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Starlight signatureThe black dots on each of these stars, simulated as yellow spheres, are extrasolar planets found by the Kepler spacecraft that pass in front of, or transit, their parent star. Below, recordings of starlight show a characteristic decrease, or minieclipse, each time a planet passes in front of the star.NASA, Borucki et al

NASA’s planet-hunting Kepler mission is off to a precocious start. The first six weeks of observations recorded by the spacefaring telescope, combined with follow-up studies from the ground, have revealed five previously unknown extrasolar planets—one body roughly the size of Neptune and four low-density versions of Jupiter. All reside within roasting distance of their parent stars.

The findings appear to reinforce hints from ground-based observations that stars have relatively few close-in planets with a mass between that of Saturn and Neptune, says Kepler scientist Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Lead mission scientist William Borucki of NASA’s Ames Research Center in Mountain View, Calif., and his colleagues announced the findings on January 4 at the winter meeting of the American Astronomical Society in Washington, D.C. The team also describes its results online January 7 in Science.

Astronomers say that the early results from the mission, which detects planets by recording tiny decreases in starlight whenever one of the orbs transits, or passes across the face of its parent star, also bode well for achieving Kepler’s main goal: finding Earthlike planets in or near the habitable zone of sunlike stars.

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Sizing them upThe five planets newly discovered by Kepler dwarf Earth. NASA, Borucki et al.

Launched in March 2009 and expected to last 3½ years, Kepler “has already established that Earth-size transiting planets can be found,” says theorist Sara Seager of MIT, a member of the discovery team. “We are salivating over the upcoming data and Kepler discoveries.”

The least-massive planet found by Kepler during its early observations, dubbed Kepler-4b, has a radius and density comparable to both Neptune and GJ 436b, a Neptune-like transiting planet observed by the European COROT satellite in 2007.

Even though Kepler-4b is blasted with 800,000 times more radiation from its parent star than is Neptune or GJ 436b, all three orbs are similar in size. That suggests that Kepler-4b has a denser, sturdier composition, with either a higher ratio of rock to water or a lower ratio of hydrogen to helium gas, Borucki and his colleagues note.

Although Kepler began observations only in May 2009, its ability to find a variety of transiting planets has already lent considerable significance to something it did not detect: a planet less massive than Jupiter but considerably heavier than Neptune. (Saturn has about a third the mass of Jupiter.) In the standard model of planet formation, Sasselov notes, the recipe to make a gas giant like Jupiter or Saturn requires that a rocky or icy core several times heavier than Earth must coalesce within the planet-forming disk around a young star. Only a core that heavy can snare the vast amount of hydrogen and helium gas needed to build a primarily gaseous planet like Saturn or Jupiter. If the core forms too late, the star will have already swept away most of they hydrogen and helium gas and a planet more like Neptune will remain instead.

The early Kepler results could be indicating “a clear separation in how you form gas giant planets like Saturn and Jupiter, and how you form planets made primarily of heavier materials, like Uranus and Neptune,” says Sasselov.

Seager says that, for now, the data are inconclusive. “It’s a tantalizing suggestion,” she says. “Kepler should be able to rule this idea out or confirm it with the next batch of planets.”

At the presentation, Borucki also noted that Kepler has identified another 100 planetary candidates. His team is now analyzing those to determine which ones, if any, might be actual extrasolar planets. 

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