S. Deiries/ESO

ScienceShot: Most Comets May Be Solar System Aliens

Comet McNaught, shown here in a 2007 Earth flyby, may need a passport when it returns later this month. New research suggests that it and trillions of its cousins were actually born light-years away. Computer simulations reported online today in Science reveal that the Oort cloud—a swarm of comets, including McNaught, that orbit the sun far beyond Pluto—contains too many comets for them to have been generated locally. In fact, the simulation suggests, over 90% of the comets in the Oort cloud probably originated around other stars. Billions of years ago, the researchers explain, our solar system congealed along with many others in relatively close proximity inside a nebula—a vast cloud of dust and gas that acts as sort of a galactic hatchery. As the sun condensed and migrated out of the nebula, it carried the rest of our solar system with it. But its gravity also snagged a huge number of comets from the Oort clouds surrounding its siblings in the process. So welcome back, McNaught, regardless of where you came from.

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