Snacking in space: Star dines on planet

Depiction of a planet in a death plunge. 2001 Lynette Cook


A star has been caught red-handed with the digested remains of one or more of its own planets.

Studying the light from the star HD 82943, known to have two planets, astronomers have found that its atmosphere contains a fragile lithium isotope. Thats surprising because stars make very little of the isotope, lithium-6, and any in HD 82943 at the stars birth would have burned up within 30 million years, when it was still young and extremely hot.

Somehow, the star acquired the lithium-6 well after its birth. The most likely explanation is that HD 82943 engulfed one or more planets, assert Garik Israelian of the Astrophysics Institute of the Canary Islands in Tenerife, Spain, and his colleagues in the May 10 Nature.

Planets arise from the disks of gas and dust that once surrounded their parent stars. Unlike stars, however, planets never get hot enough to convert their initial supply of lithium into larger, heavier atoms.

To determine the mass of the planet or planets that HD 82943 consumed, astronomers would need to know the victims composition. Thats because rocky, Earthlike planets contain much more lithium than gaseous, Jupiterlike planets do. The researchers used the Very Large Telescope in Paranal, Chile, to measure the lithium-6 and calculated that the star swallowed a gaseous body twice as heavy as Jupiter or a rocky orb about three times as massive as Earth. Or the star could have feasted on several planets.

Once a star gets an infusion of lithium-6, theres no guarantee that the isotope will remain intact. Our sun would burn any lithium-6 deposited in its relatively cool atmosphere by mixing it with much hotter layers of gas nearer its core. However, HD 82943 has a slightly higher surface temperature and is about 5 percent more massive than the sun. It maintains a better separation between its atmosphere and deeper, hotter layers, enabling the star to retain newly arrived lithium-6 for billions of years.

The idea of stellar cannibalism is in accord with the locations of the 60-plus extrasolar planets that astronomers have found since 1995. The most successful planet-hunting method preferentially detects the closest orbiting planets, but astronomers have been surprised by just how close some of these orbs get to their parents. A few lie within a blistering range, as close as one-tenth the separation between our own sun and its closest planet, Mercury (SN: 8/8/98, p. 88).

According to theorists, planets arent born that close to their stars but can migrate inward–and sometimes end up as star food. The complex gravitational interactions of HD 82943s two remaining planets could have hastened the migration into the star of a third, inner planet, Israelian says.

Doug N.C. Lin of the University of California, Santa Cruz agrees that the data point to cannibalism. For a planet to have deposited its lithium in the stars atmosphere rather than deeper down, gravity must have torn the planet apart before it was consumed, he notes.

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