The findings were reported at the American Astronomical Society meeting in Washington, D.C., by Gregory Laughlin, an assistant professor of astronomy and astrophysics at the University of California, Santa Cruz; John Chambers of NASA Ames Research Center; and Debra Fischer of the University of California, Berkeley.

Planet hunters have detected nearly 80 planets orbiting nearby stars, but most of them have elongated, or "eccentric," orbits. The two planets around 47 UMa, which is located in the Big Dipper constellation, have nearly circular orbits, like those of Earth and other planets in our solar system. Their orbits are farther from the star than Mars is from the Sun, but closer than Jupiter.

The striking similarity between the pair of planets orbiting 47 UMa and the Jupiter-Saturn pair in our solar system led Laughlin, Chambers, and Fischer to investigate whether smaller, Earth-sized planets could have formed and survived in the habitable zone around 47 UMa.

The habitable zone is the region surrounding a star where liquid water could exist on the surface of a planet -- a region roughly equivalent to the space between the orbits of Venus and Mars in the solar system. Earth-like (or "terrestrial") planets are too small to be detected with present-day planet hunting techniques.

The researchers performed a large set of computer simulations and found that Earth-sized planets have a very hard time forming in 47 UMa's habitable zone. The combined gravitational forces of the two large outer planets conspire to prevent Earth-sized planets from building up in orbits where temperatures are clement, Laughlin said.

"Our simulations suggest that terrestrial planets can readily form around 47 UMa in orbits that are roughly half the size of Earth's orbit," he said. "Out in the habitable zone, an Earth-sized planet can survive in a stable orbit, but it is very hard to see how such a planet could be assembled."

During the formation of a planetary system, terrestrial planets such as Earth or Mars are believed to form from successive collisions of small asteroid-sized bodies which stick together to form progressively larger bodies called "planetary embryos."

This process is known as planetary accretion. It is likely that our solar system went through a such a phase, in which hundreds of moon-sized planetary embryos emerged from numerous collisions among a much larger number of small precursor bodies, Laughlin said.

The researchers used a highly efficient computer program developed by Chambers to simulate the further development of planetary accretion from this stage. A typical calculation followed the long-term evolution of a swarm of 280 moon-sized planetary embryos in the presence of the two giant planets orbiting 47 UMa, and spanned a time frame of 50 million years near the beginning of 47 UMa's history.

The simulations showed that embryos starting in the habitable zone tend to jostle each other into orbits where the gravitational tugs from the two outer planets either fling the embryos from the system or drive them into the star itself.

On the other hand, the embryos that started inside the habitable zone were always able to consolidate into several Earth-sized planets, all having an orbital period of half a year or less.

At the end of several simulations, a single tiny survivor was left in the habitable zone. In other simulations, the habitable zone was entirely cleared, Chambers said.

The survival of isolated remnant embryos in the habitable zone of 47 UMa suggests a possible parallel to the asteroid belt in our own solar system. Within the asteroid belt, many orbits are stable, but certain locations within the belt contain unstable "resonances" where objects experience rapid orbital instability. Planetary embryos tend to scatter each other into these unstable zones, leaving behind a smattering of survivors -- the asteroids -- which are much smaller than the terrestrial planets.

"Because these two giant planets orbiting 47 UMa are more than twice as close to the star as Jupiter and Saturn are to the Sun, the 47 UMa system looks like an overweight, scaled-down version of the solar system. Any terrestrial planets or an analogue to the asteroid belt around 47 UMa would likely be about twice as close to the star as well," Chambers said.

The researchers' assessment of habitable planet formation was part of a larger theoretical investigation of the two planets orbiting 47 UMa, in which the team narrowed the range of possible orbital configurations that the planets might occupy. They also showed that the two outer planets have undergone very little orbital modification since their formation. The research was funded by the NASA Origins of Solar Systems Program.

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