Planet Unwinds in 'Death March'
An artist's impression of the extrasolar planet HD 189733b, seen here with its parent star looming behind--astronomers said its sunset looks similar to a hazy red sunset on Earth. The planet is slightly larger than our own solar System's Jupiter, and its atmosphere is a scorching eight hundred degrees Celsius.
Credit: ESA/NASA/Frederic Pont, Geneva University Observatory

The gravity of an alien star is causing its planet to unwind, research now reveals.

The world in question is WASP-12b, the only known planet circling the yellow sun-like star WASP-12 some 867 light-years from Earth. This gas giant is so close to its host star that it takes just 26 hours to orbit it and complete a year.

Unsurprisingly, given its closeness to its star, WASP-12b is scorching hot. At some 4,000 degrees F (2,200 degrees C), it's nearly half as hot as the surface of our sun.

Strangely, WASP-12b, which is roughly 1.4 times Jupiter's mass, is unexpectedly large compared with most gas giants of similar mass and distance from their stars. Its diameter is some 3.6 times Jupiter's.

Now scientists find the heat from the star is not enough to explain the planet's inflated size. Instead, the gravity of the star appears responsible.

On Earth, the moon's gravity causes tides. On WASP-12b, the tidal forces due to its star are far greater, and the energy is dissipated in the planet in the form of heat.

"Whereas tidal force on the Earth leads to a few meter changes in the height of the ocean surface, that on WASP-12b is 10 million times larger," said researcher Shu-lin Li, an astrophysicist at Peking University in Beijing.

In fact, WASP-12b is getting inflated so much that its atmosphere is escaping, losing an amount roughly equal to a thousandth of our moon's mass every year.

"We are witnessing the ongoing disruption and death march of a planet," Li said.

The researchers predict this gas is flowing toward the host star, forming a tenuous disk that should emit heat detectable by the Hubble or Spitzer Space Telescopes. They also calculate that an as-yet unseen planet roughly 5 to 10 times Earth's mass — a "super-Earth" — might be influencing WASP-12b's orbit.

The scientists detailed their findings in the February 25 issue of the journal Nature.