Extra-Solar Planets

The discovery of the first extra-solar planet surrounding a main-sequence star was announced in 1995, based on very precise radial velocity (Doppler) measurements. A total of 34 such planets were known by the end of March 2000, and their numbers are growing steadily. The newly-discovered systems confirm some of the features predicted by standard theories of star and planet formation, but systems with massive planets having very small orbital radii and large eccentricities are common and were generally unexpected. Other techniques being used to search for planetary signatures include accurate measurement of positional (astrometric) displacements, gravitational microlensing, and pulsar timing, the latter resulting in the detection of the first planetary mass bodies beyond our Solar System in 1992. The transit of a planet across the face of the host star provides significant physical diagnostics, and the first such detection was announced in 1999. Protoplanetary disks, which represent an important evolutionary stage for understanding planet formation, are being imaged from space. In contrast, direct imaging of extra-solar planets represents an enormous challenge. Long-term efforts are directed towards infrared space interferometry, the detection of Earth-mass planets, and measurement of their spectral characteristics. Theoretical atmospheric models provide predictions of planetary temperatures, radii, albedos, chemical condensates, and spectral features as a function of mass, composition and distance from the host star. Efforts to characterise planets occupying the `habitable zone', in which liquid water may be present, and indicators of the presence of life, are advancing quantitatively.