Like the faint hint of garlic after an Italian meal, traces of a fragile isotope have hinted at the diet of stars. Lithium lingering in the atmosphere of the star HD82943 is the first solid evidence that Jupiter-sized planets can be gobbled up by their parent stars, say astronomers in the 10 May issue of Nature. But other experts are not sure this is the only possible explanation for the results.
In the past several years, more than 50 planets weighing slightly more than Jupiter have been discovered around stars other than the sun. Life for the giant planets is less stable than in our solar system. Most such planets swing much closer to their parent star, at much higher speeds, and on eccentric orbits. Many astronomers have suggested that some of these wayward planets could crash headlong into their stellar host, or spiral inward until they are slowly engulfed. But they have no direct proof that it happens.
Astronomers might, however, be able to see scraps left over from the planetary feast. All planets contain trace amounts of lithium, but the delicate element vaporizes instantly in the raging nuclear fires at the center of stars. If a planet crashes into a star, its lithium can linger for millions of years in the relatively cool atmosphere before currents of hot gas sweep it into oblivion at the star's core.
On 7 June 2000, a team led by Garik Israelian, an astronomer at the Astrophysics Institute of the Canary Islands, detected an isotope of lithium, Li-6, in the atmosphere of HD82943. According to Israelian's calculations, only one in every 10 billion atoms in the star is Li-6. A star as hot and old as HD82943 would have consumed all its Li-6 long ago. To have even that trace amount of Li-6 left in its atmosphere, the star must have recently devoured a passing planet weighing at least as much as Jupiter. "This is a clear indication of engulfment," Israelian says.
Not everyone is so sure. "I don't think we are seeing the bones from the meal here," says astrophysicist Alan Boss of The Carnegie Institute in Washington, D.C. The complication is that Li-6 has a more stable sibling, Li-7, that has been seen in several other stars that don't have orbiting planets. Boss wonders whether there might be other sources for the lithium in HD82943. "It is a very muddled situation," Boss says.