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Exoplanets orbiting around stars.

A chart shows five of the six exoplanets discovered with backward orbits. The last object at lower right, WASP 5b, has a "normal" orbital path.

Image courtesy A. C. Cameron, ESO

An exoplanet orbiting around a star.

An artist's conception of WASP 8b, a backward orbiting exoplanet. Image courtesy L. Calçada, ESO

Brian Handwerk

for National Geographic News

Published April 14, 2010

In a batch of 27 planets found outside our solar system, half a dozen have "wrong way" orbits, astronomers have announced.

Each of the six extrasolar planets, or exoplanets, orbits in the opposite direction as its star's rotation. Such backward motion is unlike anything seen in our solar system.

(Related: "'Backward' Planet Has Density of Foam Coffee Cups.")

What's more, the discovery of these odd exoplanets challenges a leading theory for the formation of planets known as hot Jupiters, which in turn means that fewer Earthlike planets may exist in the universe. (See "The Search for New Earths" in National Geographic magazine.)

"This is a real bomb we are dropping into the field of exoplanets," study co-author Amaury Triaud, a Ph.D. student at the Observatory of Geneva in Switzerland, said in a press statement.

Odd Planets Orbit Backward and Tilted

First discovered 15 years ago, hot Jupiters are exoplanets with masses greater than Jupiter's that orbit very close to their parent stars.

In general, it's believed that all planets form out of disks of dust and gas that swirl around some young stars. (Read "Newborn Planet Found Orbiting Young Star.")

These disks should rotate in the same directions as the stars inside, the theory goes. Planets that coalesce out of the disks should therefore share the same orbital direction and should move within a single, tabletop-like plane around their stars.

For hot Jupiters, the leading theory has been that the massive planets take shape in the cold, outer reaches of their star systems. Gravitational interactions with material in the disks guide the massive planets to close-in orbits within just a few million years.

"Until recently, that was kind of the canonical picture on how these hot Jupiters migrated to their current locations," Avi Mandell, an exoplanet expert at NASA's Goddard Space Flight Center in Maryland, told National Geographic News.

But recently astronomers with the Wide Angle Search for Planets (WASP) program found nine new hot Jupiters that transit—or pass in front of—their stars, as seen from Earth. (Related: "Five New Planets Found; Hotter Than Molten Lava.")

Combined with older observations of transiting hot Jupiters, two of the new finds become part of a group of six "wrong way" exoplanets.

In addition, those six planets are part of a larger group of hot Jupiters tracked in the study with orbits that are no longer in the same "tabletop" plane as the rotation axes of their parent stars.

"It's quite an extraordinary result," Mandell said. "[It] means that most of these exoplanets could have been formed in a way that's different from what had been [most commonly] theorized before."

Hot Jupiter Planets Form via Tug-of-War?

The researchers don't know exactly how the oddball planets formed, said study co-author Andrew Collier Cameron of the University of St. Andrews in Scotland. But there is another theory for how hot Jupiters form.

Hot Jupiters might close in on their stars much more slowly than predicted, over a period of hundreds of millions of years, due to a gravitational tug-of-war with a distant star or companion planet.

This gravity battle tilts a planet into a different orbital plane, the theory goes, and elongates the planet's orbital path. On this eccentric orbit, the planet periodically passes much closer than normal to its host star. (Related: "Eccentric Exoplanet Gets Hot Flashes.")

The gravitational push and pull between the planet and its star bleeds off some energy during the close pass, so that the planet's orbit decays, and eventually the planet settles into a tight, tilted orbit.

"Rampaging Jupiter" Would Blow Away Earthlike Planets

Cameron, who presented the findings this week at the British Royal Astronomical Society's 2010 National Astronomy Meeting in Glasgow, said the theory could also solve at least one other vexing problem with hot Jupiters.

"The [previous theory] works well for getting the planets into their orbits, but it's absolutely lousy for stopping them once they get there," he said. "Nobody has really come up with a satisfactory explanation for why they don't smash into the star at the end."

In the new theory, a planet's rate of orbital decay has decreased by the time the planet gets really close to its star, allowing the planet to ease into a new orbit rather than spiral toward a collision.

However, the new theory would spell doom for any Earthlike planets that might try to take shape alongside a hot Jupiter.

If hot Jupiters move into their tight orbits in a few million years, as previously suggested, rocky Earthlike worlds would have plenty of time afterward to form in more distant orbits.

But if the hot Jupiter is on a slower migration, "you have a rampaging Jupiter on a cometlike orbit that's whizzing close to the star and then out again. ... ," Cameron explained.

"It's going to be crossing the feeding zone where terrestrial planets would form, and its gravity means that the planetary debris [needed] to form terrestrial planets is going to be completely blown away."

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