Planetary wrecking balls: how Jupiter might have destroyed Earth

'Hot Jupiters' are Jupiter-mass planets orbiting close to stars. A study suggests that they might have been kicked inward from their original orbit, destroying or ejecting other planets.?

It's lonely, being a hot Jupiter.

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But that seems to be the lot for Jupiter-scale extrasolar planets orbiting within a whisker of sun-like stars, according to a new study. It shows that such planets are likely to be the only surviving children of their host star.

The work not only sheds light on how such systems form, it also gives humans one more reason to appreciate their own Jupiter. Had the solar system's largest planet followed the same developmental process that hot Jupiters apparently have, Earth would have been either pulverized or sent hurtling into interstellar space.

?That would be bad for us,? observes Jason Steffen, a researcher at the Fermi National Accelerator Laboratory's Center for Particle Astrophysics in Batavia, Ill.

While finding an Earth-mass planet in a star's so-called habitable zone remains the holy grail of efforts to detect extrasolar planets, studying oddballs such as hot Jupiters provides insights into the processes that create the wide range of solar-system configurations researchers have so far uncovered.

The study, published Monday in the Proceedings of the National Academy of Sciences, aimed to answer a question astronomers and astrophysicists have been asking since the first hot Jupiter was discovered in 1995: How do such systems wind up with that configuration?

Gas giants orbiting other stars at distances that would fall well inside of Mercury's orbit were the first extrasolar planets discovered. Because of their mass and their close-in orbit, hot Jupiters' effects on their parent stars are more pronounced than in other systems. Once researchers had identified these planets as gas giants, the chin-scratching began.

In our solar system, Jupiter and the other outer gas planets formed beyond what researchers have dubbed the solar system's frost line: a region in the early sun's disk of dust and gas where water, ammonia, methane, and other hydrogen-bearing compounds freeze into ice grains. Inside the frost line, the rocky planets formed.

Two competing scenarios emerged to explain how Jupiter-like gas giants migrated inward. The new report has led one team member to come to a definitive conclusion in the debate. ?

The earliest explanation suggested that a hot Jupiter forms beyond the frost line, but gravity from a passing star, or perhaps another massive companion planet, kicks the Jupiter into a highly elliptical orbit around its star. Each time the planet passes close to the star, its orbit is gradually reshaped until the orbit is far less elliptical orbit and so close that its ?year? can be as fast as 19 hours.

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