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Kepler stars are bigger than thought, planet-hunting telescope finds

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A large exoplanet gas giant experiences powerful aurorae caused by energetic particles streaming from its host star. (CFA)

A team checking up on results from NASA's planet-hunting Kepler space telescope finds that most of Kepler's target stars -- and therefore any orbiting planets -- are bigger than expected. The discovery makes the search for small, Earth-like worlds more difficult. The team used a relatively large ground-based telescope at Kitt Peak National Observatory to probe 268 stars of the nearly 3,000 target stars Kepler was watching. The telescope, which was launched in 2009, last month lost use of its pointing system and currently is not operating. Troubleshooting efforts remain under way.

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The telescope works by measuring slight changes in the amount of light coming from selected sun-like stars. The idea is that some planets passing by will temporarily blot out a smidgen of light, relative to the telescope's line of sight. The percentage of light blocked relates directly to the size of a transiting planet or planets.

For example, a telescope positioned to peer into our solar system would see about 1 percent of the sun's light dimmed during Jupiter's transits.

"If we want to know the radius of the planet very accurately, we need to know the radius of the star that that planet transits. It's as simple as that," astronomer Steve Howell, with NASA's Ames Research Center, said at the American Astronomical Society meeting in Indianapolis this week.

Howell and colleagues found that most of the Kepler stars they studied were slightly larger than original estimates and about one-quarter of the stars were at least 35 percent larger than expected.

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"That means the exoplanets are larger than we thought," Howell said, adding that Kepler's confirmed planets, which currently number 132, would have had follow-up work done to accurately pinpoint the host stars' sizes.

The analysis, however, is relevant to the 2,740 planet candidates still awaiting confirmation.

By implication, the new results reduce the number of potential Earth-size planets found by Kepler, noted astronomer Mark Everett, with the National Optical Astronomy Observatory, which operates the Kitt Peak telescopes, among others.

Another implication of the research is that bigger stars' so-called "habitable zones" -- the regions where orbiting planets could have surface temperatures suitable for liquid water, a condition believed to be necessary for life -- are farther away than original estimates because bigger stars are brighter and radiate more heat.

"You would need to have planets that are in slightly longer period orbits to stay in the habitable zone," of stars that are slightly bigger than expected, Howell said.

It also means that some planets believed to be rocky worlds, based on how close they are to their parent stars, may actually be icy, gas bodies.

"I think there have been hints ever since the beginning of the (Kepler) mission when we started doing planet confirmations and getting spectra of even the very first stars that the brightest stars, which we started with, were larger and more evolved than we thought they were," Howell told Discovery News.

"So I think we weren't so surprised by this, but the large, uniform sample really shows that the effect is apparently true," he said. "There's a statistical statement here that you just can't ignore."

The research will be published in the Astrophysical Journal.

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