By Ron Cowen
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, a flurry of new studies reveals. 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 by the disk should revolve in the same direction. But in an overcrowded planetary 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 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 the chance to resettle into nearly circular orbits with little or no tilt, says Frédéric Pont of the University of Exeter in England.