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A hot exoplanet is pictured orbiting close to its sun.

An artist's impression of an extrasolar planet slightly larger than Jupiter orbiting close to its star.

Image courtesy ESA, NASA and Frédéric Pont, Geneva University Observatory

Anne Minard

for National Geographic News

Published February 24, 2010

One of the hottest known planets outside our solar system is slowly being "eaten" by its parent star, astronomers report.

First described in 2008, the extrasolar planet—or exoplanet—WASP-12b is a Jupiter-like world that orbits its host star so tightly a year lasts just 26 hours.

This closeness means that a combination of heat from the star and from a gravitational tug-of-war called tidal heating brings the surface temperature to more than 4,700 degrees Fahrenheit (2,600 degrees Celsius).

New data show that WASP-12b's atmosphere is also being puffed up by the star's heat to the point that some of its gases are escaping.

But rather than being blown away by stellar winds, the lost atmosphere might be getting pulled toward the star to form a hot ring around the star, said study leader Shu-lin Li, an astronomer at Peking University in Beijing.

In fact, WASP-12b is losing mass so fast it will likely disappear before its aging star has a chance to swallow the planet whole. (Related: "New 'Impossible' Planet May Be on a Death Spiral.")

The sunlike host star, known as WASP-12, is now about two billion years old, which means it's nearing the end of its life, said Heather Knutson, an astronomer and exoplanet researcher at the University of California, Berkeley.

Normally, dying sunlike stars grow into red giants, and their outer atmospheres expand to engulf nearby planets, said Knutson, who was not involved in the study. WASP-12 is predicted to become a red giant within about a hundred million years.

But "this star will not have WASP-12b to 'eat,' because the planet would have been totally disrupted long before then, within ten million years,” said study co-author Douglas Lin of the University of California, Santa Cruz.

Hot Planet Feeding Hotter Ring

Since 1995 astronomers have discovered more than 90 hot Jupiters—extrasolar planets with masses greater than Jupiter's that closely orbit their stars. It's thought that these gas giants initially form farther out in their star systems and migrate inward for as yet inexplicable reasons.

Among the known hot Jupiters, WASP-12b stands out because it is extremely close to its star. In addition, despite the fact that it's had nearly two billion years to settle into a circular orbit, the planet's path is unusually elongated.

WASP-12b's eccentric orbit means that the star's gravitational pull on the planet changes over time, generating heat through friction.

That can have a dramatic effect on a planet, said study co-author Jonathan Fortney, also an astronomer at UC Santa Cruz. Jupiter's moon Io, for instance, gets so much heat from tides that rock deep inside the moon melts, creating volcanic activity. (Related: "'Super Earth' May Really Be New Planet Type: Super-Io.")

"For a hot Jupiter, the energy goes into inflating the planet," Fortney said. "It makes it a lot bigger."

In fact, WASP-12b has expanded to six times Jupiter's volume, even though the planet is only about 1.4 times Jupiter's mass. (Related: "'Backward' Planet Has Density of Foam Coffee Cups.")

Writing this week in the journal Nature, Li's team proposes that some of the gas is getting pushed out so far from the planet that it's getting caught in the star's gravitational pull and could be forming a ring around the star.

"We have not seen any evidence of this, but we're suggesting that observers should look for this disk," Fortney said.

The ring around the star should be extremely hot, Li's team calculates—up to 7,232 degrees Fahrenheit (4,000 degrees Celsius)—so it should emit detectable infrared radiation.

WASP-12b an Extreme Survivor

Overall, WASP-12b presents scientists with a rare chance to study a planet that can survive, for any length of time, so close to a star, said UC Berkeley's Knutson.

"They're in such extreme environments," she said. "You can imagine scenarios in which there are other planets we didn't see that got that close and didn't make it."

Even more unusual, WASP-12b's eccentric path means that, despite its tight orbit, the gas giant might have a smaller neighbor.

Most planets that orbit so close to their stars develop much more circular orbits, the study authors note. But other bodies orbiting nearby can exert graviational influences on a planet, keeping its orbit eccentric.

For WASP-12b to have an elongated path, the authors write, the star system "may also contain a detectable resonant super-Earth, as a hypothetical perturber that continually stirs up WASP-12b's eccentricity."

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