An artist's concept of an extrasolar planet. Photo: Greg Bacon, STScI

Discoveries of planets around other stars are becoming almost routine. The latest success comes from the ace team of planet-hunters at the Geneva Observatory in Switzerland, who claim to have found no less than eight new, very low mass companions to solar-type stars. The masses of these objects appear to range from less than that of Saturn to about 15 times that of Jupiter.

The new results were obtained by means of high-precision radial-velocity measurements with the CORALIE spectrometer at the Swiss 1.2 metre Leonhard Euler telescope at the ESO La Silla Observatory in Chile. The detections are based on changes in the velocity of the central star due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. By detailed analysis of the measured velocity variations, astronomers can deduce the planet's orbit and minimum mass.

The new objects are quite diverse. While six of them are small enough to be classed as bona-fide exoplanets, two are apparently very low mass brown dwarfs (sub-stellar objects without a nuclear energy source in their interiors).

A representation of the sizes and shapes of the orbits of the eight new planetary and brown-dwarf candidates. The colours indicate the deduced minimum masses: about one Saturn mass or less (red); between 1 and 3 Jupiter masses (green); above 10 Jupiter masses (blue). The dashed line indicates the size of the Earth's orbit (radius 150 million km). Photo: ESO

The smallest discovery is a sub-Saturnian planet in orbit around HD 168746, a solar-like star which lies 140 light years away in the constellation of Scutum. The new planet follows a circular orbit with a period of only 6.4 days. It is only the third exoplanet detected which seems to be smaller than Saturn.

Two planets slightly more massive than Saturn travel around the stars HD 83443 (in the constellation of Vela) and HD 108147 (in Crux). The companion of HD 83443 has the shortest period (2.986 days) of any known exoplanet. This means it lies only 5.7 million km (3.6 million mls or 0.038 AU) from the star. A small change in the star's mean velocity variation suggests that another low-mass companion may also exist. The object orbiting HD 108147 follows a fairly eccentric (e = 0.56) path, despite its fairly short period of 10.88 days.

Three planets with 1.07, 2.2 and 2.96 times the mass of Jupiter orbit around HD 52265 (in Monoceros), HD 82943 (Hydra) and HD 169830 (Sagittarius). Their high orbital eccentricities (0.38, 0.61 and 0.34) and intermediate periods (119, 443 and 230 days) are fairly typical of the exoplanets so far discovered.

La Silla Observatory. Photo: ESO

Moving up the size scale, the Swiss team found evidence for two low mass brown dwarf companions to HD 162020 (in Scorpius) and HD 202206 (Capricorn). The object around HD 162020 orbits the star in 8.43 days on a moderately eccentric orbit and its inferred minimum mass is 13.7 times that of Jupiter. The second candidate is at least 14.7 Jupiter masses and follows a fairly eccentric orbit with a 259 day period.

Since the first discovery of an exoplanet around the star 51 Pegasi in 1995 (by Michel Mayor and Didier Queloz of the Swiss team), the count has now risen to 43. Curiously, only three of these lie in the minimum size range of 10 - 15 Jupiter masses. This strongly points towards different formation and evolution processes for giant planets and brown dwarfs.

It may also be significant that most of the stars around which giant planets have been found so far show an excess of heavy elements in their atmospheres when compared to the majority of stars in the solar vicinity.

The high precision radial velocity survey with CORALIE in the southern hemisphere is intended to make a complete inventory of giant exoplanets orbiting about 1600 Sun-like stars in our galactic neighbourhood.