  The coolest known brown dwarfs are our best analogs to extrasolar gas-giant
planets. The prolific detections of such cold substellar objects in the past
two years has spurred intensive followup, but the lack of accurate distances is
a key gap in our understanding. We present a large sample of precise distances
based on homogeneous mid-infrared astrometry that robustly establish absolute
fluxes, luminosities, and temperatures. The coolest brown dwarfs have
temperatures of 400-450 K and masses ~5-20 times that of Jupiter, showing they
bridge the gap between hotter brown dwarfs and gas-giant planets. At these
extremes, spectral energy distributions no longer follow a simple
correspondence with temperature, suggesting an increasing role of other
physical parameters such as surface gravity, vertical mixing, clouds, and
metallicity.
