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Kepler telescope's astonishing haul: 54 planet-candidates in 'habitable zone'

The Kepler space telescope is designed to look for planets like Earth that could have life. But no one expected it to find 54 planet-candidates at Goldilocks distances from their stars – not too warm or cold for life as we know it – in its first four months of operation.

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"It is an amazing system," says Jack Lissauer, a member of the Kepler team at NASA's Ames Research Center at Moffett Field, Calif., and the lead author of a paper describing the discovery in the Thursday issue of the journal Nature.

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The five innermost "planets are close in, and we never thought we'd see this many planets that aren't real, real tiny, this close to one another," he said. The five range in size from about twice the radius of Earth to slightly more than four times Earth's radius.

The densities the team calculated for the two innermost planets suggest they are a combination of rock, water, and perhaps gas. The three outermost planets of the five, however, are so large for their mass that "a substantial fraction of their volume must be made of hydrogen and helium gases," Dr. Lissauer says.

It's a planetary system "of a type we had no idea existed," he says.

This discovery alone may have been worth the price of admission, suggests Dr. Fischer.

It's "an absolutely staggering result," she says. "With five low-mass planets in the system, this discovery is every bit as momentous as 51 Peg was" – a reference to the discovery in 1995 of the first planet orbiting a sun-like star, 51 Pegasi.

Next step: confirmation

Indeed, 170 of the 155,000 stars Kepler is monitoring appear to be multiple-planet-candidate systems.

Based on the large number of planet-candidates Kepler has uncovered and the small patch of sky it is observing, "the stars around us have a huge number of planets and candidates for us to look at," says William Borucki, the mission's chief scientist. "If we find that Earth-size planets are common in the habitable zones of stars, it's very likely that life is common around these stars."

NASA launched Kepler in March of 2009. The craft is designed to detect extrasolar planets by recording the subtle dimming that occurs as a planet orbits across the face of its host star. This so-called "transit approach" to planet detecting can provide information about a planet's size.

A second approach – the so-called "radial velocity" method – detects a planet by the gravitational tug it imparts to its host star, which shows up as wobbles the star's spectrum. Radial velocity measurements can yield information about the planet's mass.

Kepler's finds are now awaiting confirmation by radial velocity measurements. If the candidates are planets, the radial velocity measurements will help yield the planets' bulk densities and thus a rudimentary idea of their bulk compositions.

Kepler's effort is one the public can join. Fischer explains that the researchers have established Planethunters.org – a website where people can look at graphs of the light output from the stars Kepler is observing and hunt for the telltale signs of a planet candidate. So far, she says, some 16,000 participants worldwide have identified hundreds of "solid" transiting-planet candidates, as well as previously undiscovered eclipsing binary stars.

IN PICTURES: Planets

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