Introduction, general background

The discovery of extrasolar planets is arguably the most exciting development in astrophysics during the past decade, rivaled only by the discovery of the cosmic acceleration. The unexpected variety of giant exo-planets, some very close to their stars, many with high orbital eccentricity, has sparked a new generation of observers and theorists to address the question of how planets form in the context of protostellar accretion disks. Planets are now known to migrate and maybe even be ejected, via planet-disk and planet-planet interactions. We are beginning to discover how our Solar system fits into a broader community of planetary systems, many with very different properties. Microlensing-based searches play a critical role by probing for cool planets with masses down to that of Earth. Of key interest is how planets are distributed according to mass and orbital distance (Figure 1) as this information provides a crucial test for theories of planet formation. The core accretion model of planet formation (Ida & Lin 2005) predicts a large reservoir of small cool planets that have depleted their store for planetesimals but have not yet reached the mass threshold for runaway gas accretion and so grown into gas giants.

There is a wide variety of planets and at first sight it appears that our system is very special. However, our view of the whole picture is still blurred by observational biases inherent to the detection techniques using transits and radial velocities. Both methods are more sensitive to massive planets close to their parent star. Doppler measurements and the space transit missions such as COROT can already or will shortly be able to detect Neptune-mass planets close to their parent star. Direct detections fill the other extreme of very large separations which are unknown in our solar system. It is therefore necessary to use different techniques, each probing different areas of the planet-mass versus orbital distance parameter space.