Distant planets show Earth's fiery future

Posted by Emma Woollacott

NASA says its discovered two Earth-sized planets that have been swallowed up by their star - and survived.

Following yesterday's announcement that two Earth-sized planets have been discovered orbiting a white dwarf similar to our own sun, the Kepler mission has now found two others that give a glimpse of the fate that may await Earth.

The planets are orbiting a dying star that has passed the red giant stage; and, says NASA, must have been engulfed by their star when it swelled up to many times its original size.

It's about five million years until we can expect the same fate on Earth. But, eventually, our sun will swell up to become a red giant, an inflated star that has used up most of its fuel. It will grow large enough to swallow Mercury, Venus, Earth and Mars.

You might think - and most scientists did - that no planet could survive the process.

But, says NASA, KOI 55.01 and KOI 55.02 not only made it through the inferno but may also have helped to strip the star of most of its fiery envelope in the process.

"When our sun swells up to become a red giant, it will engulf the Earth," says Elizabeth Green, an associate astronomer at the University of Arizona's Steward Observatory.

"If a tiny planet like the Earth spends one billion years in an environment like that, it will just evaporate. Only planets with masses very much larger than the Earth, like Jupiter or Saturn, could possibly survive."

The two planets, named KOI 55.01 and KOI 55.02, circle their host star in extremely tight orbits. Having migrated so close, they probably plunged deep into the star's envelope during the red giant phase, but survived.

In the most plausible configuration, says NASA, the two bodies would respectively have radii of 0.76 and 0.87 times Earth radius, making them the smallest planets so far detected around an active star other than our sun.

The host star, KOI 55, is what astronomers call a subdwarf B star: It consists of the exposed core of a red giant that has lost nearly its entire envelope - possibly with the help of the two planets.

"As the star puffs up and engulfs the planet, the planet has to plow through the star's hot atmosphere and that causes friction, sending it spiraling toward the star," says Green.

"As it's doing that, it helps strip atmosphere off the star. At the same time, the friction with the star's envelope also strips the gaseous and liquid layers off the planet, leaving behind only some part of the solid core, scorched but still there."