(See a Virtual Solar System.)
Astronomer David Charbonneau of Harvard University in Cambridge, Massachusetts, points out that HD 69830 is the only known star that is comparable in mass and age to the sun that also has an asteroid belt.
"It's a really neat system," said Charbonneau, who wrote a commentary accompanying the Nature study.
"It's the first one to have both planets and asteroid belt. There are a lot of similarities to our own solar system and a lot of differences, too."
The European researchers first discovered the three planets in October 2003.
Between then and January of this year, the scientists made 74 more observations.
They concluded that the planets are made mostly of rock fragments that had collided and fused to form solid cores. According to this model of planet formation, gases would have then collected around each core to form an atmosphere.
Planet Has Water?
The innermost planet appears to orbit HD 69830 once every nine days and is about ten times as massive as Earth.
The second planet orbits about once a month and has a mass around 12 times that of Earth.
The outermost planet is about 18 times the mass of Earth and orbits HD 69830 once every 197 days. The researchers say this planet is near the inner edge of the star's habitable zone.
Lead study author Lovis said the outermost planet probably is enveloped by hydrogen, which makes it look more like Uranus and Neptune than Earth.
"Nonetheless," he said, "this discovery opens the way to the detection of even smaller planets in the near future."
Mayor called it the discovery "a small step in the direction of the research for possible life in other parts of the universe."
"As we believe that rocky planets are the most suitable place for the development of life, the detection of this exciting system has also to be considered in that frame," he said.
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