First Rock From Outside the Solar System Sails Past Earth

Astronomers were stunned to see the first known object flung through our cosmic neighborhood from interstellar space.

View Images

A/2017 U1 is most likely of interstellar origin. 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 19, 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.”


The gleeful frenzy began on October 19, when University of Hawaii Institute for Astronomy postdoctoral researcher Rob Weryk saw something strange. In images from the university’s Pan-STARRS 1 telescope, he spotted a speck in the night sky that seemed to be moving far too fast to comfortably orbit the sun.

RELATED: SOLAR SYSTEM 101 How many planets are in the solar system? How did it form in the Milky Way galaxy? Learn facts about the solar system's genesis, plus its planets, moons, and asteroids.

Weryk quickly notified his colleague Marco Micheli, who also had come across the strange object with a European Space Agency telescope in the Canary Islands. With both sets of images in hand, they traced the object's trajectory and calculated its speed.

"If you take the orbit and you backtrack it to the point before it enters the solar system, it was moving at about 26 kilometers per second (58,000 miles an hour), which is quite fast," says Weryk.

Based on their data, Weryk and a growing community of astronomers quickly suspected that this object was ejected long ago from an as-yet unknown star system, passing through our solar system in a chance event by way of the constellation Lyra. (Learn more about the Voyager probes, humankind's first forays into interstellar space.)

View Images

This animation shows the path of A/2017 U1, which is an asteroid—or perhaps a comet—as it passed through our inner solar system in September and October 2017. From analysis of its motion, scientists calculate that it probably originated from outside of our solar system.

But nailing down A/2017 U1’s ballistic arc proved tricky. As the preliminary data dribbled out, a few astronomers noted that if you exclude some of the less certain measurements, the remaining data traced an orbit around the sun, albeit an extremely stretched one.

Once astronomers knew where in the sky to look for A/2017 U1, though, many more people scrambled to spot the object. Further observations left little doubt that A/2017 U1 was moving too fast to be a local: By October 26, 500 Problems calling function 'fn:length'

Problems calling function 'fn:length'

Cannot serve request to /content/news/en_US/2017/10/interstellar-solar-system-asteroid-comet-space-science.html on this server