Flurry of planets found at full tilt: violent interactions may have shaped extrasolar systems.

Call them the wrong-way planets. Several giant, extrasolar planets, all residing within sizzling distance of their parent stars, have orbits so tilted that the planets travel backward relative to their parent stars' rotation, new studies reveal. The misalignments attest to rough-and-tumble histories and may suggest that life flourished on Earth because the solar system avoided the brunt of close gravitational encounters between planets.

According to the most popular formation theory, planets coalesce from a swirling disk of gas and dust that surrounds young stars. Since the disk rotates in the same direction as the star, the planets spawned should revolve in that direction as well. But in an overcrowded system, where a gravitational game of billiards is all but inevitable, orbits can get scrambled. A close encounter between planetary siblings can push one body outward while flinging the other inward, elongating and tilting the inner planet's orbit.

In this scenario, the Earth's solar system may have been unusually lucky. Either it avoided catastrophic gravitational encounters between massive planets or it suffered such interactions so long ago that most of the planets had time to resettle into nearly circular orbits with little or no tilt, says Frederic Pont of the University of Exeter in England.

"The presence of advanced life on Earth may be contingent on our planetary system having avoided the brunt of planet-planet scatter," keeping Earth on a circular, Goldilocks-style orbit--neither too hot nor too cold for known forms of life, he speculates.

In one of the new studies, posted online August 24 at arXiv.org, Pont and colleagues examined the orbit of the planet COROT-lb. This close-in planet, like the others in the new studies, periodically passes across the face of its star as seen from Earth, blocking a fraction of the light and allowing a telescope to measure orbital tilt. By observing the spectra from the star, Pont's team found that the orbital axis of COROT-lb was tilted at an angle of about 77 degrees with respect to the star's spin axis.

Two independent studies, also posted at arXiv.org, find exoplanets with tilts at roughly 150 and 180 degrees. And another team, reporting at the Dynamics of Disks and Planets meeting in Cambridge, England, has detected two more close-in exoplanets with substantially tilted orbits.

"This has been the most exciting observational result of the summer and certainly the meeting," says Eric Ford of the University of Florida in Gainesville.

With the new findings, between 25 and 50 percent of all exoplanets whose angles of inclination have been measured have tilts exceeding 30 degrees. Earth has the greatest orbital tilt of planets in the solar system, with an angle of 7.1 degrees.

The newly found tilts, Pont says, represent "a spectacular upheaval of the standard view of close-in planet formation ... and probably indicate instead catastrophic encounters between several planets."

Ford speculates that the solar system is special not because it avoided planetary pinball, but because the encounters happened while the disk of rocky debris still survived. The disk's gravity would have damped elongated and inclined orbits, returning the solar system's planets "to the nearly circular and coplanar orbits that we enjoy today," Ford suggests.