Credit: Robert Barker, Cornell UniversityThis artists' rendering shows XO-1b, which is similar, yet much smaller than XO-3b.

June 26, 2009 - Mauna Kea, Hawaii
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An exoplanet with a steeply titled orbit around the plane of a distant star's equator has been discovered by an international team of astronomers, and it is being hailed as a finding that contradicts theories about how planetary systems form.

Observations conducted of planet XO-3b at the W. M. Keck Observatory on the Big Island of Hawaii's Mauna Kea have determined the angle of the orbit to be about 37 degrees from the star’s equator. The so called "misalignment" of the planet’s orbit contradicts simple theories of planet formation, according to astronomer Geoff Marcy of University of California Berkeley, who coauthored the discovery paper.

Marcy explains that in traditional theories of planet formation "a young star is surrounded by a flattened disk of gas and dust, like a fried egg with the yellow yolk, the star, in the middle and the white, the gas and dust, extending outward from the equator of the yolk." The planets form by collecting the dust and gas together within that disk. The theories serve to explain how the planets in our Solar System reside in a flat plane that slices through the equator of the Sun. Other planetary systems show a similar architecture.

"What is shocking about this planetary system is that the planet orbits in a plane that is grossly misaligned with its star’s equator," Marcy said.

XO-3b is about 13 times as massive as Jupiter, yet orbits its star in just 3.5 days. (Jupiter takes almost 12 years to make one orbit.) The planet most likely formed away from its current orbit, farther out from the star, then migrated inward to its present position.

A W.M. Keck Observatory media release says that planet formation theory suggests the gravitational attraction of other planets as well as debris in the disk might tug on planets, slightly disrupting their orbit. Close encounters between or among planets, however, has enough force to significantly change the planet’s trajectory.

Marcy says that in the case of XO-3b, it seems "some other planet gravitationally yanked on this poor planet, jerking it out of its original circular orbit."

Marcy added that it "suffered from a gravitational close encounter. It survived, but was left in a wacky orbit."

XO-3b was discovered during its transit directly in front of the star it orbits. Of the more than 350 exoplanets discovered so far, fewer than two dozen have been discovered through this transit method.

Astronomers John Asher Johnson of the University of Hawaii and Andrew Howard of the University of California Berkeley used the Keck I telescope’s High Resolution Echelle Spectrometer, or HIRES, to confirm the distinct spectral signature of XO-3b's orbit.