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”

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.

[ J D / B K K ]


Steinn Sigurdsson, 814-863-6038, steinn@astro.psu.edu)
John H. Debes, 814-863-7948, debes@astro.psu.edu)
Barbara K. Kennedy (PIO), 814-863-4682, science@psu.edu


This research has built upon the work of many other astronomers. Martin
Duncan, of the Queens University Department of Physics, and Jack Lissauer,
of NASA Ames Research Center, noted the possibility of change in stability
when the central mass of a system changes; Charles Alcock, of the University
of Pennsylvania Astronomy Department and colleagues were instrumental
in calculating the survivability of Oort clouds around other stars and
pointing out the link between cometary impacts and the DAZ phenomenon;
the dynamical evolution of comet clouds was treated by Jack Hills, of
Los Alamos National Laboratory; and the star G29-38 was first discovered
to have an infrared excess due to dust by Ben Zuckerman and Eric E. Becklin,
of the UCLA Astronomy Department. The current research could not have
been possible without the work of these astronomers and many others.