Extra-solar planets

Abstract

The discovery of the first extra-solar planet surrounding amain-sequence star was announced in 1995, based on very preciseradial velocity (Doppler) measurements. A total of 34 suchplanets were known by the end of March 2000, and their numbersare growing steadily. The newly discovered systems confirm someof the features predicted by standard theories of star andplanet formation, but systems with massive planets, having verysmall orbital radii and large eccentricities, are common and weregenerally unexpected.Other techniques being used to search for planetary signaturesinclude accurate measurement of positional (astrometric)displacements, gravitational microlensing and pulsar timing,the latter resulting in the detection of the first planetarymass bodies beyond our solar system in 1992. The transit of aplanet across the face of the host star provides significantphysical diagnostics, and the first such detection was announcedin 1999. Protoplanetary disks, which represent an importantevolutionary stage for understanding planet formation, are beingimaged from space. In contrast, direct imaging of extra-solarplanets represents an enormous challenge. Long-term efforts aredirected towards infrared space interferometry, the detection ofEarth-mass planets, and measurement of their spectralcharacteristics.Theoretical atmospheric models provide predictions of planetarytemperatures, radii, albedos, chemical condensates and spectralfeatures as a function of mass, composition and distance fromthe host star. Efforts to characterize planets occupying the`habitable zone', in which liquid water may be present, andindicators of the presence of life are advancingquantitatively.