D. Naef , M. Mayor, F. Pepe, D. Queloz, N.C. Santos, S. Udry, M. Burnet

Geneva Observatory, 51 ch des Maillettes, 1290 Sauverny, Switzerland

We report here the detection of a planetary companion orbiting the young dwarf Gliese-Jahreiss 3021 (GJ 3021). The radial-velocity orbit obtained from our 50 measurements has a period of 133.8 +/- 0.2 days, an eccentricity of 0.5 +/- 0.02 and a semi-amplitude of 164 +/- 4 m/s. The inferred minimum mass for the companion is 3.32 Jupiter mass. The orbital separation between the planet and its parent star ranges from 0.25 to 0.75 AU. This planet resides within the habitable zone most of the time. The equilibrium temperature ranges from about 260 Kelvin (apastron) to 440 Kelvin (periastron).

This planetary companion has been discovered with the CORALIE echelle spectrograph mounted on the 1.2m "Leonard Euler" Swiss telescope at ESO-La Silla observatory (Chile). It is the fifth detection of the Geneva Southern Planet Search after only 18 months of operations.

GJ 3021 (HD 1237, HIP 1292) is a bright (V=6.59) G6 dwarf in the constellation of Hydrus. The precise parallax from the Hipparcos satellite sets the star at a distance of 17.8 pc from the Sun. It is a chromospherically active star (log R'_HK = -4.44, Henry et al. 1996). With this activity indicator, we estimate an age of 0.6 Gyr using the calibration by Donahue (1993). Such a young age is supported by the fast stellar rotation (Vsini=5.5 km/s) and the ROSAT X-ray luminosity of GJ 3021 (L_X > 10E29 erg/s) much too high for an old G dwarf. GJ 3021 has a high metal content ([Fe/H]=0.2).

The large residuals around the best Keplerian solution (O-C=19 m/s) are most probably related to the star activity level. GJ 3021 is not the first active star hosting a planet. HD 130322 (Udry et al. 2000), and HD 192263 (Santos et al. 2000) are also chromospherically active cool stars with giant planets. It has recently been shown (Saar et al. 1998, Santos et al. 2000) that the radial-velocity jitter due to activity-related processes is getting smaller for redder solar-type stars. So it remains possible to search for planets around late-type active stars with the radial-velocity technique.

We are greatful to F. Kienzle, C. Melo and Y. Revaz for additional measurements obtained during their own observing runs.

More details and the phase-folded curve of the radial-velocity measurements can be obtained on our planet-search web page on the Geneva Observatory web site:
http://obswww.unige.ch/~udry/planet/planet.html