White Dwarf Hints at Our Solar System's End

A debris disk spied recently around a distant dead star is likely the remains of an asteroid that was vaporized when the star died, scientists say.

The discovery, detailed in the Dec. 22 issue of the journal Science, could be 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 or more planets, the finding also provides evidence that planetary systems can form around massive stars.

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

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

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

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

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

This means it is likely that one or more of the planets originally surrounding the star survived the swelling red giant phase to disrupt the asteroid, G?nsicke said.

This star and its possible planetary system provide a model of what our solar system will look like in a few billion years.

"It looks similar to how our solar system will look once the Sun ends its life," G?nsicke said.

When the Sun becomes a red giant, it will grow to somewhere between the present-day orbits of Earth and Mars.

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

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

G29-38is now about 75 percent of the mass of our Sun, but it was originally four or five solar masses. Astronomers have been uncertain whether or not planets could form around massive stars, since they don't live as long.

The discovery of this disk around the white dwarf is good evidence of the existence of 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.

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