4 Concluding remarks

We have presented giant-planet candidates around the stars HD28185 and HD213240. These two planets are both in long period orbits, and have minimum masses of 5.7 and 4.5 $M_{{\rm Jup}}$, respectively.

The main difference between these two systems is the eccentricity of the planetary orbits: while the planet around HD213240 is quite typical of the now discovered long period planets (usually with high eccentricity), HD28185 is one of the first long period but low eccentricity (lower than 0.1 and compatible with zero) planets found to date. The orbit of the planet orbiting HD28185 is compatible with a circular trajectory with a period of 383 days (close to 1 Earth year). Its distance from the star, 155 million km, is almost equal to the distance between the Sun and the Earth. With the exception of the planet around HD27442 (P=415days and e=0.06 - Butler et al. 2001), circular or almost circular orbits amongst exoplanets had so far only been found for short-period systems, contrary to what is the case for the giant planets in our own Solar System.

The eccentricity distribution of planetary orbits is, today, one of the most interesting problems. As discussed in Udry et al. (2001) and Mayor & Santos (2001), with a few exceptions, there are no clear differences between the eccentricity distributions of planetary and stellar binary systems. This poses the problem of understanding how systems that were formed as the result of different physical processes (formation in a disk vs. instability collapse) may have similar eccentricity distributions.

For masses lower than $\sim$20 $M_{{\rm Jup}}$, it has been shown that the interaction with a gas disk has the effect of damping the orbital eccentricity (Papaloizou et al. 2001). This suggests that other processes, like the interaction between planets in a multiple system (e.g. Rasio & Ford 1996; Ford et al. 2001), or the influence of a distant stellar companion (e.g. Mazeh et al. 1997), may play an important role in defining the "final'' orbital configuration.

Furthermore, another question is raised: what is the difference between these systems and our own Solar System? Although no real Solar System analogs were found to date, companions like the ones found orbiting HD27442 and HD28185, with relatively distant (more than $\sim$1AU) giant planets in circular or almost circular orbits, may indeed help answer this question.

It has been shown that stars with giant planets are metal-rich when compared to "single'' field dwarfs (Gonzalez et al. 1998; Santos et al. 2001). The Sun itself is in the "metal-poor tail'' of the metallicity distribution of stars with planets (Santos et al. 2001). This fact make us wonder if the Solar System had or not a similar origin with the now found extra-solar planets, or at least a different evolution process. Although still too early to make any further statements, it is interesting to mention that both objects presented in this paper (HD28185 with $\rm [Fe/H]=+0.24$ and HD213240 with $\rm [Fe/H]=+0.16$) as well as HD27442 ( $\rm [Fe/H]=+0.22$ from the spectroscopic study of Randich et al. 1999) seem to follow the general trend and are metal-rich with respect to the Sun.

The planet around HD28185 is the first to be discovered around a middle age solar type dwarf, in a circular orbit, and located in the "habitable zone'' where temperatures like those on the Earth are possible. Given the temperature of the star, at a distance of 1AU the equilibrium temperature of the planet is 250K. Still, it is most probably a giant, gaseous planet and thus an unlikely place for the development of life. Nevertheless, we can speculate that it may be orbited by one or more moons on which a more bio-friendly environment could have developed.

Acknowledgements

We wish to thank the Swiss National Science Foundation (Swiss NSF) for the continuous support to this project. We would also like to thank Bernard Pernier for the useful measurements, and our referee, W. Cochran, for the nice comments and suggestions. Support from Fundação para a Ciência e Tecnologia, Portugal, to N.C.S. in the form of a scholarship is gratefully acknowledged.