What will happen after sun vaporizes Earth? Scorched planets hold clues.
Scientists say they've found two planets that survived being swallowed by a red-giant star. Earth won't be so fortunate when our sun becomes a red giant in 5 billion years, but the find shows what can happen to solar systems after such dramatic events.
Forget this season's final episode of "Survivor." The ultimate survivors appear to be two small planet-candidates engulfed for a billion years inside the searing envelope of a red-giant star. And they emerged to tell the tale.Skip to next paragraph
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The planets are a glimpse at what can happen to a solar system when a star begins its death throes, becoming bloated and red as it consumes the last of the hydrogen fuel in its core. The same fate awaits our sun in about 5 billion years.
The two planet-candidates announced Wednesday are among the tiniest yet revealed by data from NASA's planet-hunting Kepler spacecraft. And they hold the potential to shed light not only on how planets could survive such a torching, but also how they might affect the evolution of red-giant stars themselves.
"On many levels, it's very cool," says Elizabeth Green, a researcher with the University of Arizona's Steward Observatory and a member of the team reporting its observations in the Dec. 22 issue of the journal Nature.
A red giant originates as a star roughly like our sun – between 0.5 and 8 times the sun’s mass. As the star exhausts its hydrogen fuel, its core collapses. The heat of that event causes remaining hydrogen in the outer shell to begin fusion, and the star’s outer layer, or photosphere, expands.
By the time the red-giant phase of our sun ends, the Earth, Venus, and Mercury are likely to be vaporized. But scientists have examples of other objects – planets and brown-dwarf stars – that survived being enveloped by red-giant stars they orbited.
None of them, however, is like the ones reported Tuesday. All the previous examples were bigger objects that orbited farther from their parent stars to begin with. For that reason, they didn't spiral as deeply into their stars’ photospheres. When these stars’ red-giant phase ended – and the stars shrank back to become helium-burning so-called subdwarf B stars – the planets survived.
By contrast, the objects reported Tuesday appear to have traveled far deeper into the red-giant's photosphere and survived only as tiny remnants.
Indeed, the planet-candidates orbit so close to their subdwarf B star, named KIC 05807616, that their years are 5.8 hours and 8.2 hours long, respectively. With one side constantly facing the star, the planets’ sun-side faces would roast at between 14,000 and 16,000 degrees Fahrenheit.
So how did the planet-candidates survive such a blistering? The team suggests that the objects may represent the rocky cores of stripped-down gas-giant planets that once orbited farther away.