Towards the detection of earth-like 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 about 80 such planets were known by the end of March 2002, and their numbers are growing steadily. Space missions under development or consideration by the European Space Agency and NASA are expected to contribute to the detection and characterization of Earth-like systems over the next 10-20 years: via astrometry through the detection of systems like our own Solar System (Jupiter mass objects at a few AU from their parent star) in very large numbers (GAIA); via Earth-mass object detection through photometric transit measurements (Eddington and Kepler); and via interferometric detection of Earth-like systems (Darwin and TPF). Efforts to characterise planets occupying the 'habitable zone', in which liquid water may be present, and indicators of the presence of life, are meanwhile advancing quantitatively.