David A. Aguilar (CfA)

A planetary nebula surrounds a white dwarf star with a hypothetical habitable planet in orbit.

By Irene Klotz
Discovery

Earth-sized planets that host life should be far easier to find around parent stars that are white dwarfs, the ultimate incarnations of stars like the sun, a new study shows.

Alien worlds could circle dying white dwarf stars, this NBCNews.com report explains.

White dwarfs are the dense stellar cores that remain after a sunlike star runs out of fuel and goes through its expanding, red giant phase, a process that will consume its inner planets. In our solar system, for example, Mercury, Venus and possibly Earth will be destroyed when the sun evolves into a red giant some 4.5 billion years from now.

But the system won't necessarily be doomed.

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Outer planets may migrate inward, closer to the star, and new worlds may form. Not all will be in stable orbits, but an Earth-sized world located about 1 million miles away from a host white dwarf star would have a temperature roughly the same as Earth’s. At that distance, the planet could have liquid water on its surface, a condition believed to be necessary for life.

Scientists are developing techniques to scan the atmospheres of planets beyond the solar system for oxygen and other chemical signs of life. It's a laborious and time-consuming process to separating out light passing through a planet’s atmosphere from all the background starlight.

But Earth-sized planets circling white dwarf stars, which are themselves about as big as Earth, make for much bigger needles in extrasolar planet haystacks.

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Avi Loeb, a theorist at the Harvard-Smithsonian Center for Astrophysics, figures the upcoming James Webb Space Telescope, a successor to the Hubble observatory, would need only about five hours of observing time to look for biomarkers in the atmosphere of a planet circling in a white dwarf’s habitable zone.

“Usually the background star is so much brighter, it’s so much bigger than the planet that absorption (of light) due to the atmosphere is a very small signal that you have to fish out of the much more prominent emission from the background star,” Loeb told Discovery News.

“In the case of the white dwarf, it’s sort of the best of all circumstances, where the object that is blocking the star is of the same size as the star itself. That offers the best prospect for detecting the absorption due to the atmosphere, relative to the background light,” he said.

Like NASA’s planet-hunting Kepler Space Telescope, the technique would only work for white dwarf systems that are aligned relative to the observatory’s line of sight so that orbiting planets pass in front of, or transit, their parent stars.

While the star is eclipsed, some light will pass through the planet’s atmosphere -- if it has one -- and leave telltale chemical fingerprints that can be detected by instruments in a telescope.

“If we happen to be situated so that we can see an eclipse, then the planet would block a substantial fraction of the light from the white dwarf. Then we can basically use the light that is passing through the atmosphere to figure out what the atmosphere is made of,” Loeb said.

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Of key interest would be detections of oxygen, which on Earth is a clear sign of life from photosynthesis by plants.

“The only reason we have oxygen in the atmosphere right now is because of life,” Loeb said. “If you remove life from Earth, then within 1 million years or so, the oxygen will be completely depleted. It will make all kinds of molecules of oxidized metals, for example, and it will be consumed from the atmosphere.”

Scans of exoplanets’ atmospheres also could find water vapor and other potential biomarkers.

While there is not yet any direct evidence of planets circling white dwarfs, astronomers believe they exist. Previous studies have shown that as many as 30 percent of white dwarf stars have heavy elements on their surfaces, presumably from rocky bodies that broke up relatively soon after the white dwarf formed.

PICTURES: Exquisite Exoplanetary Art

A planet could find a stable orbit in white dwarfs’ habitable zone, one that would have it circle its parent star in just 10 hours.

“I’m not saying that we definitely know that such planets are there, but it’s quite plausible that the system after a while cleans itself up and for over a billion years or more, it may have stable Earth-mass planets,” Loeb said.

“It sounds like a reasonable extrapolation for what we’ve seen,” added astronomer Marc Kuchner, with the Exoplanets and Stellar Astrophysics Laboratory at NASA’s Goddard Space Flight Center in Greenbelt, Md.

“There’s no reason to think you wouldn’t find one now and then," Kuchner told Discovery News.

The research will be published in an upcoming edition of the Monthly Notices of the Royal Astronomical Society and is available online via the arXiv preprint service.