Hidden in the mass of data from the Kepler telescope is a planetary system that has two of its apparent planets share the same orbit around their star. If the discovery is confirmed, it would support the theory that Earth once shared its orbit with a Mars-sized body, labeled Theia, that later collided with it, creating the moon.

The two planets are part of a four-planet system dubbed KOI-730 that circle their sun-like parent star every 9.8 days at exactly the same orbital distance, one permanently about 60 degrees ahead of the other. In their night sky, each would appear as a constant, blazing light.

When one body orbits a star or large object, there are two Lagrange points along the planet's orbit where a third body can orbit stably. These lie 60 degrees ahead of and 60 degrees behind the smaller object. A groups of asteroids called Trojans, for example, lie at these gravitaional sweet spots along Jupiter's orbit.

"Systems like this are not common, as this is the only one we have seen," says Jack Lissauer of NASA's Ames Research Center in Mountain View, California. Lissauer and colleagues describe the KOI-730 system in a paper submitted to the Astrophysical Journal (arxiv.org/abs/1102.0543).

Will KOI-730's co-orbiting planets collide to form a moon someday? "That would be spectacular," says Richard Gott of Princeton. That may be so, but simulations by Bob Vanderbei at Princeton suggest the planets will continue to orbit in lockstep with each other for the next 2.22 million years at least.

What do you think?