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

During the last 5 years, more than 40 exoplanets have been detected around stars in the solar vicinity, but only one system with multiple planets has been unambiguously identified -- around the star Upsilon Andromedae. Today at the International Astronomical Union General Assembly in Manchester, UK, we report the discovery of a second extra-solar planetary system with two Saturnian planets around the star HD 83443.

The new system was detected by means of high-precision radial-velocity measurements with the CORALIE high-resolution spectrograph at the 1.2-metre Leonhard Euler Swiss telescope (ESO-La Silla) as part of the Geneva planet-search project conducted by the CORALIE team.

HD 83443 is a fairly bright star of 8.2 visual magnitude, slightly less massive than the Sun (0.8 solar masses). It lies the constellation Vela, and is 141 light years away from the solar system.

The new planetary system around HD 83443 is unusual in more than one way, because it has two very low mass gaseous giant planets, and both orbit very close to the star.

Their main characteristics:

HD 83443 b. Period 2.9853 day; minimum mass (m2 sin i) 0.35 Jupiter masses; eccentricity 0.08; semi-major axis (a) 0.038 astronomical units; K velocity 56 m/s.

HD 83443 c. Period 29.83 day; minimum mass (m2 sin i) 0.15 Jupiter masses; eccentricity 0.42; semi-major axis (a) 0.17 astronomical units; K velocity 14 m/s.

This inner planet is the planet with the shortest period and with the smallest separation from the parent star among the known candidates. Its period expressed in sideral time is very close to an integer: 2.9935 sideral days. As a consequence every 3 days almost the same radial velocity is observed and several months are needed to fill in observations at different phases of the orbit. After a whole season of measurements clumps of observations are still present at three phases.

Early on, abnormal behaviour was detected in the residuals of the short-period Keplerian solution showing an apparent periodicity of about one month. New measurements added after the announcement of HD 83443 b have fully confirmed the presence of the second planet. Interestingly, HD 83443 c is the planet with the smallest projected mass discovered to date. The system HD 83443 is thus composed of 2 planets close to the star and less massive than Saturn:

• m2 sin i (HD 83443 b) 1.17 Saturn masses
• m2 sin i (HD 83443 c) 0.50 Saturn masses.

Looking at such a system with two planets on so close orbits immediately raises concerns about the system's dynamical stability. A preliminary analysis of its dynamical behaviour clearly confirms that the system is stable. Nevertheless, dynamical perturbations between the planets are far from being negligible and in particular explain the non-circularity of the inner orbit despite its very short period. This system once more emphasizes the impressive diversity of extra-solar planets compared to our own solar system.

New planets from both celestial hemispheres
In addition to the new planet HD83443c described above, we can also announce new planetary candidates discovered in our planet-search programmes in both hemispheres.

La Silla observatory. Photo: ESO

From the North: One additional planet has been detected from the Haute-Provence Observatory around the star HD 190228. It has been discovered by using the ELODIE spectrometer on the 1.93-metre telescope. In addition to HD 190228b, the ELODIE programme has already revealed four planetary companions including 51 Pegasi, the first extra-solar planet to be detected, and one very low mass brown dwarf (m2 sini =19 Jupiter masses).

From the South: Four new planetary candidates have been detected around HD 6434, HD 19994, HD 92788 and HD 121504. As for the HD 83443 system, these new planets have been discovered by the CORALIE team as a result of the fruitful systematic radial-velocity survey carried out at La Silla Observatory (ESO, Chile). In about two years, 19 planets and very low mass brown dwarfs (m2 sin i less than or equal to 15 Jupiter masses} have already been detected with the CORALIE spectrometer fed by the small 1.2-m Euler Swiss telescope.

The main stellar characteristics as well as some orbital parameters and inferred planetary properties for the 4 systems, rather similar to what was observed for the previously detected planets, are summarized below:

HD 6434 - spectrum G3IV; distance 40.3 parsecs (131 light years); period 22.09 days; eccentricity 0.3; m2sin i (minimum mass) 0.48 Jupiter mases; semimajor axis (a) 0.15 AU; site - ESO.

HD 19994 - spectrum F8V; distance 22.2 parsecs (72 light years); period 454.2 days; eccentricity 0.2; m2sin i (minimum mass) 1.8 Jupiter mases; semimajor axis (a) 1.3 AU; site - ESO.

HD 92788 - spectrum G5; distance 32.3 parsecs (105 light years); period 340.8 days; eccentricity 0.36; m2sin i (minimum mass) 3.8 Jupiter mases; semimajor axis (a) 0.94 AU; site - ESO.

HD 121504 - spectrum G2V; distance 44.4 parsecs (145 light years); period 64.62 days; eccentricity 0.13; m2sin i (minimum mass) 0.89 Jupiter mases; semimajor axis (a) 0.32 AU; site - ESO.

HD 190228 - spectrum G5IV; distance 62.1 parsecs (203 light years); period 1127 days; eccentricity 0.43; m2sin i (minimum mass) 5.0 Jupiter mases; semimajor axis (a) 2.3 AU; site - ESO.

To date, the ELODIE and CORALIE exoplanet searches have revealed 25 companions to solar-type stars with masses smaller than 20 Jupiter masses. The quest for exoplanets is now revealing not only more and more candidates, but also planets with smaller and smaller masses.