Following the discovery of a giant planet orbiting the Solar-type star 51 Peg (Mayor & Queloz 1995), planet hunters have unveiled the presence of about 200 exo-worlds.1 Most of these discoveries were made using the radial-velocity technique, and include ~20 multi-planetary systems, several confirmed transiting planets (e.g. Charbonneau et al. 2000; Konacki et al. 2003; Bouchy et al. 2004), as well as the first Neptune-mass planets (e.g. Santos et al. 2004b; McArthur et al. 2004).

Globally, these findings brought to light the existence of planets with a huge variety of characteristics, eliciting unexpected questions about the processes of giant-planet formation. For instance, some of the planets are on eccentric orbits (Naef et al. 2001), which would be more typical of some comets in the Solar System, while some behemoths have more than 15 times the mass of Jupiter (Udry et al. 2002). As a result, the definition of a planet has itself been put into question; see e.g. the discussion in Santos et al. (2005a).

1 See the table at for continuous updates. Before these discoveries, only planets around a pulsar had been detected (Wolszczan & Frail 1992). Given the violent supernova explosion that gave rise to the pulsar, however, it is believed that these are probably second-generation planets.

The Metal-rich Universe, eds. G. Israelian and G. Meynet. Published by Cambridge University Press. © Cambridge University Press 2008.

With the number of detected exoplanets increasing very fast, current results are already giving us the chance to undertake the first statistical studies of their properties, as well as of their host stars (e.g. Cumming et al. 1999; Zucker et al. 2002; Udry et al. 2003; Santos et al. 2003; Eggenberber et al. 2004; Halbwachs et al. 2005). These studies, together with theoretical models, are now helping us to better our understanding of the way planets are formed and evolve. In this paper we will review one of the major observational constraints regarding this issue: the study of the metallicity of stars with giant planets.

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