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Astron. Astrophys. 338, L67-L70 (1998)

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1. Introduction

The still recent discovery of the first extrasolar planet, around 51 Peg (Mayor & Queloz, 1995), has since been followed by many more. The count currently runs to 11 very low mass companions (Marcy & Butler, 1998; Queloz, 1999), with minimum masses (M [FORMULA]) which range between 0.5 and 10 times the mass of Jupiter. Asides from their Jupiter-like masses, which largely reflect the sensitivity threshold of current radial velocity programs, the known extra-solar planets are a very diverse class. Some of them have large eccentricities when others have nearly circular orbits, and their periods range between 3.3 days and 4.4 years. Giant planets can thus have very much shorter periods than in our solar system, which clearly does not represent the only possible outcome of planetary system formation and evolution.

To date on the other hand, planets have mostly been looked for around solar type stars, and, pulsar companions asides, they have only been found orbiting such stars. This reflects to some extent an understandable desire to identify close analogs to our own solar system, which could perhaps contain life sustaining planets. Also, the selection function of the radial velocity planet searches has a relatively sharp optimum around spectral class G. Essentially all stars hotter than approximately F5 have fast rotation (Wolf et al., 1982), so that it is impossible to measure their radial velocity to the [FORMULA] 10m.s-1 accuracy needed to detect planets. At the other end of the mass spectrum, most M dwarfs have slow rotation (Delfosse et al., 1998a) and their velocity can be measured accurately, as we discuss below. Their luminosities however are much lower than those of solar type stars. At a given distance a much longer integration time is thus needed to obtain a given radial velocity precision on an M dwarf than on a G dwarf. All planet search programs have thus understandably concentrated on solar type stars.

G dwarfs however only represent a small fraction of the disk stellar population, with the lower mass M dwarfs outnumbering them by about an order of magnitude (Gliese & Jahreiss, 1991). It is thus likely that most planets in our galaxy orbit stars whose mass and luminosity are significantly lower than the Sun's (Boss, 1995), unless some as yet unidentified physical process restricts planet formation to the environment of sufficiently massive stars. It is clearly important to establish whether such a mechanism exists.

For the last three years, we have been monitoring the radial velocities of a nominally volume limited sample of 125 nearby M dwarfs. The two main goals of this large observing program ([FORMULA] 30 nights/year) are to establish the controversial (e.g. Kroupa, 1995, and Reid & Gizis, 1997, for two contrasted views) multiplicity statistics of field M dwarf systems, and to pin down the still uncertain mass-luminosity relation at the bottom of the main sequence. Delfosse et al (1998b) present preliminary results for the stellar companion search, with 12 new components found in these nearby M dwarf systems, including the third detached M dwarf eclipsing binary (Delfosse et al., 1998c). A byproduct of this program, related to the angular momentum dissipation of very low-mass stars, is described in Delfosse et al (1998a).

Even though this was not the main focus of the program, we also realised from the start that for most of these stars we obtain radial velocity precisions which are sufficient to detect giant planets, if any exists around them. We present in this letter the first such detection, around Gl 876 (BD[FORMULA]6290, LHS 530, Ross 780, HIP 113020), a V=10.2 M4 dwarf (Reid et al., 1995) at d = 4.702[FORMULA]0.046 pc (ESA, 1997).

Delfosse et al. (1998a) present in detail the observed sample, while Delfosse et al. (1998b) discuss the observing and analysis technique at length. We therefore only briefly summarize this information in Sect. 2. We then proceed to discuss in Sect. 3 the radial velocity detection of the planetary companion of GL 876. In Sect. 4 we consider the implications of this detection and suggest some possible follow-up observations.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998
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