(1) Geneva Observatory, 51 ch des Maillettes, 1290 Sauverny, Switzerland
(2) JPL, 4800 Oak Grove drive, Pasadena, CA 91109
(3) St-Luc Observatory, St-Luc, Switzerland
A new extra-solar planet has been discovered in orbit around HD 75289, a solar-type star of the southern sky. With a minimum mass m_2sini=0.397E-3 solar mass, this planet is the lightest one discovered around a solar-type star. That mass corresponds to 0.42 times the mass of Jupiter or 1.4 times the mass of Saturn. Despite of the low mass of the planet, the velocity variation is precisely determined. The error on the minimum mass is less than 4%.

The planet has been discovered with the 1.2-m "Leonard Euler" Swiss telescope at ESO-La Silla Observatory, as part of the CORALIE planet-search programme. 69 precise velocity measurements on 21 cycles have permitted to derive accurate orbital parameters: the orbital period p=3.5097 +/- 0.0023 days, the separation a=0.046 AU, the orbit is circular and the amplitude of the velocity variation K=54 +/- 1 m/s.

HD 75289 is a bright (V=6.35) G0V star in the VELA (SAILS) constellation. Its age derived from evolutionary tracks or an indirect estimator based on activity is comparable to the age of the Sun. The precise astrometric parallaxe from the HIPPARCOS satellite sets the star at a distance of 29 pc from the Sun. Such a distance invalidates the supergiant classification usually given for this star, as for example in the Bright Star Catalogue. HD 75289 is slightly metal-rich compared to the Sun ([Fe/H]=0.29) as most of the other stars with giant planets. Its metal content is about twice the solar composition.

The star is a moderate rotator (vsini=4.4 km/s) without any sign of chromospheric activity. It is also very stable in luminosity as measured with the HIPPARCOS satellite (sigma(V)=7.3 mmag). The mean bisector of the stellar lines does not exhibit any variation either, ruling out possible non-planetary interpretations of the observed radial-velocity variations like rotating stellar spots or active regions in the stellar atmosphere.

This discovery adds a new system to the group of 51 Peg-type short-period planets (P<=10 days). Ordered by mass we now have:

    Name        mass [M_Jup]     period [days]
    ----        ------------     -------------
   HD 75289        0.42             3.51
   51 peg          0.44             4.23
   HD 187123       0.57             3.1
   Ups And         0.61             4.62
   HD 217107       1.20             7.11
   Tau Boo         3.60             3.31

It is remarkable to note that 2/3 of these planets have masses lighter than 1 M_Jup.

All these short-period planetary systems have a non-negligible probability (5-10%) of a planet transit in front of the star. Such a transit, if observed, will allow the direct measurement of the mean density of the planet. A photometric monitoring of HD 75289 has been initiated by our Danish collegues at La Silla (J. Clausen E. Olsen, and B. Helt) to search for a possible transit.

We are greatful to B. Vandenbussche and F. Kienzle 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