Astronomers find hottest planet ever discovered

By Clara Moskowitz

updated 3:07 p.m. ET Oct. 16, 2008

In the hunt for extrasolar planets, a new find is shattering records left and right.

A planet called WASP-12b is the hottest planet ever discovered (about 4,000 degrees Fahrenheit, or 2,200 degrees Celsius), and orbits its star faster and closer in than any other known world.

This sizzling monster whips its way around its parent star about once a day (for comparison, the fastest-circling planet in the solar system, Mercury, orbits the sun once every 88 days).

To make such swift progress, the planet circles extremely close-in to its star — about 2 percent of the distance from the Earth to the sun, in fact, or 2 million miles (3.4 million kilometers).

"WASP-12b is incredibly interesting, because we're at a stage in the study of exoplanets where we're finding new examples all the time," said Don Pollacco of Queen's University in Northern Ireland, who is a project scientist for the SuperWASP (Super Wide Angle Search for Planets) project that discovered WASp-12b. "It was exciting because it was the shortest period and the hottest planet, but I suspect there are even shorter period planets, and hotter planets to come."

WASP-12b is a gaseous planet, about 1.5 times the mass of Jupiter, and almost twice the size.

The planet, which orbits a star 870 light years from Earth, is especially notable because it pushes the bounds of how close planets can ever come to their stars without being destroyed.

"There is a limit because as a planet gets closer to its star, the radiation field gets more and more intense, and at some point that whole planet will be evaporated by its star," Pollacco told SPACE.com. "Before, some people thought it was impossible to find planets that had 1-day periods. I think it's so early in the whole subject, and it takes a number of objects before you can start setting limits."

The planet is also so hot that its temperature matches that of some stars. This planet, however, is definitely not a star because its mass isn't nearly large enough for the internal thermonuclear reactions that define stars.

WASP-12b is one of only about 50 extrasolar planets that have been detected through the transit method, meaning they were found by measuring the dip in brightness of their parent star as they pass in front of it and block part of its light.

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"It's an incredibly hard way to detect planets, because the size of this dip when it moves across the star is very small," Pollacco said. "These objects are difficulty to find, but they're incredibly valuable when you do find them because they tell you so much."

The transit method allows astronomers to not only note the presence of a planet, but estimate its size, mass and density. And by estimating its distance from its star, researchers can deduce its rough temperature, because the closer in an object is, the hotter it gets.

All the information scientists have so far about WASP-12b indicates that this fiery ball cozily circling its star is an odd case. Yet discoveries like this raise the question, are planets like this in fact more common in the universe than planets like Earth?

"Is our solar system the freak, or are these other solar systems the freaks?" Pollacco said. "Who knows? I suspect that for life to evolve as we know it, you have to have a special set of circumstances come together to produce very specific conditions."

The SuperWASP project, based in the UK, uses telescopes in Spain's Canary Islands and in South Africa to scan the sky searching for distant planets that cross in front of their stars.

The discovery of WASP-12b was first announced in April 2008, though its distinction as the hottest and fastest-orbiting exoplanet was confirmed Oct. 11 at the annual meeting of the American Astronomical Society's Division for Planetary Sciences by co-discoverer Leslie Hebb of the University of St. Andrews in Scotland.

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