May 18, 2006 — Scientists looking for planets around stars beyond our sun have found a system that contains an asteroid belt and three Neptune-sized worlds, one of which orbits in a zone where liquid surface water could exist.
Although the planet’s location could theoretically support life, researchers believe it is wrapped in an extensive hydrogen atmosphere and probably not suitable for life as we know it. Nevertheless, the finding, which is reported in this week’s issue of Nature, is considered a key development in the continuing push to find Earth-like worlds elsewhere in the universe.
The trio of planets circle HD 69830, a pale sun-like star located about 41 light years away in the constellation Puppis. Astronomers using the infrared Spitzer Space Telescope previously discovered that HD 69830 probably has an asteroid belt in orbit. If true, the star would be the first similar in mass and age to the sun to have one.
The researchers also predicted that a planet’s gravitational tug was helping to keep the belt in order. With that hint and a sophisticated light-splitting spectrograph on the European Space Agency’s 3.6-meter telescope in La Silla, Chile, astronomers spent two years studying miniscule wobbles in the star’s orbit. They found three planets orbiting within the same distance that Earth orbits the sun.
Surprisingly, the hunt turned up no sign of a large Jupiter-class planet, making HD 69830 the first extrasolar planetary system without a massive planet.
"The planetary system around HD 69830 clearly represents a Rosetta stone in our understanding of how planets form," said Swiss astronomer Michel Mayor, a co-author of the paper. "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."
Although HD 69830’s planets are still 10 to 18 times bigger than Earth, the discovery is encouraging to researchers who are refining their planet-hunting techniques to find smaller, more Earth-like worlds.
"It implies that further low-mass planets will be spotted orbiting other stars," writes Harvard University astronomer David Charbonneau, in a related article in Nature.
Computer simulations indicate the innermost planet is probably rocky, like Earth. The middle one is a combination of rock and gas and the outer planet, which is the one predicted to lie in the zone of habitability, is estimated to have a rocky-ice core and a massive envelope of gas. Researchers believe the system is stable.
