Astrophile: Evaporating planet shows how worlds vanish

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: Evaporating exoplanet
Distance: 1500 light years

1. Preheat star to 2000 kelvin.
2. Take one Mercury-sized rocky world and place in a 16-hour-long orbit.
3. Bake for 10 billion years or until obliterated.

It's a recipe that won't win any culinary awards, but when turned into a computer model, it could explain the strange sighting last year of a rapidly disappearing exoplanet around the small sunlike star KIC 12557548. The model hints that this world is now about the size of Earth's moon and may even be down to its naked iron core, which would make it the smallest known exoplanet. If true, the tiny remnant could help us better understand what's inside our home world.

Dubbed KIC 12557548b, the planet was discovered using NASA's Kepler space telescope. Kepler spots exoplanets by looking for the regular dimming of stars caused by orbiting planets that transit them ? pass in front of their star ? as seen from Earth. The more light is blocked, the larger the planet.

KIC 12557548b is small and so close to its star that its year lasts just 16 hours. But the dimming caused by the planet varies wildly, suggesting it is surrounded by a gigantic, ever-changing cloud of material.

"It basically looks like a comet that's very erratic, with a long trailing tail of dust," says Eugene Chiang of the University of California, Berkeley, who was part of a team that found the planet last year. The team thinks the rocky surface is being evaporated away by the intense heat of its star.

Chiang and colleague Daniel Perez-Becker have now come up with a general model of how rocky planets evaporate, based in part on Kepler data. Assuming KIC 12557548b started out about the same size as Mercury, they found it should take roughly 10 billion years to vaporise completely, and that the process happens at different rates over time.

Bubbling away

"If you start with a Mercury-sized object, it can sit there very happily for billions of years, very slowly bubbling away," says Chiang. Eventually though, enough of the planet is vaporised into space that its gravity is greatly reduced, making it even easier for material to escape and speeding up its destruction.

This runaway phase lasts for the last one per cent of the planet's life, and it's thought KIC 12557548b has just 70 million or so years left. "When you compare it to the age of the star, almost certainly billions of years, it's just a blink of an eye," says Chiang.

If the planet is already stripped to its core, it should be possible to look at the surrounding cloud and identify its components, adds Ignas Snellen of the University of Leiden in the Netherlands, who was not involved in the work. "This would be the first time we can see a core, which is exciting," he says.

Chiang and Perez-Becker's model also suggests that some ancient planets were vaporised before we could ever observe them, while other Mercury-sized worlds are just teetering on the edge of destruction. Spotting such doomed planets will prove difficult ? we only see KIC 12557548b because its dust cloud covers an area about the size of Jupiter.

Saul Rappaport of the Massachusetts Institute of Technology, who led the original discovery of the evaporating world, thinks Kepler is up to the task. The real trick is proving an object is an evaporating planet rather than a fainter star that orbits the target star, and which can look like a transiting world.

Rappaport's team have just completed new observations of KIC 12557548b with the Hubble Space Telescope and are analysing the data to try and rule out this scenario. But he thinks the new model makes the case for an evaporating planet much stronger: "It seems better and better."

Journal reference: arxiv.org/abs/1302.2147

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