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.
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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.

All four hot, Jupiter-like planets discovered by Kepler have densities lower than that predicted for such giant, gaseous planets. One of these bodies, Kepler-7b, has one of the lowest densities—0.17 grams per cubic centimeter—of any known extrasolar planet. (By comparison, Jupiter’s average density is 1.33 grams per cubic centimeter, slightly higher than that of water, but Jupiter lies much farther from the sun than does Kepler-7b from its star.)

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.

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