Star-hugging planet is hottest and fastest found

The hottest planet ever found is a sizzling 2250 °C – as hot as some stars. The find could challenge models of how close planets can sidle up to their host stars.

The new planet, known as WASP-12b, is 1.5 times as massive as Jupiter. Incredibly, it takes just over a day to circle its host star, orbiting at 1/40^th the distance between the Earth and the Sun. The tight embrace heats WASP-12b to an estimated 2250 °Celsius – about half as hot as the surface of the Sun, and as hot as some stars.

That makes the planet the hottest yet discovered, as well as the planet with the fastest orbit, says Leslie Hebb of the University of St Andrews in the UK.

Hebb and colleagues found the giant in a large survey called the Super Wide Angle Search for Planets (SuperWASP). The collaboration uses two sets of telescopes, one in Spain's Canary Islands and the other in South Africa, to search for signs of 'transiting' planets, which pass in front of and dim their host stars as seen from Earth.

Extrasolar planets are too dim compared to their host stars to directly measure the infrared light – or heat – they emit. But astronomers know the planets' size and orbital distances from the transit observations. From that, they can work out how much starlight falls on the planets and thus take their temperature.

The competition for the hottest planet is tight. WASP-12b only just beats out the last record-holder, HD 149026b, whose blacker-than-charcoal surface is a searing 2040 °Celsius.

Puffy planet

But WASP-12b's speedy orbit might be a harder record to break. Astronomers believe Jupiter-sized exoplanets form farther from their stars and then migrate to closer orbits. That's because there could not have been enough gas and dust so close to the stars to amass such giant worlds.

Most observed exoplanets have orbital periods of three days or longer, Hebb says, suggesting that some mechanism may prevent the planets from migrating even closer to their stars.

"When the planets form and migrate inward, something is causing them to stop and preferentially stop with a period of three days," Hebb told New Scientist. "I was surprised that the period could be so much shorter."

WASP-12b's size may also be a challenge to explain. The planet's width is 1.8 times that of Jupiter, larger than gas giants are thought to grow.

"The planet radius is suspiciously large," notes Sara Seager of MIT. "While observation is leading theory, it's uncomfortable to have a planet with a radius that cannot be accommodated by theory."

So far the team does not have an explanation, but radiation from WASP-12b's host star could be puffing up the planet, Hebb says. The planet's composition, which might be rich in metals like its host star, may enhance the effect. Planets rich in heavy elements are expected to be less dense than their 'metal-poor' cousins.

The team next plans to look for ultraviolet light emanating from WASP-12b. Such observations could reveal whether the planet's atmosphere is being stripped or evaporated away by its host star.

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