But that bonanza has been a headache for theoreticians, he said, because many of the newly discovered star systems defy existing models of how planets form. (Related: "New Planet Found; Star's Fourth World Stumps Astronomers.")

Current theory holds that planets are made from disks of gas and dust left over after star birth.

In our solar system, it's long been thought that the large, gassy planets such as Jupiter and Saturn initially took shape in the far reaches and then migrated inward, as gravitational drag from leftover gas and dust eroded their orbits.

The migration process halted when most of the gas and