Dead Stars Harbor Asteroids

LONG BEACH, Calif. ? New observations of chewed-up asteroids around old dead ?stars called white dwarfs bolster the idea that the Earth and other rocky planets in our solar system are far from alone in the universe.

Astronomers used NASA's Spitzer Space Telescope to peer at six such white dwarf stars and found the signature of asteroid debris circling the stars. An analysis of the light coming from the systems show the rings are made of some of the same materials as rocky bodies in our own solar system.

"It strengthens suspicions that Earth-like planets are common," said Michael Jura of UCLA here Monday at the 213th meeting of the American Astronomical Society.

Planet building

Asteroids and planets form from the dust and gas that swirls around young stars. As the dust sticks together, it forms clumps that eventually become full-fledged planets, according to the leading theory. Asteroids are the "leftover building blocks that didn?t get incorporated in the planets," Jura said.

As stars like our own sun near the end of their life, they puff up into red giants that consume their innermost planets and jostle the orbits of outer planets and asteroids. Eventually the stars blow off their outer layers and shrink down into white dwarfs.

Occasionally, a perturbed asteroid will wander too close to the white dwarf, whose gravity rips the rocky body to shreds, forming debris. (This is similar to what happened to Comet Shoemaker-Levy 9 when it was torn apart by Jupiter's gravity before impacting the planet in 1994.)

Spitzer had previously observed shredded asteroid pieces around two white dwarfs; the new observations bring the total count (of white dwarfs with asteroid debris) to eight.

"Now we?ve got a bigger sample of these polluted white dwarfs, so we know these types of events are not extremely rare," Jura said, noting that they about 1 percent of white dwarfs are estimated to have these signatures.

The details

All eight white dwarf systems observed showed signatures of a glassy silicate mineral similar to olivine, which is common on Earth.

"This is one clue that the rocky material around these stars has evolved very much like our own," Jura said.

The spectra (light broken into its components) of the asteroid debris also showed no carbon signature ? what Jura calls a "carbon desert" ? another similarity to the asteroids and rocky planets in our own solar system, which have relatively little carbon.

A single asteroid is thought to have broken apart within the last million years or so in each of the eight white-dwarf systems. The biggest was once about 124 miles (200 km) in diameter, a bit larger across than Los Angeles County.

Continued observations of this white dwarf debris could help astronomers further figure out the composition of exoplanets, which so far has proven difficult.

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