The observed orbital charateristics of planets are the direct outcome of their formation processes and of their evolutions. Therefore, these characteristics may be used to retrace their formation mechanisms and to constrain theories of planetary formation. The recent spectroscopic studies of stars where planets have been detected have shown that the host star itself may also bear marks from some processes occuring during planetary formation (Gonzalez 1997). More specifically a large number of planets with short-period orbits have surprisingly metal rich host stars. These planets, very close to their stars, are usually referred as 51 Peg like, or "hot Jupiters". The metallicity of their host star is much higher than the "average field star" and is not the result of a selection process in the survey samples (Marcy et al. 1999b). Typical metallicities similar to field stars may be assumed for the stars from various surveys, since these star samples have not been selected from any metallicity criteria.
If we look in more detail at all the planets with semi-major axis less than 1 AU, where the number of detections is significant and not strongly high-mass biased, we observe a relation between the semi-major axes of the planetary orbits and the metal content of their host stars. All planets with semi-major axes less than 0.08 AU seem to have a star with an unusually very high metal content compared to other stars with planets (see Fig. 4). Actually, a comparison of the two distribution using the Kolmogorov-Smirnov test indicates a 99% probability that the two distributions are indeed different.
The unusual metal content of the short orbit planets had been pointed out shortly after the detection of 51 Peg (Mayor & Queloz 1995). But now, with the large number of detections of similar systems and others with slightly larger semi-major axes, we observe a typical distance (or period) for which this unusual high metal content is systematically observed. A possible explanation may be related to some very specific processes occuring during the formation of these very close systems. However the large incertainties on the estimation of the age of these systems and the small mass range of primaries are noteworthy. Therefore it is difficult to completely rule out a stellar population effect.
The migration theory (Lin & Papaloizou 1986; Lin et al. 1996; Ward 1997; Trilling et al. 1998) is one of theories that has been called for to explain the existence of very close planets that were not described by the "classic" solar-system planetary formation model (Boss 1995; Lissauer 1995). But so far, we have a poor understanding of the way the planet stops its migration. The two different metallicity distributions pointed out in this article are perhaps a new clue to a better understanding of the migration process or the likelyhood of an in-situ formation (Bodenheimer et al. 1999).
Others scenarios involving strong gravity interactions with other planets have been proposed as a possible origin for small planetary orbits (Weidenschilling & Marzari 1996; Rasio & Ford 1996). Since these models are purely driven by dynamical interactions it seems a priori difficult to expect any metallicity enhancement effect. Moreover such scenarios do not really explain the very small orbit planets like 51 Peg. However, if one believes that the high metal content of the star is the end-result of a planet swallowed by the star (Sandquist et al. 1998), the gravity interaction is a possible means to send planets into their stars.
The precision of surveys from which the planets have been found so far has been limited to the detection of systems with a -amplitude (K) larger than 25 m s-1. Therefore it is still premature to compare the mass of the two sets of planets because there is a direct relationship between the amplitude of the radial velocity curve, the semi-major axis, and the minimum mass of the planet that can be detected. However if we limit our comparison to a sample free of such bias, including planets only having semi-major axes smaller than 0.3 AU, we may be inclined to believe that metal rich stars tend to host on average less massive planets than solar stars or metal poor stars.
The distribution of mass of all planets that have been detected can also be studied with a restricted sample of planetary systems in order to avoid a biased selection towards small orbits and massive systems. We see that with a sample restricted to minimum masses greater than 1 MJ and semi-major axes smaller than 1.3 AU, the number of planets per mass bin is almost constant from 1 to 5 MJ and then drops suddenly for more massive companions. This reinforces the idea that a maximum planet mass may lie somewhere close to 5-7 MJ as pointed out earlier by Mayor et al. 1998.
New discoveries and improved detection precision will allow us to get a better picture of the relation between the mass and certain orbital characteritics of planets and some peculiarities seen in the atmosphere of their host stars. This will perhaps enhance our understanding of the mechanisms of planetary formation.
© European Southern Observatory (ESO) 2000
Online publication: January 31, 2000