First Rock From Outside the Solar System Sails Past Earth

Approaching from above, it was closest to the sun on September 9. Traveling at 27 miles per second (44 kilometers per second), the comet is headed away from Earth and the sun on its way out of the solar system.

Astronomers around the world are scrambling to study an object unlike anything they’ve ever seen: a chunk of rock and ice seemingly fired our way from another solar system.

Discovered on October 18, the object is several hundred feet across and is currently speeding away from us at more than 98,000 miles an hour. At that speed, the space rock is moving fast enough to outrun the sun’s gravitational tug—implying that it was never part of our solar system to begin with.

The find marks a historic first for astronomers studying how stars and planets form. Scientists had long expected that the process of planetary formation results in chunks of ice and rock that, when given a nudge, could be flung into interstellar space. And in previous surveys, they had seen hints of this kind of interstellar material in the form of dust-size particles.

This object, known as A/2017 U1, is the first interloper of appreciable size that has flown through our cosmic neighborhood. The object could give scientists an unprecedented, if fleeting, opportunity to stare straight at the leftovers of an alien planet.

“This has been crazy-cool—for the asteroid community, this is as big as the gravitational-wave announcement,” says NASA astronomer Joseph Masiero, referencing the recent detections of ripples in space-time that have been amazing astrophysicists.

“This the first piece of evidence we’ve seen of how planets are built around other stars.”