BILLIONS of years ago, something kicked planet XO-3b into a cock-eyed orbit. The culprit may have been another planet, which would mean planets can bounce each other around like cosmic billiard balls.
Guillame H茅brard of the Institute of Astrophysics in Paris and colleagues detected an unusual colour shift as XO-3b passed in front of its star. The pattern suggests that its 3.2-day orbit is tilted by 70 degrees (see diagram). 鈥淚f confirmed, this might be the first planet of this type,鈥� says H茅brard.
Every other known planet follows an orbit that lines up with the equator of its star. Astronomers think they coalesced from rotating discs of material and were gradually dragged inwards to their orbits by friction with leftover gases. 鈥淸The team鈥檚 discovery] suggests a completely different kind of mechanism,鈥� says Fred Rasio of Northwestern University in Chicago. Like asteroids in our solar system, planets whose paths cross could knock each other out of the orbital plane, he explains.
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The second planet in this cosmic hit-and-run must have been huge, because XO-3b has the mass of 12 Jupiters. 鈥淣ormally, the little guy is one who gets kicked around,鈥� says Alan Boss of the Carnegie Institution of Washington. Yet such an event should cause a gravitational wobble in the star, which has not yet been detected.
