Image: Infrared image
John Debes, et al.  /  Penn State
In this highly processed image of a distant white dwarf star, the glare of the star itself has been reduced, revealing a point of infrared light toward the upper right that may be a planet or a brown dwarf. Additional observations are likely to show whether the object moves with the white dwarf and is a companion, or if it is a background object moving at a different pace.
By Senior Science Writer
updated 5/10/2004 1:24:00 PM ET 2004-05-10T17:24:00

In a preliminary analysis of new data, astronomers say they may have imaged a planet outside our solar system for the first time by using a tricky new method to ferret out dim objects from the light of a star.

The researchers are very cautious not to claim any discovery yet. The faint point of light, captured by the Hubble Space Telescope, might instead be a background star or a very distant galaxy. Follow-up observations are required to confirm the discovery.

The scientists say there is a high probability the object is a planet, however. If so, it would not resemble anything in our solar system.

The possible planet is huge, something less than 10 times as massive as Jupiter. It orbits a white dwarf star, which is dramatically different from our sun. White dwarfs are burnt-out corpses of formerly massive stars.

Three candidates
The object is one of three planet candidates found in the new study around white dwarf stars between 30 and 55 light-years away. The other two candidates appear to be even more massive, about 15 to 20 times as heavy as Jupiter. That would put them at the boundary between massive planets and failed stars known as brown dwarfs. Stellar pairs and even triplets are common, so astronomers would not be surprised to find a brown dwarf circling a white dwarf.

Each of the newly spotted objects and its corresponding white dwarf are separated by more than the distance from Neptune to the sun.

"This is all dependent on seeing whether these are background objects or not," Penn State graduate student John Debes said in an interview. "If not, then there is a very strong chance that at least one is a planet."

A firm answer could come later this year with one more snapshot of each candidate that would show whether it moves across the sky with its presumed host star, instead of moving at a different pace. The latter scenario would reveal it to be another star or galaxy that is in the more distant background.

Stretching the envelope
Scientists have been detecting planets around other stars since 1995. More than 120 are known. But most have been found indirectly by noting a gravitationally induced wobble in the host star. No extrasolar planets have been photographed directly, and many researchers have said the first picture would not likely come until next-generation space observatories are launched.

Several groups are working on creative ways to image extrasolar planets from current ground- or space-based telescopes, however.

The challenge is to spot a planet that by nature is small and dim compared to the overwhelming light of the star it orbits.

Given the hurdle, Debes and his colleagues, Penn State professor Steinn Sigurdsson and NASA researcher Bruce Woodgate, surveyed seven white dwarf stars. These are more massive than sunlike stars but many times dimmer. The three white dwarfs with candidate planets are between 1 billion and 3 billion years old.

Tricks with infrared
The observations were made with Hubble's infrared NICMOS camera. The infrared light from the presumed planets is not reflected light from the host stars but instead represents heat emitted by the giant worlds.

Image: White dwarf
John Debes, et al.  /  Penn State
Researchers started out with this picture of a white dwarf star.
The subtle detections involved using a trick developed by other researchers. After taking the first image of a white dwarf and its surroundings, Hubble was rolled slightly in space and a second image was made. By comparing the two images, scattered light created by instrument imperfections can be removed, Debes explained. That way the star is reduced more closely to a point-like source. Remaining nearby points of light then emerge as either dim orbiting companions or background objects.

An analysis of these faint sources then relies on other theorists' predictions of how much infrared radiation should come from planets of certain mass.

Debes presented the preliminary results at a meeting of astronomers here last week at the Space Telescope Science Institute, which operates Hubble for NASA.

Caution for now
"There are significant uncertainties" in the models relied upon to predict the planets' characteristics, Debes cautions. But additional telescope time will take all the mystery out of it. "The good news is we can take these [follow-up] observations in three to six months and know for sure."

One of the candidates is bright enough to be re-imaged from the ground. The other two would require additional investigation by Hubble. Debes is in the process of applying for the telescope time.

White dwarf stars do not present conditions favorable to hospitable worlds. But learning about planets around them should help researchers improve theories of how planets form, Debes said, by expanding or constraining the environmental conditions necessary to make planets of various kinds.

Planets around white dwarfs might sound odd, but other planets have been found in strange places.

Previous work led by Sigurdsson suggests a gas giant planet was apparently captured into an orbit around a white dwarf and a neutron star, in one of the most bizarre cosmic setups ever proposed. Sigurdsson said that planet is the oldest known, having formed 12.7 billion years ago when the universe was in its infancy. And a separate research group last year determined that three worlds roughly comparable to Earth in size orbit a rapidly spinning neutron star.

Debes and Sigurdsson also previously created a computer model to explain dust found around one white dwarf, using comets as the dust source. The comets, they theorized, where gravitationally flung toward the star by hypothetical planets, and the comets then broke up to create the dust. That idea motivated the current planet search.

The researchers would like to expand the planet search over the next several years to other white dwarf stars. The observations are only practical with Hubble, Debes said.

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