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

TL;DR

This study confirms a planetary mass companion around a young star through astrometry and spectroscopy, providing insights into planetary formation at large orbital distances.

Contribution

It provides the first confirmation of a co-moving planetary mass companion at large separation around a young star using follow-up astrometry and spectroscopy.

Findings

Confirmed the companion is co-moving with the star.

Estimated the companion's temperature as 1800+/-200 K.

Ruled out additional planets more massive than 1-8 Mjup at certain separations.

Abstract

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.