Solar System's Fate Predicted by Nearby White Dwarf?

John Roach
for National Geographic News
December 21, 2006
In an example of galactic foreshadowing, scientists have gazed out at the stars and glimpsed how our solar system might look in several billion years.

An unusual ring of metal-rich gas orbiting a white dwarf—the remains of a burned-out star of modest mass—has been spied within the constellation Virgo.

The white dwarf, called SDSS 1228+1040, sits about 463 light-years from our solar system (explore an online star chart).

Scientists believe the solar remnant formed from a star that was about four to five times as massive as the sun.

Interactions between at least one planetlike object and an asteroid that once orbited the white dwarf at a great distance are responsible for the ring of metal-rich gas.

In a paper appearing in tomorrow's issue of the journal Science, astronomers describe how the process that likely formed this unusual system could provide insight into how our solar system will eventually meet its end.

White Dwarf

When small to medium-size stars burn up all their hydrogen, they expand like balloons into so-called red giants, destroying everything in their path.

These dying stars can become hundreds to thousands of times the size of our sun.

Eventually the outer layers of a red giant disperse and leave behind a dense core—a white dwarf—that usually contains roughly the same mass of our sun in a body the size of Earth.

As it expanded, the red giant version of SDSS 1228+1040 probably destroyed everything in its orbit out to about 500 million miles (800 million kilometers), according to the scientists.

What's so unusual about SDSS 1228+1040 is the metal-rich disk of gas that orbits close to the white dwarf, roughly 500,000 miles (800,000 kilometers).

"The stuff we see [in the disk] can't have been there before," said Boris Gansicke, an astrophysicist at the University of Warwick in the United Kingdom and the paper's lead author.

"It must have been brought to this place after the white dwarf formed."

He and his colleagues believe that the gas originated from a 30-mile-wide (50-kilometer-wide) asteroid that they think was kicked out of a stable orbit by an interaction with a large planetlike object.

The larger body forced the asteroid close enough to the white dwarf to be broken up by the star's gravity.

"It was just shredded to pebbles, and the white dwarf is quite hot still, so the material got evaporated to form the disk," Gansicke said.

Solar System Model

In a few billion years, Gansicke and colleagues expect our solar system to look similar to SDSS 1228+1040.

During our sun's red giant phase, the inner planets Mercury, Venus, and probably Earth will be destroyed, Gansicke noted.

But as the sun swells, it will lose about half its mass, and its weakened gravitational pull will allow Mars, the asteroid belt, and the outer planets to escape to wider orbits.

(Related news: "Early Life Fed on Organic Haze, Study Suggests" [November 6, 2006].)

After the dying sun becomes a white dwarf, one of the asteroids in the surviving asteroid belt may be knocked out of orbit by Jupiter's gravity and sent on a course close to the dead star.

That asteroid will eventually meet the same fate as the one that came too close to SDSS 1228+1040.

"We believe in five to eight billion years, the solar system will look very much like the white dwarf" in Virgo, Gansicke said.

"It will mainly have the asteroids going around it and a couple of planets."

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