ABSTRACT
Exoplanets are planets outside the Solar System. The first such planet was discovered in 1995 by Mayor and Queloz (1995). The planet, similar in mass to Jupiter, was found orbiting a relatively ordinary star, 51 Pegasus. In the intervening period over a hundred exoplanets have been discovered, nearly all being detected indirectly, using the gravitational influence they exert on their associated central stars. A fascinating account of exoplanets and their discovery is given in Mayor and Frei (2003). From the properties of the exoplanets found up to now it appears that
the theory of planetary development constructed for the planets of the Solar System may need to be reformulated. The exoplanets are not at all like the nine local planets that we know so well. A first step in the process of understanding the exoplanets might be to try to classify them with respect to their known properties and this will be the aim in this chapter. The data in Table 15.1 (taken with permission from Mayor and Frei, 2003), gives the mass (in Jupiter mass, mass), the period (in earth days, period) and the eccentricity (eccent) of the exoplanets discovered up until October 2002. We shall investigate the structure of these data using several methods of cluster analysis.
