Update on 5 March: Lead researcher Jack Lissauer says: 鈥淔urther study of the light curve of this target produced an alternative interpretation wherein one of the co-orbital candidates (KOI 730.03) has a period that is twice what we originally estimated. We think that this new interpretation, without co-orbital candidates, is more likely to be correct. We will continue to acquire Kepler data and ground-based observations 鈥� so we can reach a better understanding of this interesting, multi-resonant, system.鈥�
Buried in the flood of data from the Kepler telescope is a planetary system unlike any seen before. Two of its apparent planets share the same orbit around their star. If the discovery is confirmed, it would bolster a theory that Earth once shared its orbit with a Mars-sized body that later crashed into it, resulting in the moon鈥檚 formation.
The two planets are part of a four-planet system dubbed KOI-730. They circle their sun-like parent star every 9.8 days at exactly the same orbital distance.
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Gravitational 鈥渟weet spots鈥� make this possible. When one body (such as a planet) orbits a much more massive body (a star), there are two Lagrange points along the planet鈥檚 orbit where a third body can orbit stably. These lie 60 degrees ahead of and 60 degrees behind the smaller object. For example, groups of asteroids called Trojans lie at these points along Jupiter鈥檚 orbit.
In theory, matter in a disc of material around a newborn star could coalesce into so-called 鈥渃o-orbiting鈥� planets, but no one had spotted evidence of this before. 鈥淪ystems like this are not common, as this is the only one we have seen,鈥� says of NASA鈥檚 Ames Research Center in Mountain View, California. Lissauer and colleagues describe the KOI-730 system in a paper submitted to the Astrophysical Journal ().
and at Princeton University say we may even have evidence of the phenomenon in our own cosmic backyard. The moon is thought to have formed about 50 million years after the birth of the solar system, from the debris of a collision between a Mars-sized body and Earth. Simulations suggest the impactor, dubbed Theia, must have come in at a low speed. According to Gott and Belbruno, this could only have happened if Theia had originated in a leading or trailing Lagrange point along Earth鈥檚 orbit. The new finds 鈥渟how the kind of thing we imagined can happen鈥�, Gott says.
Will KOI-730鈥檚 co-orbiting planets collide to form a moon someday? 鈥淭hat would be spectacular,鈥� says Gott. 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.
Read more: 鈥�Planetary zoo arises from flood of alien data鈥�
