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Polluted stars suggest other Earth-like worlds Earth-like planets should be a fairly common feature of other solar systems in our galaxy, a new study of stellar senior citizens suggests.
Image: Asteroid belt
An artist's impression of a massive asteroid belt in orbit around a star. The new work with SDSS data shows that similar rubble around many white dwarfs contaminates these stars with rocky material and water.
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Earth-like planets should be a fairly common feature of other solar systems in our galaxy, a new study of stellar senior citizens suggests.

More than 90 percent of stars in the Milky Way, including our own sun, end their lives as a white dwarfs. Traditionally, these dense stellar remains haven't been the first place that astronomers look for signs of planets outside our own solar system. Instead, exoplanet searches have focused on stars like our own sun.

But tantalizing new results suggest that these elderly stars might also be a rich source of information on the potential for other planetary systems out there in the galaxy.

White dwarfs should essentially be composed of pure hydrogen and helium atmospheres. Any elements heavier than helium ("metals" in astronomical parlance) present in a white dwarf atmosphere have to be pollutants from some external source.

For decades, astronomers attributed this metallic pollution to the interstellar medium, the thin gas that permeates the space between stars. The idea was that white dwarfs were old stars that had been on several orbits around the Milky Way and had picked up bits of the interstellar medium as they went around, explained Jay Farihi of the University of Leicester who studied the white dwarf contamination.

"But it turns out that [this explanation] doesn't really fit the data," Fahiri told

What's going on
Farihi has looked at white dwarfs with NASA's infrared Spitzer Space Telescope for five years, and those observations showed that the white dwarfs "have dust right on top of them," Farihi said. "It's almost certainly raining down on their atmospheres."

Farihi and his colleagues looked at the positions of these white dwarfs within the Milky Way and estimated whether the impurities they saw in the stars' atmospheres could be explained by sweeping up the interstellar medium.

"And the answer is a resounding 'No, it doesn't make sense,'" Farihi said.

To get a better look at the pollution in the white dwarf atmospheres, Farihi and his colleagues used data from the Sloan Digital Sky Survey, which has taken the spectrum, or light signature, of 1 million cosmic objects.

They found that the types of metals seen in the stellar atmospheres, such as silicon, magnesium and iron, suggest a rocky origin. The exact source of the rocky debris isn't known, but Farihi says there are two possibilities: the debris could come from an asteroid belt similar to our own, which essential represents a planet that didn't form, or the pieces of a shattered planet.

The new work, presented this week at the Royal Astronomical Society meeting in Glasgow, Scotland, indicates that at least 3 percent and possibly as much as 20 percent of all white dwarfs are contaminated by rocky material. This in turn suggests that a similar proportion of stars like the sun, as well as some a little more massive, such as Vega, that eventually become white dwarfs host planetary systems.

Signs of water
Another recent estimate suggested that about 15 percent of the stars in the Milky Way hosted systems like our own.

Interestingly, there are also indications that some of the rocky material polluting the white dwarfs contained water.

The white dwarfs studied had helium atmospheres, but showed trace amounts of hydrogen, one of the two elements that make up water. If the hydrogen and the metals came from different sources, the stars that contained them both should be rare, Farihi explained. But they were actually fairly common, suggesting that the hydrogen and metals have the same source.

"The rocks that delivered the metals probably delivered the hydrogen," Farihi said. The hydrogen suggests that the minerals that contained the metals also contained water, an essential chemical for life as we know it.

Finding an oxygen signature in the atmospheres of these white dwarfs would help bolster that interpretation, but Farihi says the team would need to use the Hubble telescope to find that signature. They have request time on the space telescope and are waiting to hear back to see if their proposal is granted telescope time.