Toasty planets circle stellar heart

Roasted remains orbit former red giant

It’s rude to play with your food, even if you’re a star.

TOASTY PLANETS Two Earth-sized remains of planets circle a dying star, as seen in this illustration. The planets were downsized during the star’s red giant phase, when it expanded and engulfed the spheres, astronomers suggest in a new study. S. Charpinet

One of the stars in the Kepler space telescope’s crosshairs has apparently regurgitated the Earth-sized remains of two planets ingested when the star temporarily ballooned into a red giant. Now, the rocky survivors are whizzing around the star’s small, pulsing heart, each completing its orbit in less than 10 hours.

At least, that’s the interpretation offered by an international team thatreports the oddball planetary system in the Dec. 22 Nature. While some astronomers are skeptical of the explanation, if the story holds up it could explain how a rare type of star forms and foreshadow the fates of enormous planets engulfed by their hosts.

“The idea that a planet could survive while being immersed in a star is pretty spectacular,” says Eliza Kempton, an astronomer at the University of California, Santa Cruz. “It’s a really interesting discovery if it does pan out.”

The star is a rare beast, a hot B subdwarf, and the two roasted planets — KOI 55.01 and 55.02 — are the purported cores of partially digested planets that started off larger than Jupiter and are now just toasty crisps a bit smaller than Earth. They’re squished right next to the star, orbiting at less than 1 percent the distance between Earth and the sun.

Normally, a star like the sun evolves into a red giant, then collapses into a white dwarf. But sometimes, the giant sheds its stellar skin before contracting, leaving behind a bright, beating heart — the hot B subdwarf. Scientists don’t know what provokes some red giants to molt, but hypothesize that a companion — whether it’s a second star or a planet — might instigate the transformation. A culprit planet would need to be at least several times as large as Jupiter, large enough to leave behind the surviving remnants, says astrophysicist and study coauthor Stephane Charpinet of the University of Toulouse in France.  

To spy the tenacious travelers, scientists monitored enigmatic signals muddying the star’s normal pulsations. Like bells, stars vibrate and ring at certain tones. This one, called KIC 05807616, had several subtle tones that couldn’t be explained by the star alone, Charpinet says. He and the team ruled out other explanations such as star spots or a stellar companion before deciding that planets were the most likely culprits of the star’s abnormal jingles.

But some scientists are skeptical about the team’s interpretation, pointing to a number of assumptions the team used to calculate the size and properties of the proposed planets and the need for additional observations of other similar stars.

“I’m not convinced,” says astronomer John Johnson of Caltech. “This doesn’t have to be wrong, but I have my doubts.”

Planets have been discovered in inhospitable locations before. The first extrasolar planet to be discovered was found orbiting a pulsar, the dense, rapidly spinning remains of a star that has gone supernova.  Planet remains have also been found in the atmospheres of white dwarfs. And on December 19, scientists reported on the discovery of a Jupiter-sized planet orbiting a binary star system that included the same type of hot B subdwarf.

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