Their latest models point to the formation of planet-like dust clouds around these old and dim infernoes. The prospects for finding candidate planets around such burnt-up parent stars now appears likely, especially by pointing future infrared telescopes to any White Dwarfs that prove particularly rich in heavy metals.

White dwarf stars have long fascinated astronomers, but haven't previously caught the imagination of planet finders. To understand white dwarfs is to see the lifecycle of the stars.

Unlike the permanent and perpetually shining night sky, stars during their lifespan pass through radical changes, both in their spectral colors and brightness. A fledgling star aggregates dust, then fuses or burns the available light elements to create heavier metals. As the earliest stars ignite and deplete these metals, they burn towards what is known as an expanded red giant stage. Dramatic events arise as the stellar spectra then shifts from red to white, and collapse 100-fold or more to so-called dwarf phases. Finally, in some cases, the largest stars morph again to neutron stars or massive black holes.

But for the new breed of planet hunters, white dwarfs are like 'dying' stars, having burnt up the major portions of their fusionable elements (hydrogen and helium). Having expanded rapidly to red giants, any inner planets would likely have been engulfed, and long ago escaped any detection. But that predictable lifecycle is now coming under revision. During this massive and catastrophic solar system shakeup, the Penn State research points to a chance for reforming new planets.

Debes and Sigurdsson have predicted a kind of white dwarf having an unexpectedly warm dust cloud -- and perhaps reborn planets rising from the red giant's ashes. If they prove correct, these new planets would show an infrared signature -- their heat of formation becoming a tell-tale marker. The first step and clue in their favor, as in most planet finder surveys, is quite simple: reduce the vastness of where to look.

"Extrapolating to the whole galaxy, we expect there to be many billions of white dwarfs in total in the Milky Way," says Debes. "Current estimates hold that as many as 2 percent of white dwarfs may have such planets. Our research suggests that white dwarfs with dust should also have planets. So in some sense dust around a white dwarf would be like a little flag to observers telling them where to look for planets."

Prior to the analysis of Debes and Sigurdsson, planet finders might have overlooked white dwarf candidates because, as stated above, even if the stellar system might have started with inner planets, their destruction would be inevitable. Until any outer planets have a chance to realign, the solar system's survival is doubtful in transition to the more compact white dwarf stage.

However, the newly imaged mysterious dust cloud surrounding the star, G29-38, located in the constellation Pisces (the Fishes) has riveted planet-hunters attention, suggesting the reality that White Dwarfs could harbor planets."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."

Seeing Red If imaging a white dwarf spectroscopically isn't challenging enough, hunting for planets a million to a billion times dimmer is truly daunting. In their white dwarf stage, the aged star has exhausted most of its nuclear fuel, and thus even if its inner planets somehow could survive past a red giant phase, the star itself might prove less attractive to astronomers because of its low intrinsic luminosity.

The combined dimness of the parent and any planets makes their combination an unlikely candidate for ground based observers. But the Penn State scientists have turned this liability to their advantage, by unveiling possible planetary dust clouds around a dimmer parent white dwarf.

"Since they orbit around a very dim white dwarf," says Sigurdsson, "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."

To predict detection of these new possible planets, the Penn State research combined these dramatic astronomical events into a new observational strategy for planet finders. The heat of renewed planetary formation would show up strongly relative to an even dimmer parent star. Thus, freshly-made planetary fragments, having an even hotter character, may prove detectable using infrared observatories.

By using advanced color spectroscopy (mainly in the infrared bands), planet hunters can now begin to look for relative differences between old parent stars and phoenix-like outer planets as bright contenders in future sky searches. According to Debes, "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.

Heavy Metal The Penn State researchers also made a proposition for why parent stars may have heavy elements, like anomalous metals seen in what should have only a light (helium or hydrogen) atmospheric corona.

The presence of such metal-rich atmospheres, or DAZ white dwarfs, is particularly intriguing. "People have thought about what puts metals onto the surface of a white dwarf for a long time," says Debes. "The problem is that you don't expect to see anything but hydrogen, or helium, in the atmosphere of a white dwarf--anything heavier than helium should essentially settle out of the atmosphere and fall to the inner region of the white dwarf." Previous studies showed that such heavy elements would sink to inner regions of the gravitationally strong core, and not appear in the atmosphere or corona.

The Penn State research, however, provides a way for wayward comets to carry in such metals and effectively pollute the atmosphere of white dwarfs. Using a "sling shot" effect of such newly formed planets, "our models provide a channel for greatly enhancing the flux of comets from the outer cometary belt to the inner parts of the white dwarf system - the region scoured clean by the red giant phase that precedes the white dwarf phase of stellar evolution," says Debes.

"Planets scattered into the outer system perturb the comets and sling them into the inner system, there the planets scattered inwards "catch" the comets and relay them deep into the inner system, much like a cutoff relay in baseball. Once in the inner system the comets are torn apart by the strong gravitational field of the white dwarf, creating a debris and dust cloud which may be observable, this dust then rains down onto the white dwarf showing up as the anomalous metal spectral lines."

Based on this theory it could be predicted that white dwarfs, which have metals present in their atmospheres, could be considered candidates for having some system of planets around them. "There are on the order of a couple of thousand white dwarfs that have been identified spectroscopically," says Debes, "so if our estimate is correct, you would expect perhaps 40 systems that would have the reborn planet scenario."

What's Next In this artist's rendition, SIRTF is seen in its Earth-trailing orbit around the Sun. This innovative orbit produces many advantages, from a more benign thermal environment for the super-cooled detectors, to a better view of the open sky, away from the Earth and the Moon. Credit: SIRTF CalTech One place likely to target in future white dwarf surveys will be to look for heavy metals in the corona, then pinpoint hotter planetary dust clouds or collision remnants from outer planets. "There are several predictions that our hypothesis makes", says Sigurdsson, "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."

Two future orbiting observatories will strongly feature infrared capabilities. According to Debes: "Any planet, even a newly reborn planet would still be very dim and hard to observe. However, future NASA space missions such as SIRTF should be sensitive enough to detect a planet similar in size to Jupiter."

Also, says Debes, "Future advanced space missions, such as GAIA, will be able to observe few hundred thousand of those white dwarfs, though most are too faint to observe with currently available technology. Further advanced missions, such as the European Space Agency's Darwin mission would be able to do detailed measurements of the planet's atmosphere."

Finding 40 candidate planetary systems among the billions of white dwarfs will depend on better instruments and the kind of markers uncovered by the Penn State research. Debes concludes, "No planet except those in our Solar System have been properly observed with infrared spectroscopy so to some extent we can only make very educated guesses about what we would see."

The Penn State research features John Debes as the first author of this paper, done in collaboration with Steinn Sigurdsson.

Related Links
Penn State Astrobiology
Constellation Pisces (the Fishes)
European Space Agency's Darwin mission
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