Extrasolar planets at full tilt

The sun’s planetary system may have escaped or rebounded from violent interactions that studies show played a key role in shaping other 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, 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.

OFF KILTER A gravitational encounter between two neighboring extrasolar planets, shown in this artist’s illustration, may hurl one of the two planets towards their parent star, tilting the planetary orbit in the process. Such violent gravitational scattering may account for recent findings that several extrasolar planets that tightly orbit their star have highly tilted orbits. KASI/CBNU/ARCSEC

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.

“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 life as we know it, he speculates.

In one of the new studies, posted online August 24 at arXiv.org, Pont and his colleagues examined the orbit of the planet COROT-Exo-1b, the first extrasolar planet discovered by the European satellite COROT. 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 tiny fraction of the starlight. Because of this passage, a telescope can measure the tilt of its orbit. By observing the spectra from the star, Pont’s team found that the orbital axis of COROT-Exo-1b was tilted at an angle of about 77 degrees with respect to the star’s spin axis.

On August 12, a team led by David Anderson of Keele University in England reported online at arXiv.org that another close-in planet, called WASP-17b, has an even more inclined orbit, with a tilt of roughly 150 degrees. The findings suggest the planet is almost certainly traveling backward relative to the star’s rotation, says study coauthor Andrew Collier Cameron of St. Andrews University in Scotland.

Just a day later, on August 13, Joshua Winn of MIT and his colleagues reported online at arXiv.org that yet another close-in planet, HAT-P-7b, is either in a polar orbit or moving backward around its parent star, with an orbit tilted at about 180 degrees. Winn and his collaborators also detected signs of another, more distant object, either a massive planet or a companion star, whose gravity may have hurled HAT-P-7b into its strange, backward orbit. The team will report the findings in an upcoming Astrophysical Journal Letters.

In a study of the extrasolar planet HD 80606b, which has the most highly elongated orbit of any extrasolar planet known, a team led by Winn found that the planet orbits at a tilt of somewhere between 14 and 142 degrees. The team’s report, posted online at arXiv.org on July 30, will appear in an upcoming Astrophysical Journal.

The party line had been that any close-in planet would naturally rotate in the same direction as its parent star, Winn says. And the first few observations of these planets’ orbits had borne that out. Those early findings meshed with the widely accepted theory of how planets adopt close-in orbits about their parent stars. According to the simplest version of that theory, planets are born in regions of the disk much farther from their star, but as they give up rotational energy to the disk, the bodies slowly spiral inward, typically preserving their initial direction of motion and orbit. 

But then Winn and others started finding a plethora of planets that the researchers say don’t fit the standard migration theory. “This has been the summer of tilted planets,” he says. 

The trend continues. Last week, at the Dynamics of Disks and Planets meeting in Cambridge, England, Amaury Triaud of the Geneva Observatory in Sauverny, Switzerland, and his colleagues reported the detection of two additional close-in extrasolar planets with substantially tilted orbits.

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

Including the new findings, between 25 and 50 percent of all extrasolar planets whose angles of inclination have been measured have tilts exceeding 30 degrees. Of the planets in the solar system, Earth has the greatest orbital tilt relative to the sun’s axis of rotation, at an angle of 7.1 degrees.

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

All this is old hat to Ford and Fred Rasio of Northwestern University in Evanston, Ill., as well as other theorists who have been advocating for years that gravitational encounters between planets play a critical role. Although it will take time for all the new discoveries to be published and compared with models, says Ford, “my impression is that several people at this very meeting were surprised by the findings and began to realize that planet scattering is likely very important for determining the final architecture of planetary systems.”

Even for the sedate, orderly solar system, planet-planet interactions have been important, Ford says. He cites the violent events responsible for the formation of Mercury and for the formation of Earth’s moon, believed to be created when a Mars-sized body crashed into the young Earth.

He speculates that what makes the solar system special is not that it entirely avoided planetary pinball, but that it happened relatively early. Early enough, in fact, that the short-lived, massive disk of rocky debris survived after the last strong planet-scattering event. The disk’s gravity would have damped back down elongated and inclined orbits, returning the solar system’s planets “to the nearly circular and coplanar orbits that we enjoy today,” Ford suggests.

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