17080

Located at 2.2 pc comma the virgul250K Y2 (sub-)brown dwarf WISE J085510.83-071442.5 is the fourth closest known system to Earth. Studying this object provides unique insights into brown dwarf (and exoplanet) atmospheric properties comma thermal evolution comma multiplicity comma and planet-hosting prospects. We propose to use HST to obtain one the most accurate annual parallax of this - or any - planetary-mass brown dwarf to date comma achieving an unprecedented accuracy of 200 micro-arcseconds comma and constraining its absolute space motion to equivalent accuracy. More importantly comma we will be able to search for faint companions co-moving with the target comma either resolved or through astrometric perturbations of its linear motion comma the latter probing down to a few Earth masses. HST is the optimal facility for precision astrometric measurement of WISE 0855-0714 comma as this source is too cool and faint at optical and near-infrared wavelengths to observe with ground-based facilities or Gaia. Our observations will improve current astrometric precision of this source by a factor of 40 and anchor future JWST measurements of this and other cold brown dwarfs comma while providing high-precision distance and motion measurements and tight constraints on planetary-mass companions for one of the closest ''stellar object.. to the Sun.