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 the 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.

Grazing transit

The team considered another possibility: that the dimmings were caused by transits of a giant planet with a variable orbit. In that case, some transits might put the entire planet in front of the star, whereas others would be grazing transits, with only part of the planet blocking starlight. That could explain the variability.

However, this explanation requires a second planet to gravitationally perturb the first planet’s path – and such tugs would also change the timing of the transits. So far the time between transits has stayed constant to within one part in 100,000 – far too regular to accommodate a second-planet explanation.

Alternatively, if there was a binary star very nearby in the sky, its light could contaminate measurements of the star’s light, mimicking transit events – but the team could find no sign of such a binary near the star of interest. That leaves the evaporating-planet idea as the only viable explanation the team could come up with.

“The idea is very provocative,” says Jonathan Fortney of the University of California, Santa Cruz, who was not involved in the study. But he says further work is needed before all other possibilities can be ruled out.

Mercury boils dry

The planet is probably about the size of Mercury, the team say. If it were larger, its gravity would prevent the gas and dust from leaking into space to form the cloud, whereas a smaller planet would evaporate so quickly that we would be implausibly lucky to spot it during this phase.

Normally Kepler would not be able to detect a planet as small as Mercury, because it dims its star too little. If the team are right, the telescope only picked up this one because of the dust cloud, which blocks much more light than the planet alone would.

Assuming the planet is Mercury-sized, it should evaporate away to nothing in about 200 million years. This may offer a preview of things to come in our own solar system billions of years from now, when the sun is predicted to bloat into a red giant.

Mercury will eventually be engulfed by the expanding sun, but for a brief period before being swallowed, its surface may boil and produce a dusty cloud around the planet like the one Kepler has observed. “There will come a point where our Mercury will go through this phase, but it will be pretty short-lived,” says Chiang.

The team hope to obtain time on the Hubble Space Telescope to measure the spectrum of starlight passing through the cloud. If it reveals common rock-forming elements like silicon and magnesium, it would bolster the evaporating-planet explanation.

Reference: arxiv.org/abs/1201.2662