We report the discovery of a planetary-mass companion, HD
106906 b, with the new Magellan Adaptive Optics (MagAO) + Clio2 system. The
companion is detected with Clio2 in three bands: J, KS, and L′, and lies at a
projected separation of 7.1” (650 AU). It is confirmed to be comoving with its
13±2 Myr-old F5 host using Hubble Space Telescope/Advanced Camera for Surveys
astrometry over a time baseline of 8.3 yr. DUSTY and COND evolutionary models
predict the companion’s luminosity corresponds to a mass of 11±2MJup, making it
one of the most widely separated planetary-mass companions known.