Spacecraft Eavesdrops on Distant Stars

Sound waves could offer clues about other planetary systems.

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By Ron Cowen, Science News

Using sensitive light detectors to record a cosmic orchestra—sound waves emanating from thousands of distant stars—NASA's Kepler spacecraft is hitting the high notes, the low notes and everything in between. The amplitude and frequency of each sound wave, which Kepler detects as tiny flickers in starlight, has already enabled the craft to pin down the age and size of a sunlike star to unprecedented accuracy, researchers said October 26 during a telephone briefing with reporters. Along with data gathered by a European mission, COROT, the Kepler observations could lead to new insights about stars that harbor planets, researchers noted.

"The determination of masses, radii and ages of planet-hosting stars is of paramount relevance as these parameters can be used to characterize the planet itself—for instance its mean density, which gives insight into its internal structure and composition," said Andrea Miglio of the University of Liège in Belgium.

Although the sunlike KIC 11026764, described during the briefing, isn't known to harbor planets, it demonstrates that researchers can use Kepler to precisely determine the diameter and age of stars that do host planets, said Travis Metcalfe of the National Center for Atmospheric Research in Boulder, Colo. That information can in turn advance Kepler's main mission of finding planets as small as Earth by detecting the tiny amount of light they block each time they pass in front of their parent stars.

The amount of light blocked during each minieclipse and its duration depends on the relative diameters of the parent star and the planet. By measuring the star's diameter, researchers can then determine the diameter of the orbiting planet and more importantly, accurately calculate the planet's density—an indication of the planet's composition and, perhaps, its potential for habitability.

It was particularly easy to record the flickering generated by sound waves within KIC 11026764 because the star is one of the brightest Kepler has examined. To find the star's age, Kepler scientists compared sound waves that traveled all the way to the star's core before heading toward the surface with those that did not. The hallmark of an aging star is that a significant amount of the hydrogen at its core has fused into helium, a denser material. Sound waves traveling through a denser medium have a higher speed, which enabled Metcalfe's team to peg the star at 5.94 billion years old with an accuracy of 15 percent. Previous age estimates for stars were at best known only to within 50 percent. The researchers also posted their findings online October 21.

"We're learning details about the internal structure of stars that we really can't learn about in any other way," said Kepler researcher Ron Gilliland of the Space Telescope Science Institute in Baltimore.

Because planets are born soon after their parent stars, the age of a stellar parent indicates the age of its planetary progeny. With Kepler's ability to determine age so precisely, the craft is expected to reveal the chronological evolution of planets, from young whippersnappers to the elderly.

Researchers also reported that Kepler had examined starquakes in more than 1,000 red giant stars—elderly, puffed-up versions of the sun. Information from those ongoing studies, posted online October 25 by Daniel Huber of the University of Sydney and his colleagues, could give researchers a better handle on the sun's fate when it becomes ared giant in some 5 billion years, such as its composition and how much it will expand.

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