Planetary siblings.
Three Neptune-sized worlds and a dense belt of asteroids orbit a small nearby star in this artist's view.

Planet Hunters Triple Their Pleasure

A new system of three extrasolar planets is delighting astronomers by virtue of something that it lacks: a Jupiter-size bully to throw its gravitational weight around. The worlds, roughly the mass of Neptune and perhaps made mostly of rock and ice, revolve in orderly, undisturbed paths around a small star just 41 light-years away. The star also appears to host a band of asteroids, giving the system a tantalizing hint of home. What's most exciting about the find, astronomers say, is that "super-Earths" with solid surfaces are now likely to be plentiful in the universe.

The discoveries raise the census of known exoplanets above 190, nearly all of them found by detecting the slight wobbles of stars like our sun (ScienceNOW, 25 August 2004). The wobbles arise from the back-and-forth gravitational tugs of planets as they orbit. A team led by astronomers Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland monitored a southern hemisphere star called HD 69830 and saw signs of two planets, with estimated masses 10 to 15 times that of Earth. The planets are both baked by the star's heat: They take just 9 days and 31 days to orbit, placing them closer to the star than Mercury is to the sun.

Further analysis pointed to barely discernable tuggings from a third world, at least 18 times as massive as Earth and orbiting in 197 days. Models suggest the two inner worlds are solid, while the third planet probably has a thick mantle of gas blanketing a large core of rock and ice. Although the orbit of this outer world is smaller than Venus's orbit around our sun, it travels in a mild realm where liquid water might exist on the surfaces of rocky satellites, says lead author Christophe Lovis, a graduate student at Geneva Observatory. The astronomers describe the system in the 18 May issue of Nature.

When Lovis's colleagues simulated the long-term motions of the planets, they found two regions where asteroids can drift calmly around the star without getting ejected by the planets' gravitational jockeying. Indeed, the system appears to host an asteroid belt far more massive than the one in our solar system, according to a detection last year of warm dust there by the infrared Spitzer Space Telescope.

The family of exoplanets--kept stable by the absence of a disruptive giant world--is "gorgeous and unique," says planet-hunter Geoffrey Marcy of the University of California, Berkeley. "Their existence, especially in a system, suggests that Earth-like planets are common." Among the 19 known systems of planets circling sunlike stars, it's the first consisting solely of modestly sized worlds, he notes, moving scientists a step closer to finding life-friendly abodes among the stars.

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