$\begin{figure} \par\epsfig{figure=De293_f1.eps,width=0.8\hsize} \end{figure}$

Figure 1: HD 80606 radial-velocity data. Crosses: Elodie-OHP measurements. Open squares: Hires-KECK measurements. a) Temporal velocities. b) Residuals around the solution. c) Phase-folded velocities.

The fitted orbital eccentricity is the highest found so far for an extrasolar planet orbiting a solar-type star. The orbital eccentricities for extrasolar planets with period longer than 100 days almost cover the full possible range (Mayor & Udry 2000; Udry & Mayor 2001): from nearly circular (see e.g. the recently announced planet around HD 28185, P = 385 days, e = 0.06, ESO PR) to nearly unity, as in the case of HD 80606. The distribution of the eccentricities of the planetary orbits might be a keystone in understanding the formation processes of planets, as was pointed out early in the study of extrasolar planets by Mazeh et al. (1997b).

Before discussing any mechanism that could have generated the eccentricity of HD 80606, it is interesting to note that the eccentricity distribution of the planets with long orbital periods found so far is strikingly similar to that of the binary orbits (Heacox 1999; Stepinski & Black 2000; Stepinski & Black 2001; Mayor & Udry 2000; Mazeh & Zucker 2000). In particular, the high eccentricity of HD 80606 is very similar to one of the highest eccentricity found so far for a spectroscopic binary - 0.975(Duquennoy et al. 1992). The similarity of the two eccentricity distributions does not prove that the planets and the low-mass stellar companions come from the same population. The large gap between the mass distribution of the planets and that of the stellar companions (Jorissen et al. 2001; Zucker & Mazeh 2001) and the differences in the metallicity distributions for stars with and without planets (Santos et al. 2001) strongly suggests that we are dealing with two distinct populations. Nevertheless, we might need to look for mechanism(s) that can produce a range of eccentricities from zero up to unity for the two populations.

A mechanism to generate eccentric orbits could be the gravitational interaction of a planet (and a binary) with a disk (Artymowicz et al. 1991; Artymowicz 1992). However, a recent study (Papaloizou et al. 2001) suggests that for a standard disk model this can happen only for massive companions, at least in the range of brown dwarf masses. For companions with planetary masses the disk probably acts to damp the eccentricity growth, and therefore can not explain the observed high eccentricities.

Another possible mechanism is the gravitational interaction with another planet(s). This could be via dynamical instability (e.g. Weidenschilling & Marzari 1996; Rasio & Ford 1996; Lin & Ida 1997; Ford et al. 2001) or through some resonant interaction with a disk and another planet (Murray et al. 2001). The instabilities naturally lead to high eccentricities, specially if they involve ejection of another planet out of the system. The resonant interaction, on the other hand, seems to need some fine tuning for generating eccentricities as high as the one found here.

A possible clue to the origin of the particularly high eccentricity found here could have been found in the fact that HD 80606 resides in a stellar wide binary. At least one other planet, the one orbiting 16 Cyg B, was found with a high eccentricity in a wide binary. A few studies (Mazeh et al. 1997a; Holman et al. 1997) have suggested that the high eccentricity of 16 Cyg B is because of the gravitational interaction with the distant stellar companion. In this model, the ``tidal'' interaction of the distant companion induces an eccentricity modulation into the planetary orbit on a long timescale. The present phase of the cycle is close to the highest point of the eccentricity modulation.

However, this interpretation does not apply here. This is so because the modulation timescale induced by HD 80607 is of the order of 1 Gyr (e.g. Mazeh & Shaham 1979). This is very long relative to the relativistic periastron passage modulation, which is of the order of 1 Myr. The precession of the longitude of the periastron induced by the relativistic effect completely suppresses the third-body modulation. To hold on to the third-body interpretation we have to assume an additional body in the system, in an orbit around HD 80606 with a period of the order of 100 yrs. The present radial-velocity measurements can not rule out such a companion. To be consistent, this model has to apply for all the planets with high eccentricity, above, say, 0.6 - an eccentricity that does not seem to be that rare anymore. Note also that this model requires a large angle between the plane of motion of the planet and that of the perturbating body.

In short, as stressed by Mayor et al. (2000) and Stepinski & Black (2001), we need a consistent model that will account for the distribution of eccentricities of the planetary orbits and for its similarity to the distribution for stellar companions. It seems that further observations and theoretical work are needed to reach a consensus about such a model.

4 Discussion