A Dynamical Mass of $70 \pm 5$ Jupiter Masses for Gliese 229B, the First T Dwarf

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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 $70 \pm 5$ Jupiter masses 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 Jupiter masses, 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.