  Giant planets are usually thought to form within a few tens of AU of their
host stars, and hence it came as a surprise when we found what appeared to be a
planetary mass (~0.008 Msun) companion around the 5 Myr-old solar mass star
1RXS J160929.1-210524 in the Upper Scorpius association. At the time, we took
the object's membership in Upper Scorpius -- established from near-infrared, H-
and K-band spectroscopy -- and its proximity (2.2", or 330 AU) to the primary
as strong evidence for companionship, but could not verify their common proper
motion. Here, we present follow-up astrometric measurements that confirm that
the companion is indeed co-moving with the primary star, which we interpret as
evidence that it is a truly bound planetary mass companion. We also present new
J-band spectroscopy and 3.0-3.8 microns photometry of the companion. Based on a
comparison with model spectra, these new measurements are consistent with the
previous estimate of the companion effective temperature of 1800+/-200 K. We
present a new estimate of the companion mass based on evolution models and the
calculated bolometric luminosity of the companion; we obtain a value of 0.008
(-0.002/+0.003) Msun, again consistent with our previous result. Finally, we
present angular differential imaging observations of the system allowing us to
rule out additional planets in the system more massive than 1, 2 and 8 Mjup at
projected separations larger than 3" (~440 AU), 0.7" (~100 AU) and 0.35" (~50
AU), respectively. This companion is the least massive known to date at such a
large orbital distance; it shows that objects in the planetary mass range exist
at orbital separations of several hundred AU, posing a serious challenge for
current formation models.
