Old Stars Have Planets for Lunch

Imagine planet Earth and everything on it being swallowed and burned by the sun. Not a pretty prospect, but it's probably the eventual fate of all planets, including our own, that circle their stars in a tight orbit, astrophysicists say in the current Monthly Notices of the Royal Astronomical Society. Many old stars, they say, show symptoms of indigestion from having gobbled up planets: bloating, unusually large infrared luminosity, and excess lithium.

Stars power themselves through most of their lives by fusing hydrogen into helium. But eventually, the hydrogen runs out, leaving only helium in the stellar core. A thin shell of hydrogen continues to burn, heating the star's atmosphere and causing it to expand into a so-called red giant, whose radius can be 1000 times larger than the original star's. "Virtually anything within one astronomical unit [the distance from Earth to the sun] will get engulfed," says Massachusetts Institute of Technology astrophysicist Fred Rasio. But until astronomers began finding planets around other stars, no one calculated how swallowing nearby objects would affect a star, says theoretical astrophysicist Mario Livio of the Space Telescope Science Institute in Baltimore.

Now, Livio and co-worker Lionel Siess of the Grenoble Observatory in France have adapted a sophisticated stellar evolution model to compute the aftermath of a red giant devouring a nearby planet. They find that the engulfed planet adds energy and angular momentum to the star, causing it to expand even further, shine brightly in the infrared, and spin fast. The planetary material also contains lots of lithium that would be absorbed by the star. Because 4% to 8% of all red giants radiate unusually bright infrared light and are abundant in lithium, Livio and Siess suggest that these may have had a planet on their menu recently. "There is no clear signature" that this is what happened in all these stars, says Rasio, "but it is quite plausible."