In cosmic graveyards around the galaxy lay the remains of stars that reflect the fate of our Sun. They're kind of scary to look at. Sad and shriveled skeletons of their former selves, these so-called white dwarfs are also thought to represent the death of any habitable planets that might have circled the stars in their youth.
A new and slightly less deadly view of white dwarfs may emerge, though, based in part on a fresh analysis of one strange stellar corpse that is kicking up a little dust in its grave.
When normal stars like the Sun get old, they swell to hundreds of times their regular size -- a stage in stellar evolution called the red giant phase. Then they run out of fuel, lose most of their mass, burn out and die. In the process, they clean out their nearby surroundings, vaporizing any dust, rocks and planets that might be there.
Our Earth is probably destined for such a fate.
Strange exception
Hundreds of dead white dwarf stars have been studied in the infrared wavelength, which is a measurement of heat rather than normal light. All are devoid of dust, except one, a white dwarf named G29-38. It's a mystery that has existed since 1987, when UCLA researchers Ben Zuckerman and Eric Becklin first spotted the ring of debris.
Dust disks are prevalent around young stars about to give birth to planets. A middle-aged star like the Sun can be ringed by dust, too, because asteroids and comets grind into one another, continually replenishing a reservoir of material that tends, over time, to drift into the star.
Now Penn State researchers John Debes and Steinn Sigurdsson have a possible explanation for the dust around G29-38. Their computer simulation might also explain oddly high metal contents noted in some other white dwarfs. The researchers put two planets in orbit around a hypothetical dying star, programmed various orbital scenarios, then watched what would happen.
As the star loses mass, chaos sets in. Gravity plays games with the planet's orbits.
There are three outcomes: The planets collide; they get rearranged; or one is booted clear out of the system, irrevocably lost to interstellar space.
Cutoff man
In the case of G29-38, a simple rearrangement of planets, along with some well-known baseball fundamentals, could explain the one-of-a-kind dust disk. Here's how it might have happened, in the jargon of America's favorite pastime:
Assume the star is home plate, Debes suggests. As its gravity fluctuates, one planet is flung into the left field position. This new left-fielder then uses its gravity to grab comets and tosses them to a planet that's remained in the infield. The infielder serves as a cutoff-man, relaying the comets toward home plate.
Somewhere around this point things begin to look decidedly bush-league.
"After a few orbits in the inner system, the comet will then be perturbed by the inner planet and pass very close to the white dwarf," Debes says. "This process will inevitably lead to comets being shredded either by the inner planet or the white dwarf, causing the dust."
Other researchers had suggested comets as possible sources for the dust around G29-38, but the new calculations -- and the unprofessional infielding -- strengthen the theory by providing a mechanism for breaking the comets apart.
A paper detailing the work has been submitted to the Astrophysical Journal.
Zuckerman, the UCLA researcher, thinks the mass of dust around G29-38 may be more than comets could account for. "Of course, since I don't know what the correct model is, the fact that I am unimpressed with a model does not make it necessarily wrong," he said.
Brand new planets
Around other simulated white dwarfs, chaos leads to collisions.
"The two planets can interact and collide, creating a freshly formed, reborn planet," Debes says.
If stars do recreate planets while in their death throes, Debes and Sigurdsson figure the planets might be relatively easy to photograph in infrared light. Because of the heat energy left from its creation, the planet would stand out, relatively speaking, compared to the dim white dwarf it orbits.
Of the 80 or so known planets outside our solar system, none have been imaged, because they are too dim compared to their bright stars. The planets are detected indirectly by noting a gravitational wobble in their host stars. Several research groups are
to make the first photograph of an extrasolar planet."Since [the planets around a white dwarf] are young, they are relatively bright, and since they orbit around a very dim white dwarf, they are perhaps only thousands of time dimmer than their parent stars, rather than millions or billions of times dimmer," Sigurdsson said.
These same conditions, however, might make the planets rather boring. The search for other worlds is largely driven by the search for life, a guiding principle that determines where NASA spends a lot of its money. But life around a white dwarf would be excruciatingly difficult.
In addition to the low luminosity of a white dwarf, its output changes constantly over time as the dying star cools off. In fact, it might be one of the few places where advanced life stands a better chance of surviving than the microbial variety, suggests Zuckerman.
"Any habitable zone would have to be very close to the white dwarf and moving closer as time advanced," Zuckerman said. "I think the best chance for life would be little green men living in a giant spacecraft orbiting close to the white dwarf, and they move inward as the white dwarf cools. They can still gather raw materials, if need be, by traveling outward to objects -- planets, moons, whatever -- that were not destroyed when their white dwarf was a red giant."
What about Earth?
None of this is likely to improve the odds that Earth will survive the swelling of the Sun, Debes says. According to several other studies, Earth will maintain a stable orbit as the Sun expands, beginning in a few billion years. Eventually, the planet will be
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