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

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4. Discussion

The orbital elements of Gliese 876b are worth noting. In spite of the low mass of Gl 876, the ice-condensation radius at the time of planet formation around this star is [FORMULA] 4 AU, only 20% lower than around a solar type star (Boss 1995). Once again the orbital semi-major axis (a = 0.20 AU) is thus much smaller than the expected minimum radius for giant planet formation, and some orbital migration must have occcured. However the observed orbital separation is also 4 times larger than the measured semi-major axes of 51 Peg, [FORMULA] Boo and [FORMULA] And (0.04-0.05 AU). The excess of planets with such small semi-major axes is believed to result from outward torques which counteract at short distances the inward torque induced by the interaction of the planet and the protoplanetary disk (Lin et al. 1996, Trilling et al. 1998). These torques only become effective at separations significantly smaller than 0.20 AU, and cannot have played a signficant role for the Gl 876 system. It is also interesting to note that the orbit of GL 876b is eccentric (e=0.35), while interaction with an accretion disk is expected to damp any significant orbital eccentricity of a planet (Goldreich and Tremaine, 1980). Several mechanisms may be called in to explain the large orbital eccentricities of giant extrasolar planets, but in the present case the most interesting possibility is probably the chaotic interaction of several giant planets (Weidenschilling & Marzari, 1996; Rasio & Ford, 1996; Lin & Ida, 1997). A frequent final result of such a strong gravitational interaction is a planetary system with a single giant planet at a moderate semi-major axis, in an eccentric orbit. This could thus simultaneously explain the semi-major axis and the eccentricity.

Contrary to all previously confirmed planets around main sequence stars, Gl 876b orbits a star which is very different from our Sun, showing that planetary systems form around stars of widely different types. Gl 876 is much less massive than the Sun, [FORMULA] 0.3[FORMULA], and at most only [FORMULA] 150 times more massive than its planet. Its radius is only three times as large: the radius of Gl 876 is [FORMULA] 0.3[FORMULA] (Chabrier & Baraffe, 1997), while that of Jupiter is 0.10 solar radii. Gl 876 is also much cooler than the Sun, and much less luminous. From the observed I-K and V-I colours (Leggett, 1992) its effective temperature is 3100 to 3250 K (Leggett et al., 1996), compared with 6000 K for the solar surface. From its absolute V magnitude and the bolometric correction of Delfosse et al. (1998a) it bolometric magnitude is 9.42, corresponding to 1.35 [FORMULA][FORMULA]. Even though the planet of Gl 876 is twice closer to its star than Mercury is to the Sun, the stellar flux at Gl 876b is only ten times the solar flux at Jupiter, and lower than the solar flux at Mars. The apropriate albedo for Gl 876b is unclear, and, by analogy with Jupiter (e.g. Podolak et al., 1993), its thermal balance of Gl 876b may also include a substantial contribution from an internal heat source. A detailed evaluation of its effective temperature is thus beyond the scope of the present letter, but Gl 876b is clearly much too cold to possibly sustain liquid water above the 1 bar level.

Gl 876 is closer to us than all other stars orbited by known extra-solar planets, by at least a factor of 3. At d=4.7 pc, Gl 876 is the 40th closest stellar system to our Sun, and the 53rd closest star. Since M dwarfs make up [FORMULA] 80% of the solar neighbourhood population (Gliese & Jahreiss, 1991), it is only natural that the first member of this numerous class found orbited by a planet is a very nearby one, unless planetary formation would have selected against low mass stars. This discovery weakens such an hypothesis but improved statistics would obviously be needed for a reliable conclusion.

This detection represents an opportunity to confirm a radial velocity detected planet through astrometry and determine its actual mass, or at the very least to set a lower limit which is firmly planetary. Gl 876 is both at least 3 times closer to us than any other star with a detected planetary companion, and about 4 times less massive (only 0.3[FORMULA] instead of about [FORMULA] 1[FORMULA] for all previous detections). Despite its relatively short orbital period of 61 days, the astrometric reflex motion induced by its [FORMULA] 2[FORMULA] companion is therefore unusually large by extrasolar planet standards, with a minimum semi-major axis of 0.27 milliarcsecond for an edge-on orbit, and larger by 1/sin(i) for more face-on geometries. The best single measurement precision of an astrometric observations is at present 1 milliarcsecond, with the FGS instrument on HST. A detection is clearly an ambitious measurement at this time if the orbit is seen edge-on, and it would need a very determined effort. A lower limit on the inclination of [FORMULA] on the other hand will be easily obtained, and would already imply M[FORMULA]8 MJ. In addition, these observations can be accomplished over the short timescale of one orbital period, only 2 months.

Finally, it is interesting to note that the measurements of Gl 876 obtained with the new CORALIE spectrometer on the 1.2 m telescope have residual O-Cs as small as 22 m/s, for a V magnitude of 10.2. This discovery of a giant planet around a rather faint M4 dwarf illustrates that this small telescope will powerfully contribute to the search for extrasolar planets. The application of the cross-correlation technique to the full wavelength domain (300 nm) of CORALIE compensates to some extent the disadvantage of the small telescope aperture (Baranne et al. 1996), and the nearly full-time availability of the telescope for planet searches will make future period identifications much easier.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998