White Dwarf Hints at Our Solar System's End

A debrisdisk spied recently around a distant dead star is likely the remains of anasteroid that was vaporized when the star died, scientists say.

Thediscovery, detailed in the Dec. 22 issue of the journal Science, couldbe a sign of what will happen in our own solar system in a few billion years.Because the crushed asteroid was probably gravitationally lassoed in by one ormore planets, the finding also provides evidence that planetary systems canform around massive stars.

While analyzingthe light spectra of several hundred white dwarfs, astronomer Boris G?nsicke ofthe University of Warwick discovered evidence of a cool dust cloud around thewhite dwarf G29-38. Whitedwarfs are the dead stellar remains of relatively small stars like our Sun that have run out of fuel and sloughedtheir outer layers off into space.

G29-38"had very, very unusual calcium emission lines in the red end of thespectrum, which white dwarfs shouldn't have, or which most stars shouldn't have anyway," G?nsicketold SPACE.com.

The chemicalsignature of light from the white dwarf suggested it was girdled by some kindof rotating gas disk, he said.

"It's thefirst time we can really actually prove that there is a disk of debris materialgoing around the white dwarf," G?nsicke said.

He and hiscolleagues believe the disk was created by a tidally disrupted asteroid, pulled out of its orbit bya large object. They think the most likely scenario is that one or more planetsdisrupted the asteroid's orbit, causing it to tumble closer to the star. Thestar's gravity eventually ripped it apart, and the heat of the star evaporatedthe debris to form a ring of rotating gas. Such a disk would be short-livedbecause the material falls onto white dwarf, according to G?nsicke, so it musthave formed relatively recently.

This meansit is likely that one or more of the planetsoriginally surrounding the star survived the swelling redgiant phase to disrupt the asteroid, G?nsicke said.

This starand its possible planetary system provide a model of what our solar system will look like in afew billion years.

"It lookssimilar to how our solar system will look once the Sun ends its life," G?nsickesaid.

When the Sunbecomes a red giant, it will grow to somewhere between the present-dayorbits of Earth and Mars.

"So whatwill happen is that the Sun becomes a red giant, probably destroys Mercury and Venus and the Earth, but Mars, theasteroid belt, Jupiter, Saturn, and all the other planets willsurvive, and they will move maybe just a little bit further out," G?nsickesaid.

Eventually,the Sun would become a white dwarf with asteroids and the remaining planets orbiting around it. It ispossible that Jupiter could disrupt the orbit of asteroid, causing it to falltoward the Sun, forming the same disk that G?nsicke discovered, he said.

G29-38is nowabout 75 percent of the mass of our Sun, but it was originally four or five solarmasses. Astronomers have been uncertain whether or not planets could formaround massivestars, since they don't live as long.

Thediscovery of this disk around the white dwarf is good evidence of the existenceof planets, according to G?nsicke. "While we haven't found a planet directly,we have quite strong indirect evidence that there must be a planet," he said.

• Naked White Dwarf Shows its Dead Stellar Engine
• Freeze, Fry or Dry: How Long Has the Earth Got?
• Dusty Rings May Mark Planetary Systems
• Image: Beginning of the End