As with children, the development of stars is influenced by the company they keep.
Astronomers have observed a former red giant star that is orbited by two planets comparable in size to Earth, according to an article in the journal Nature. The star’s cooling outer envelope may have been stripped by the planets, leaving only an exposed core.
Red giants are among the last stages of life for sun-sized stars that have expanded to about 100 times their size. The sun will become a red giant about 5 billion years from now, engulfing Mercury and vaporizing the small planets like Earth. Today’s discovery shows that these red giants may be influenced by larger planets that are perhaps the size of Saturn or Jupiter.
“It really is a big discovery that planets can influence the destiny of a star,” said Valerie Van Grootel, one of the study’s authors and a research fellow at Universite de Liege in Belgium, in an e-mail.
The discovery was an accident, because the research team had been trying to study pulsating stars, not find new planets, said Elizabeth Green, an astronomer at the University of Arizona. The scientists used the National Aeronautics and Space Administration’s Kepler Space Telescope to observe flickers from a star that were too faint to be the celestial body itself pulsing.
The best explanation was the existence of two small planets, passing in front every 5.76 and 8.23 hours, Green said. That means those planets are closer to their star than Mercury is to the sun.
The tight orbit is the smoking gun for the planets being engulfed by the red giant. Red giants’ outer layers are so far from the inner core that they aren’t as influenced by gravity. As the planets passed through, they dispersed the envelope. That interaction probably also pulled away the gassy atmospheres surrounding the planets, leaving behind the scorched centers.
A similar phenomenon has been observed with two stars very close together. This is the first time astronomers have discovered planets contributing to a star’s evolution, the report said. Previously, it had been a mystery as to why a red giant might lose almost its entire envelope without a companion star, said Van Grootel.
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