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The kinematics of isolated brown dwarfs in the Galaxy, beyond the solarneighborhood, is virtually unknown. Microlensing has the potential to probethis hidden population, as it can measure both the mass and five of the sixphase-space coordinates (all except the radial velocity) even of a darkisolated lens. However, the measurements of both the microlens parallax andfinite-source effects are needed in order to recover the full information.Here, we combine $Spitzer$ satellite parallax measurement with the ground-basedlight curve, which exhibits strong finite-source effects, of eventOGLE-2017-BLG-0896. We find two degenerate solutions for the lens (due to theknown satellite-parallax degeneracy), which are consistent with each otherexcept for their proper motion. The lens is an isolated brown dwarf with a massof either $18\pm1M_J$ or $20\pm1M_J$. This is the lowest isolated-object massmeasurement to date, only $\sim$45\% more massive than the theoreticaldeuterium-fusion boundary at solar metallicity, which is the common definitionof a free-floating planet. The brown dwarf is located at either $3.9\pm0.1$ kpcor $4.1\pm0.1$ kpc toward the Galactic bulge, but with proper motion in theopposite direction of disk stars, with one solution suggesting it is movingwithin the Galactic plane. While it is possibly a halo brown dwarf, it mightalso represent a different, unknown population.

$Spitzer$ Microlensing Parallax for OGLE-2017-BLG-0896 Reveals a Counter-Rotating Low-Mass Brown Dwarf