NASA: First sublime planet foreshadows Mercury's fate

Now rocky planets can do this too (Image: NASA, European Space Agency, Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS))

A rocky exoplanet about the size of Mercury appears to be evaporating before our eyes.

If confirmed, this would be the first time a rocky planet has been found turning to gas, demonstrating just how wacky alien planets can be.

The provocative suggestion may also foreshadow the fate of Mercury.

"My first reaction was disbelief," says Dan Fabrycky of the University of California, Santa Cruz, who was not involved in the new analysis. 

After playing with the data himself, however, he has come around – though he is still cautious. "After turning it over in my mind a few days, I cannot come up with a more natural theoretical explanation," he says.

The evaporation was inferred from observations by NASA's Kepler space telescope. These show that a star called KIC 12557548, which is slightly smaller than the sun, is dimming every 15.685 hours precisely. 

That suggests an orbiting companion is transiting, or passing in front of the star. Unlike other transits seen by Kepler, though, the dimming in this system varies wildly from one pass to another.

The best explanation is a rocky planet about the size of Mercury that is subliming – turning directly to a gas - due to the intense radiation from its star, conclude a team led by Saul Rappaport of Massachusetts Institute of Technology.

Hot rocks

The planet's orbital period suggests it is just 1 per cent of Earth's distance from the sun, where it should attain a temperature of 2000 kelvin. 

"That's well above what you would need to vaporise pyroxene and olivine – common minerals that make up rocky planets," says team member Eugene Chiang of the University of California, Berkeley.

As a result, the subliming planet is leaking rock vapour and dust into space, the team say, forming a large cloud around the planet that blocks starlight when it passes in front of the star. 

This is similar to the way sunlight vaporises ice from comets, producing a dusty cloud called a coma. The planet may even have a comet-like tail, the team say.

The cloud fluctuates in size over time, explaining why the amount of dimming varies from one event to another, they say.

No rocky planets have been seen evaporating before, although gas giants have.