The search for planets around small stars seems to be gaining in intensity. Now a team of French and Swiss astronomers has announced the discovery of a Neptune-class planet around the star Gliese 581. The Gliese catalog lists all known stars within 25 parsecs (81.5 light years) of the Sun, making its listings of significant interest in the hunt for exoplanets and, long-term, in our thinking about robotic interstellar probes. Also catching the eye is the fact that this star is a red dwarf, confirming the notion that such stars are ripe for exoplanetary investigation.

Gliese 581 is 20.5 light years away in the constellation Libra. Red dwarfs like it make up about 70 percent of the galactic population; in fact, of the 100 stars closest to the Sun, fully 80 are red dwarfs. Earlier surveys of red dwarfs have revealed few with planets, but Stéphane Udry (Geneva Observatory), a co-author of the paper on the new find, believes the earlier surveys may have operated with insufficient precision. It took the HARPS instrument (High Accuracy Radial Velocity for Planetary Searches) mounted on the European Southern Observatory’s 3.6-meter telescope at La Silla (Chile) to track this planet down. HARPS is considered the most precise spectograph available for such study.

As to the new planet itself, its mass is about 17 times that of Earth. The world orbits its host star in 5.4 days, at a mean distance some 9 times closer than the orbit of Mercury to the Sun. In a paper on the find, the authors note that around the 3 M-class dwarfs found to have planets, a total of five planets are known. None are ‘hot Jupiters,’ while three are ‘hot Neptunes,’ in close proximity to their stars. From the paper:

“…the mass-distribution of close-in planets has two peaks centered at about the masses of Jupiter and Neptune, with the former preferentially populated around G-dwarfs and the latter around M dwarfs, reflecting how much matter remains available in the disk for accretion during the inward migration of the planet. Other theoreticians however take the view that many hot-Neptunes are actually evaporated hot-Jupiters…Better statistics on M-dwarf planets will help determine which of these mechanisms dominate.”

The paper, which can be found here (PDF warning) is Bonfils, et al., “The HARPS Search for southern extra-solar planets. VI. Neptune-mass planet around the nearby M dwarf GL 518,” Astronomy and Astrophysics 443, L15 (2005). A press release from the European Southern Observatory is also available.