Mysterious Visitor From Another Solar System Flew by Earth, Leaving Astronomers Baffled

The interstellar visitor's path through our solar system. NASA/JPL-Caltech

Astronomers may have spotted our first visitor from outside the bounds of our solar system: a hunk of ice called A/2017 U1 that a Hawaiian telescope identified on October 19. It's already headed back out away from Earth, and while scientists can study it for another few weeks, they say that after it waves goodbye, we'll never see it again.

Everything flying around space is pretty weird, but this one is extra weird, which is why astronomers are so excited about it—in fact, although scientists first dubbed it a comet, they're now not even sure what it is. The object's path has been difficult to track back in time from when it was first identified, but scientists are pretty sure it doesn't belong to our solar system. That would make it the first interstellar visitor to be observed.

And if it turns out to be truly out of this world, it would be precisely what astronomers have been looking for. "We have long suspected that these objects should exist," said Karen Meech, an astronomer at the University of Hawaii, which runs the telescope that spotted A/2017 U1, in a press release. "What's most surprising is that we've never seen interstellar objects pass through before."

Earth is no stranger to comets, of course, which in our solar system come from two main regions: the Kuiper Belt, which circles the cosmic block around Neptune, Pluto, and beyond; and the Oort Cloud, a bubble of ice lumps that surrounds the solar system at such great distances we've never even seen it. For context, the New Horizons mission that flew past Pluto is visiting a Kuiper Belt object on January 1, 2019, whereas the Voyager missions that launched 40 years ago won't escape the Oort Cloud for at least another 14,000 years.

But no matter where those comets come from, they've never had a path like this new object, except when they've been knocked off course, which doesn't seem to have happened to this speedy wanderer. (At its fastest, it clocked 27 miles per second.)

And the story gets weirder, since this adventurous little comet may not be a comet at all. As astronomers kept looking at the thing, they didn't spot a comet's trademark coma, the fuzzy blob of gas and dust surrounding a comet's icy core. That means it could actually be an interstellar asteroid instead. Needless to say, scientists aren't taking their eyes off it, in hopes of cracking its mysteries.