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THERE鈥檚 something puzzling about the 70 or so planets known to orbit other stars: the heaviest ones are too far out. All these planets are gas giants, like Jupiter or Saturn, and they are easiest to detect when they are in close orbit, so researchers expected to find them close to their stars. But an analysis of the known planets recently submitted to Astrophysical Journal Letters shows a lack of massive planets close to the stars. The authors, Shay Zucker and Tsevi Maseh, of Tel Aviv University in Israel, think astronomers may have to change their theories about the way planets form.