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December 21, 2006
Dying Star May Presage Our Solar System's Demise
New evidence suggests that white dwarf stars have planets orbiting them

by Nikhil Swaminathan

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Science Image: white dwarf ring
METALLIC RING:  An unusual disk of gaseous, metallic debris around a white dwarf star 463 light years away provides a model for what could become of our solar system
Researchers using data collected by the Sloan Digital Sky Survey--a comprehensive effort to map a quarter of the sky with a dedicated telescope at Apache Point, N.M.--have identified a cooling ember of a star ringed by a rare gaseous, metal-rich disk. This discovery, according to team leader Boris Gänsicke of the University of Warwick in England, suggests that there is a planet orbiting this once massive star. It also may provide, he says, "a glimpse into the future of our solar system," specifically how it may end.

The star in question is a white dwarf known as SDSS1228+1040. It is located 463 light-years away from Earth and is in the constellation Virgo. In its prime, the star weighed in at four to five solar masses. (Progenitor stars of white dwarfs can be up to eight times the mass of our sun). SDSS1228+1040's progenitor likely thrived as a main sequence star for about 70 million years, which is a relatively short life span compared with that of our sun--its current age is around 4.6 billion years old.

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Gänsicke points out that there has been much debate within the realm of astrophysics about whether these short stellar life spans provide the "time that is necessary to form planets from the debris of the disk that made the star in the first place." (Stars are believed to form from matter that coalesces from a disk of debris and then ignites in a nuclear fusion reaction.) The new findings suggest they are. And if these short-lived stars are able to support planetary systems of their own, they can certainly serve as models for what could happen billions of years in the future to our own solar system.

Based on the group's estimates, SDSS1228+1040 has been in the white dwarf stage for 100 million years, and its current surface temperature is thought to be a steamy 22,000 degrees Celsius. (In contrast, our sun's surface temperature is around 5,500 degrees C.) When running spectral analyses on the material surrounding the star, the team found the double-peaked emission lines of magnesium, iron and calcium. This allowed them to determine that the circumstellar material was distributed in roughly a half-a-million-mile radius around the star.

Gänsicke and his colleagues believe that the debris ring is the remains of a 50-kilometer-wide asteroid, which once orbited the star closely along with other entities. This finding "strongly suggests that there is still a planet orbiting around SDSS1228+1040 today," Gänsicke says. "[Asteroids need] the gravitation of an object much bigger than the asteroids themselves to dislodge one of them from their stable circular orbits." Once dislodged, the asteroid likely moved too close to the gravitational field of the star, where it was broken up in a process called tidal shredding. The pieces then likely evaporated into the disk seen now via radiation from the hot star.

The timeline for the star now known as SDSS1228+1040 likely went as follows, according to the Warwick team, which speculates that a similar chain of events will likely befall our sun: After the star burned all the hydrogen in its core, it likely swelled into its red-giant phase, eviscerating all material--planets included--in orbits up to 500 million miles away. Any asteroids or planets stationed beyond that radius would then be kicked into orbits farther away from the newly swelled star. (Gänsicke estimates that our sun will enter its red-giant phase in five billion to eight billion years.) Once the outer regions of the red giant are shed, the star shrinks into its white dwarf phase that is superdense--the diameter of SDSS1228+1040 is 1 percent of our sun's, but it's mass is 75 percent that of the sun--and initially very hot. From this point it gradually cools down and eventually will burn out.

Michael A. Jura, an astronomer at the University of California, Los Angeles, says the discovery could lead to new information about the makeup of "extrasolar minor planets." He is skeptical, however, that the members of our solar system will meet the same fate as the objects that surrounded SDSS1228+1040's progenitor.

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