MOUNTAIN VIEW, Calif., Feb. 24 (UPI) -- U.S. astronomers say they've discovered a distant planetary system unlike anything seen before, with two of its planets apparently sharing the exact same orbit.
The discovery, if confirmed, could add credence to a theory that Earth once shared its orbit with another cosmic body the size of Mars that later crashed into it, creating our moon, NewScientist.com reported Thursday.
The new system, discovered by the Kepler space telescope, has four planets, two of which circle their parent star at exactly the same orbital distance, one permanently orbiting about 60 degrees ahead of the other.
The phenomenon is made possible by gravitational "sweet spots," astronomers say.
When one body such as a planet orbits a much more massive body like a star, there are two so-called Lagrange points along the planet's orbit where a third body can orbit stably, either 60 degrees ahead or 60 degrees behind in the orbit.
Although "co-orbiting" planets are possible in theory, no one had spotted evidence of this before.
"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, Calif.
Some astronomers say it could have occurred in our own solar system, leading to the formation of our moon 50 million years ago in a collision of the Earth and a Mars-sized body.
Simulations suggest the impactor must have come toward Earth at a low speed, only possible if it had originated in a leading or trailing Lagrange point along Earth's orbit.
The newly found "co-orbiting" planets "show the kind of thing we imagined can happen," Princeton university astronomer Richard Gott says.
