Read also about 14 Her

Gliese 876 : THE CLOSEST EXTRASOLAR PLANET

Electronic preprint

The discovery of the first extrasolar planet by Mayor and Queloz in 1995, at Haute Provence Observatory (France), has been followed by those of a dozen other planets with Jupiter-like masses. These planets have very different orbits, with periods spanning between 3.3 days and 4.27 years. This indicates that giant planets can have much shorter periods than those of our solar system, where Jupiter, Saturn or Uranus have periods longer than 10 years. Up until now, however, all these planets have been found close to solar-type stars. This new planet, orbiting a star which is very different from our Sun, shows that planetary systems form around a wide variety of stars.

A GIANT PLANET AROUND A VERY LOW MASS STAR

Gliese 876 (or Gl 876) is a red dwarf star, 5 times less massive than the Sun (its mass is also 200 times the mass of Jupiter). It is therefore considerably less luminous: about 600 times less than the Sun. It is also very close to us, at only 15 light-years. This makes it the 40th closest star to our Sun (by number of star systems; it would be the 53rd closest star if one would instead count the components of multiple star systems). It is nonetheless much too faint to be visible to the naked eye, though it can be seen even with a small telescope.

Take a look at a finding chart generated with Starry Night showing the location of the star, which has been drawn in by hand, in the constellation of Aquarius.

Its radial velocity is accurately measured since October 1995 with the ELODIE spectrograph, built by the Haute-Provence and Geneva Observatories to detect planets, and installed at Haute-Provence Observatory. Since June 1998 it is also observed with CORALIE, an improved copy of ELODIE which has just been commissionned on the brand-new Swiss telescope at La Silla Observatory (Chile). These measurements show that a planetary mass of about 1.5 times the mass of Jupiter perturbs the movement of this star.

Radial velocity curve

The parameters of its orbit are :

Period : ~60 days
Velocity variation semi-amplitude ~200 m/s

The minimum planetary mass is ~1.5 times that of Jupiter The average radius of its orbit is 0.2 times the Earth-Sun distance. Moreover, its orbit is slightly eccentric.

Among the dozen extrasolar planets discovered up to now, this one is the closest to the Sun.

This system of a very low-mass star and its giant planet has some peculiarities. The star is only 100 times more massive than its planet and its radius is only twice as large: the radius of Gl 876 is 0.2 solar radii, while the planetary radius is 0.1 solar radii.

Gl 876 is much less luminous and much cooler than the Sun: about 3000 degrees, compared with 6000 degrees for the solar surface. Even though the planet of Gl 876 is much closer to its star than the Earth is to the Sun, its temperature is nevertheless only about -125 degrees (Celsius).

Red dwarf stars are the most numerous objects in our Galaxy: of the 150 stars closest to the Sun, for instance, 120 are red dwarfs of less than 0.5 times the mass of the Sun. The detection of a planet around one of them therefore opens exciting prospects on the number of detectable planets in the immediate solar neighbourhood.

This discovery will also be presented at a press conference organized by the Haute-Provence Observatory on July 7th, 1998 in Marseille, together with another planet discovery obtained with the ELODIE spectrograph.