Astronomers have discovered a pair of planets around a star 5,000 light-years away that resemble smaller versions of Jupiter and Saturn, hinting that solar systems like ours may be unexpectedly common. As in our own solar system, the closer of the two planets to their star is the larger one, 70 percent as massive as Jupiter; the more distant planet has 90 percent the mass of Saturn.

The star itself, dubbed OGLE-2006-BLG-109L, is dimmer than our sun and is only half its size. But the ratios between the two planets' masses and that of their star as well as their relative orbital distances are very similar to those of Jupiter and Saturn. "Basically what we found is a scaled-down analog of our solar system," says Scott Gaudi, an assistant professor of astronomy at The Ohio State University and lead author of the study published this week in Science.

Gaudi and his colleagues discovered the planets over a two-week period in early spring 2006, when their stellar parent crossed in front of a more distant star. Due to an effect called gravitational microlensing, the gravity of the nearer star magnified the light 500-fold from the more distant one. The motions of the planets caused periodic spikes in the brightness of the magnified light, which allowed the team to calculate the size of the planets and their distances from the star. The Optical Gravitational Lensing Experiment (OGLE) at the Las Campanas Observatory in Chile first detected the event.

Researchers know of other multiplanet systems, but in those systems the planets are huddled close to their stars. Microlensing events reveal planets in orbits that are more distant from their stars.

Previous to this discovery, microlensing had turned up four planets, two of them Jupiter-size. But this crossing was the first one that happened to have the right conditions to reveal the presence of smaller planets. "The first time we could find a Jupiter-–Saturn analogue, we did," Gaudis says. "And that provides us a hint ... that these kind of solar system analogues might be quite common."