Directly imaged exoplanet challenges formation models

Two years ago, astronomers in Canada directly imaged what seemed to be a gas giant planet in a very distant orbit—more than 300 times the Earth-Sun distance of one astronomical unit (AU)—around a star much like our Sun. (For comparison, Jupiter’s orbit is 5.2 AU, Neptune’s is 30 AU.) Such a scenario poses difficulties for all the major planet-formation models in current use: core accretion, gravitational instability, and fragmentation of a pre-stellar core. The main difficulty is that either much larger objects, like another star, or much smaller ones are expected at such a great distance. Now, with further observations in hand from the Gemini North telescope and its adaptive optics, University of Toronto astronomers Ray Jayawardhana, Marten van Kerkwijk, and David Lafrenière (now at the University of Montreal) have confirmed the puzzle: The planet, with about eight times the mass of Jupiter, is moving through space gravitationally bound to the parent star, known by its nickname 1RXS 1609. Besides astrometric observations, the direct imaging (shown here) along with spectroscopic and photometric data allowed the researchers to further characterize the planet and confirm that no other large planets are farther out in the system. A mere toddler at only 5 million years old, 1RXS 1609 is about 500 light-years away. Hundreds of other exoplanets have been discovered in recent years, but this one is expected to keep theorists busy for some time. (D. Lafrenière, R. Jayawardhana, M. H. van Kerkwijk, Astrophys. J. 719 , 497, 2010 http://dx.doi.org/10.1088/0004-637X/719/1/497 .)