .
Gl 229B was the first imaged brown dwarf to show clear signs of methane in its atmosphere .
Cooling models have been used to estimate a mass in the range of 20–55 M _ { Jup } , much lower than our measured value .
We argue that our high dynamical mass is unlikely to be due to perturbations from additional unseen companions or to Gl 229B being itself a binary , and we find no evidence of a previously claimed radial velocity planet around Gl 229A .
Future Gaia data releases will confirm the reliability of the absolute astrometry , though the data pass all quality checks in both Hipparcos and Gaia .
Our dynamical mass implies a very old age for Gl 229 , in some tension with kinematic and activity age indicators , and/or shortcomings in brown dwarf cooling models .
Gl 229B joins a small but growing list of T dwarfs with masses approaching the minimum mass for core hydrogen ignition .