We report here the discovery of the first planet around an ultracool dwarf
star. It is also the first extrasolar giant planet (EGP) astrometrically
discovered around a main-sequence star. The statistical significance of the
detection is shown in two ways. First, there is a 2 x 10^-8 probability that
the astrometric motion fits a parallax-and-proper-motion-only model. Second,
periodogram analysis shows a false alarm probability of 3 x 10^-5 that the
discovered period is randomly generated. The planetary mass is M2 = 6.4
(+2.6,-3.1) Jupiter-masses (MJ), and the orbital period is P = 0.744
(+0.013,-0.008) yr in the most likely model. In less likely models, companion
masses that are higher than the 13 MJ planetary mass limit are ruled out by
past radial velocity measurements unless the system radial velocity is more
than twice the current upper limits and the near-periastron orbital phase was
never observed. This new planetary system is remarkable, in part, because its
star, VB 10, is near the lower mass limit for a star. Our astrometric
observations provide a dynamical mass measurement and will in time allow us to
confront the theoretical models of formation and evolution of such systems and
their members. We thus add to the diversity of planetary systems and to the
small number of known M-dwarf planets. Planets such as VB 10b could be the most
numerous type of planets because M stars comprise >70% of all stars. To date
they have remained hidden since the dominant radial-velocity (RV)
planet-discovery technique is relatively insensitive to these dim, red systems.Comment: 1 30 page pdf file, 9 figures, accepted for publication in The
Astrophysical Journa