Some highly unusual planets orbiting other stars are calling into question theories about how solar systems are formed and suggesting that more complex theories must be developed.
European researchers reported Tuesday that some of the recently observed extrasolar planets are revolving around their stars in the opposite direction from the stars' spin.
That finding is inconsistent with the view that planets are formed by the condensation of dust from a disk surrounding a newly formed star.
Some other planets were found to have highly tilted orbits that are also at odds with conventional theory.
The findings also suggest that planets with such eccentric orbits would destroy any smaller, rocky planets, eliminating the chance that an Earth-like planet could be orbiting around that same star.
"This is a real bomb we are dropping into the field of exoplanets," astronomer Amaury Triaud of the Geneva Observatory, one of the authors of the report, said in a statement.
The new results imply that at least some of these massive planets, known as hot Jupiters because they orbit close to their suns, must reach their final orbits not through the familiar process of accumulation of dust, "but rather through the much more dramatic and exciting process of gravitational billiards," astronomer Alan Boss of the Carnegie Institution for Science in Washington said in an e-mail.
In such a billiards scenario, close encounters between planets orbiting the star would result in some planets getting sling-shotted into highly eccentric orbits, in some cases even backward (or retrograde) ones.
"The real excitement here is what it says about planetary migration," Boss said.
Added astronomer William J. Borucki of NASA's Ames Research Laboratory in Mountain View, "What's wonderful is that we can measure the direction of the orbit and compare it to the spin of the star. It's something brand new, that we are able to do this."
The results were reported at a Glasgow meeting of the Royal Astronomical Society by astronomer Andrew Cameron of the University of St. Andrews in Scotland.
His team identified nine new extrasolar planets using the Wide Angle Search for Planets project at the European Southern Observatory in Chile, which monitors millions of stars looking for sudden diminution of light intensity indicating that a planet has passed in front of them. That brings the total extrasolar planets observed so far to 452.
The team then used a combination of other telescopes to determine the orbital tilt and direction of these and 18 other recently discovered planets, all of them hot Jupiters.
They found that more than half of the planets had orbits that were misaligned with the rotation axis of their parent stars. Six of them had retrograde, or backward, orbits.
Conventional theory is that a newly formed star and its dust disk rotate in the same direction. As planets are condensed from the dust, they should continue to orbit in that direction. That these are not suggests that other forces are coming into play.
One possibility, Cameron suggested, is that the massive planets are being tugged at by more distant massive planets or companion stars that pull them into tilted or elongated orbits. Over millions of years, such tidal friction would pull them into a near circular but randomly tilted orbit, even one that is retrograde.
A dramatic side effect of this process is that it would wipe out any smaller, Earth-like planets in the system, either absorbing them into their own mass or sling-shotting them into the far reaches of the solar system.
Two of the retrograde planets have already been found to have more distant massive companions, supporting this possibility, Cameron said.
"We don't see this in our solar system, suggesting that there were different physical forces at work here," Borucki said. "Our planetary system is different. We may not be alone, but there are certainly a lot of other systems out there that are quite different from ours."