The Directly Imaged Planet Around the Young Solar Analog 1RXS J160929.1 - 210524: Confirmation of Common Proper Motion, Temperature, and Mass

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 M _sun) 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 (2farcs2 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 comoving 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 μm 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^+0.003 _-0.002 M _sun, 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 M _Jup, 2 M _Jup, and 8 M _Jup at projected separations larger than 3'' (~440 AU), 0farcs7 (~100 AU), and 0farcs35 (~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.