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When astronomers began spotting planets around distant stars in the mid-1990s, they were baffled. Many of these early discoveries involved worlds as big as Jupiter, or even bigger -- but they orbit their stars so tightly that their "years" were just days long. Nobody could imagine how a Jupiter or anything like it could form in such a hostile location, where the radiation of the parent star would have pushed the light gas -- which makes up most of such a planet's mass -- out to the farthest reaches of the solar system before it could ever coalesce.
But a handful of theorists already had a better explanation at hand. The giant planets could have formed in a much more sensible location, like the actual Jupiter did, and then migrated inward, establishing a stable orbit there. It all made sense, except for one tiny problem: this same model also suggested that a little world like Earth shouldn't exist at all; it (or more precisely, the Moon-size proto-planets that eventually assembled into the Earth) should have spiraled into the Sun more than 4 billion years ago. A star might not gobble a Jupiter whole when it moved close enough, but it could surely swallow a canapé like the proto-Earth.
"It's a problem," admits Mordecai-Mark Mac Low, an astronomer at New York's American Museum of Natural History. Or, rather, it was a problem -- but Mac Low and his collaborators may have solved it. In a paper recently submitted to the Astrophysical Journal, they argue that the old, Earth-destroying theory was generally accurate, but lacked some key details -- ones that both reshape theories about how planets form and, oh yes, allows the planet we know best to exist.
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