Planets in the hinterland: the case of 1RXS J1609b
David Lafrenière (1) Ray Jayawardhana (2) Marten van Kerkwijk (2)
(1) Université de Montréal (2) University of Toronto
In September 2008 we announced the direct imaging discovery of a young planet likely in orbit around the star 1RXS J160929.1-210524, a roughly solar-mass member of the 5 Myr old Upper Scorpius association. The evidence for the youth, temperature ( 1800 K), and mass ( 8 Mjup) of the object from near-infrared imaging and spectroscopy was very clear, but its projected separation, a whopping 330 AU, was rather surprising for such a low mass object. We have now obtained two years of follow-up astrometric measurements and the data clearly confirm that the companion is co-moving with the primary star, which we interpret as evidence that it is a truly bound companion. 1RXS J1609b is the least massive companion known to date to orbit a star at such a large distance; it shows that planetary mass objects do exist at orbital separations of several hundred AUs, posing a challenge for formation models. In this talk I will summarize the various observations of this planet, discuss its physical properties and speculate about how it could have formed.