1. Introduction: Direct imaging of planets
Several extra-solar planet candidates have been detected indirectly by radial velocity variations of stars (review by Marcy & Butler 1998), one such candidate is confirmed by a transit event (Charbonneau et al. 2000). Direct imaging detection of planets like those in our solar system but orbiting other stars is difficult due to the limited dynamical range: Planets are too faint and too close to bright stars. Planets are thought to form in circumstellar disks around young stars, which are typically hundreds of AU in size (e.g. McCaughrean & O'Dell 1996). One can try to avoid the problem of dynamical range by searching for planetary companions around nearby stars, where even the orbit of the outermost solar system planet corresponds to several arc sec, sufficient to resolve a faint object next to a bright star. However, nearby stars usually are too old, so that their planets are too faint for direct detection with current technology. Young planets are still self-luminous due to on-going accretion and/or contraction (Burrows et al. 1997, Brandner et al. 1997, Malkov et al. 1998) and sufficiently bright for direct detection. This should best be possible in the infrared bands H and K, where the brightness difference between young stars and young planets is expected to be the lowest (Burrows et al. 1997).
A few sub-stellar companions to normal stars were detected already by direct imaging: Gl 229 B (Nakajima et al. 1995), G196-3 B (Rebolo et al. 1998), and GG Tau Bb (White et al. 1999). All of these companions are brown dwarfs confirmed by spectroscopy and proper motion. The first extra-solar planet candidate (Terebey et al. 1998) has not yet been confirmed by spectroscopy or proper motion.
We started a ground-based search for planets around nearby young stars: Their planets should be relatively luminous (because nearby and young) and well separated from the star. In addition, nearby stars usually have large proper motion, so that one can decide after only a few years, whether a companion candidate is co-moving. Then, its mass can be better constrained than for a free-floating object, because age and distance of the primary is usually well-known. We report here the first ground-based direct imaging detection of an extra-solar planet candidate.
© European Southern Observatory (ESO) 2000
Online publication: January 31, 2000