GENEVA, 18 May 2006 - A solar system consisting of at least three planets has been discovered by European astronomers, including a planet that may contain liquid water.

Using the ultra-precise HARPS spectrograph on the European Southern Observatory's 3.6-m telescope at La Silla (Chile), the team discovered that a nearby star is host to three planets about the mass of Neptune. The planet furthest from the star they are orbiting could even contain liquid water. This unique system is likely to be further enriched by an asteroid belt.

"For the first time, we have discovered a planetary system composed of several Neptune-mass planets", said Christophe Lovis, from the Geneva Observatory and author of the paper presenting the results in the British journal, Nature.

During more than two years, the astronomers carefully studied HD 69830, a rather inconspicuous nearby star slightly less massive than the Sun. Located 41 light-years away towards the constellation of Puppis (the Stern). It is just visible with the naked eye.

The newly found planets have minimum masses between 10 and 18 times the mass of the Earth. The astronomers believe the inner planet has an essentially rocky composition, and a rocky/gas structure for the middle one. The outer planet has probably accreted some ice during its formation, and is likely to be made of a rocky/icy core surrounded by a quite massive envelope. Further calculations have also shown that the system is in a dynamically stable configuration.

The outer planet also appears to be located near the inner edge of the habitable zone, where liquid water can exist at the surface of rocky/icy bodies. Although this planet is probably not Earth-like due to its heavy mass, its discovery opens the way to exciting perspectives.

With three roughly equal-mass planets, one being in the habitable zone, and an asteroid belt, this planetary system shares many properties with our own solar system.

"This alone makes this system already exceptional", said Willy Benz, from Bern University, and co-author. "But the recent discovery by the Spitzer Space Telescope that the star most likely hosts an asteroid belt is adding the cherry to the cake."

"The planetary system around HD 69830 clearly represents a [major step] in our understanding of how planets form", said Michel Mayor also from Geneva Observatory, and HARPS Principal Investigator.

"No doubt it will help us better understand the huge diversity we have observed since the first extra-solar planet was found 11 years ago."