Twelve years of discovery

Abstract

What do we know about other planetary systems? Very little, if we consider individually each of the planets so far discovered. The characteristics deduced using the radial velocities method (the only method that has so far yielded a good number of results) involve three parameters: the period of revolution of the planet (directly linked with the semimajor axis of its orbit, according to Kepler’s Law); the ellipticity of the orbit; and the planet’s mass, if the inclination of the orbit is known, involving the factor sin i. Under the right circumstances, the number of planets involved in the system may also be deduced. These parameters are decided on the basis of observed variations in velocity, and depend on the mass of the star involved, estimated using spectral type/luminosity models. As for such a basic parameter as the radius of the planet, this is only known in the case of the objects observed by the transit method. With the available evidence we cannot really step into the realms of speculation about the individual nature of such planets or their composition or climate; but with more than 200 already known, statistical studies become possible, and the global properties of the population of exoplanets can be discussed.