The discovery of planets orbiting distant stars is one of the most exciting fields in astronomy today and interest in it is continuously growing. The first discovery of a massive planetary companion (exoplanet) to a main-sequence star opened new horizons to our understanding of how planets form. New research fields are fast emerging, such as the field of Astrobiology which focuses on the study of proto-stellar systems, planets and ultimately life itself.
Over the past few years, systems with massive planets at very small orbital radii have proved to be quite common despite being generally unexpected. It was originally thought that large gas-giant planets cannot form close to their host stars since the stellar wind would have `blown away’ most of the surrounding gas in the disc to a much farther distance. The theoretical formation models have had to be revised in the light of the new discoveries and the way this has been done is by incorporating the idea of protoplanetary migration in the gaseous disk. The protoplanet coevolves with the disk and can move further inwards as the disk gas material gets depleted.
The current number of confirmed exoplanets exceeds 600, with the vast majority having been discovered by radial velocity surveys. These are severely biased towards the detection of systems with massive planets (several times the mass of Jupiter) in small orbits. The only technique with the potential to detect Earth-mass planets from the ground is microlensing. This figure by K. Horne shows the regions probed by the various techniques used to detect planets.
