Jan. 8, 2002 A new way to identify planetary systems around nearby stars, and a possible new explanation for the mysterious properties of certain strange white-dwarf stars, will be presented at the annual meeting of the American Astronomical Society in Washington, DC, on 7 January. The report by Penn State astronomers John Debes and Steinn Sigurdsson details several observable signatures that the researchers predict could occur as planetary systems become unstable and chaotic when a normal star like the Sun reaches the end of its life and evolves into a white dwarf.
Previous theoretical research has shown that, when a star at the end of its life exhausts all of the nuclear fuel it needs to keep shining steadily, it goes through a slow process during which it grows to a few hundred times its original size and loses a large fraction of its mass, shrinking to a compact white dwarf. This process has been shown to have a profound effect on the dynamics of any planets that might orbit the star. Planets will be engulfed during the star's death throes if they, like Earth, are very close to the star, but planets will survive if they are about the same or greater distance from their star as Jupiter is from the Sun--about 5 Astronomical Units, or 490 million miles. Debes and Sigurdsson have found that this instability because of the star's loss of mass could happen in a significant fraction of planetary systems.
"Two planets can interact and collide, creating a freshly formed reborn planet whose characteristically elevated temperature we feel can be used to make it much easier for astronomers to directly image planets around white dwarfs--something that has not yet been achieved," Debes says. Debes and Sigurdsson predict that astronomers might be able to image these reborn planets and dust disks or infer their presence by using spectroscopy in infrared wavelengths. They also predict that white dwarfs, which would have metals present in their atmospheres, could be then considered candidates for having some system of planets around them.
Planets are intrinsically millions of times dimmer than the stars they orbit, making it very difficult to see them directly in contrast to the very bright nearby star. But young planets are easier to detect because they glow more brightly in the infrared from their heat of formation. "Planets reborn from collision around white dwarfs would have two advantages for detecting planetary systems," Sigurdsson explains. "Since they are young, they are relatively bright, and since they orbit around a very dim white dwarf, they are perhaps only thousands of times dimmer than their parent stars rather than millions or billions of times dimmer, as they would be in a stable planetary system." Debes and Sigurdsson estimate that as many as 2 percent of white dwarfs may have such planets, although there are still many uncertainties that could change the estimate of the fraction of systems containing such "reborn'' planets.
Another possibility is that other planets in the system could interact without colliding, resulting in one planet being flung into a very wide orbit, away from the white dwarf. A planet in a very wide orbit may disturb a cloud of comets similar to our Solar System's Oort Cloud, which other researchers have shown could likely survive a star's evolution into a white dwarf. "This process, along with the disturbance of the comets during the star's mass loss, can create a period of heavy bombardment of material into the inner planetary system of the resulting white dwarf star, analogous to the heavy comet bombardment phase we believe occurred in the early history of our solar system," Debes and Sigurdsson predict. "Such an influx of new comets could pollute the surface of the white dwarf with metals and would create a disk of debris in the inner regions of the white-dwarf system, which would otherwise be expected to have been scoured clean by the giant phase of the star."
This mechanism could help to explain two phenomena that involve white dwarfs: the mystery of the star G29-38, the only white dwarf known to have a dust disk, and the presence of so-called DAZ white dwarfs, which have metals present in their atmospheres, say Debes and Sigurdsson.
"The calculations and simulations we have done show that unstable old solar systems could cause dusty white dwarfs and born-again planets," Debes says, "but they need to be backed up with more observations." Sigurdsson adds, "There are several predictions that this hypothesis makes, including the presence of anomalously bright planets around white dwarfs and a correlation between metal pollution and the presence of dusty disks, which can be tested with future observations with future space and ground-based observatories."
This work was funded by a NASA Graduate Student Research Project fellowship awarded to Debes and a NASA grant (#GO-8267) to Sigurdsson.
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