Astronomy: Three Neptune-mass exoplanets at HD 69830

ESO - the European Southern Observatory - has announced on 18 May 2006 the dazzling discovery of three exoplanets around a very, very nearby star. The most inner planet is believed to orbit the star in a habitale zone, possibly containing life. Using the ultra-precise HARPS spectrograph on ESO's 3.6-m telescope at La Silla (Chile), a team of European astronomers have discovered that this nearby star is host to three Neptune-mass planets, with the minimum size of 10 - 18 times Earth. The star HD 69830 is located 41 light years away - only slightly further than the nearest star Proxima Centauri at 4,2 light years, but much nearer than most stars at billions of light years from Earth. During more than two years, astronomers carefully studied HD 69830, a rather inconspicuous nearby star slightly less massive than our Sun. Located 41 light years away towards the constellation of Puppis (the Stern), it is, with a visual magnitude of 5.95, just visible with the unaided eye. Extensive theoretical simulations favour an essentially rocky composition for the inner planet, 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 planets orbit the star in 8.67, 31.6 and 197 days. This unique system is likely further enriched by an asteroid belt, observed by the Spitzer Space Telescope "For the first time, we have discovered a planetary system composed of several Neptune-mass planets", said Christophe Lovis, from the Geneva Observatory and lead-author of the paper presenting the results. "Only ESO's HARPS instrument installed at the La Silla Observatory, Chile, made it possible to uncover these planets.", said Michel Mayor, also from Geneva Observatory, and HARPS Principal Investigator. "Without any doubt, it is presently the world's most precise planet-hunting machine." 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. "The planetary system around HD 69830 clearly represents a Rosetta stone in our understanding of how planets form.", said Mayor. "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."

Artist impression of HD 69830 system

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