Visitor from outside the solar system passed us at high speed this week

A cosmic interloper — perhaps a comet, perhaps an asteroid — hurtled through the solar system last week, stunning astronomers and sending them scurrying to their telescopes to learn more about it. 

“It’s like a visitor from another solar system,” says Humberto Campins, an asteroid researcher at the University of Central Florida, Orlando, who was not part of the discovery team. 

And that makes it one of the most important astronomical discoveries of recent times. “I would compare it to the discovery of gravitational waves,” Campins says, “with the big caveat that this is not as important as gravitational waves. [Gravitational waves] opened a new window into the universe [and] this opens a new window into interstellar space.”

The object, designated A/2017U1, was first seen by researchers using a 1.8-metre telescope called Pan-STARRS, atop Hawaii’s 3,000-metre Haleakala volcano. Rob Weryk, a postdoctoral researcher at the University of Hawaii, spotted it on 19 October, initially thinking it was just a new near-Earth asteroid, which is what Pan-STARRS is designed to detect. Then he checked prior images from the telescope. “The motion wasn’t consistent with a normal asteroid,”he recalls. 

So he consulted with another astronomer, Marco Micheli, who took follow-up images with the European Space Agency’s telescope on Tenerife in the Canary Islands. “It became clear that this object had a hyperbolic orbit, suggesting it was from outside the solar system,” he tells Cosmos. 

Hyperbolic orbits are like slingshots, in which an object zooms by quickly, then curves away never to return. Normal orbits are ellipses that follow the same course, over and over.

Weryk estimates that A/2017U1 was moving at about 26 kilometres per second when it entered the solar system, then sped up as it dived toward the sun. It passed inside the orbit of Mercury, then shot back past Earth, coming within 24 million kilometres (about 60 times the distance to the moon) where it was spotted by Pan-STARRS. Now it’s receding so rapidly that it won’t be long until it’s out of sight of even the biggest telescopes.

At present, little is known about the object except for its orbit. Even its size is open to question, although a press release from the University of Hawaii puts its diameter at less than 400 metres.{%recommended 5978%}

“It comes from the direction of [the constellation] Lyra,” Weryk says, though he notes that this doesn’t mean the solar system from which it originated is actually in that direction. 

“It’s been traveling through space for so long you don’t know how it interacted with other things in the galaxy,” he says. “So you can’t infer anything about its source based on the direction it came from. There’s going to be a lot of work done on that.”

Meanwhile, the goal is to get as much data about it as possible. “For most telescopes, it will become too faint around November 2 or 3,” Weryk says. “So we have to get as much data as we can.”

Sarah Sonnett, a researcher with the Planetary Science Institute, who is based in Huntington Beach, California, adds that if A/2017U1 is indeed a comet, it may contain planet-building material from the dawn of some other solar system’s history, giving us our first up-close look into the early chemistry of another planetary system. “Comets are essentially fossils from a planet-building era,” she says. 

If it’s an asteroid rather than a comet, its composition won’t be as primordial, but it will still be interesting. Though, Sonnett notes, “this is a very difficult object to observe. It is moving incredibly fast on the sky, [and] most telescopes have a hard time tracking an object moving at those speeds. But it’s certainly worth a try. This is the first object of its kind, and may be the last we see for a very, very long time.”

That said, models of solar system formation indicate that gravitational interactions and near collisions cause infant solar systems to throw enormous numbers of objects — ranging from comets to full-grown worlds — out into interstellar space. If so, space may be teeming with them. Maybe, Campins says, we frequently get such visitors, and A/2017U1 is simply the first to come close enough to Earth for us to notice. 

If so, he says, there will be a “next time” when, armed with what we are learning from A/2017U1, we will be prepared to learn even more.