A mysterious space rock, first spotted in 2017, bewildered astronomers — was it an icy comet, a rocky asteroid, or something entirely new? As the object, called ‘Oumuamua, hurtles away from us, the mystery may be solved: it’s accelerating like a comet.

Researchers tracked the space rock’s trajectory on its way out of this solar system, using telescopes on the ground and the powerful Hubble Space Telescope to keep watch even as the interstellar visitor faded out of sight. They discovered that ‘Oumuamua’s speed couldn’t just be the result of gravity. It was accelerating — which could be explained by gas puffing out of the sun-warmed end of a comet, the team reports today in the journal Nature,

It’s not quite like anything we’ve seen before in our Solar System

There’s debate over ‘Oumuamua’s identity because it’s not quite like anything we’ve seen before in our Solar System. Astronomers expected that the first space rock to visit us from outside our Solar System would be a ball of ice and rocks: a comet. After all, our planetary system flung icy objects into interstellar space when it was forming. Wouldn’t others, too? But there’s usually a cloud of dust and gas surrounding comets, and ‘Oumuamua didn’t appear to have one — which could mean it was made mostly out of rock and metal, like an asteroid.

In December, researchers led by Alan Fitzsimmons, an astronomer at Queen’s University Belfast, suggested ‘Oumuamua was indeed comet-like, it was just coated in a thick layer of carbon-rich grime that insulated the space rock’s icy heart. The rock’s exit from our neighborhood has settled most astronomers’ minds. It’s a comet, just an unusual one. “I love this paper because it shows I’m right. Of course I’m going to love this paper!” says Fitzsimmons, who wasn’t involved in today’s study. “The important thing about this paper — let’s be sensible for a moment — is that it’s a really careful analysis of how this object has moved through the solar system as we’ve been observing it.”

As the space rock careened away from the Sun — and us — a research team led by Marco Micheli, an astronomer at the European Space Agency center that studies near-Earth Objects, tracked it. They discovered that ‘Oumuamua was accelerating, and interactions with gravity from the Moon, the Sun, and nearby planets weren’t enough to explain it. “There was something else that was pushing ‘Oumuamua out from the sun, so it was moving faster than it should be just due to gravity alone,” Fitzsimmons says.

“That acts like little rocket thrusters.”

That’s exactly what you’d see with a comet. As the sun warms up a comet, its ice thaws to gas — just like dry ice does here on Earth. “So it’s this gas coming off of the comet that gives it a push,” says Karen Meech, an astronomer at the University of Hawaii and an author on today’s study. “That acts like little rocket thrusters.” That could explain ‘Oumuamua’s accelerations — although to be certain, the team explored other possibilities, too. Maybe the force of the sun’s radiation was giving the space rock an extra nudge. Or, maybe, the comet was magnetized, and was getting a push from the solar wind’s magnetic field.

But the best explanation was that ‘Oumuamua was a comet — sure, a weird one, but a comet. So why didn’t the researchers see that characteristic cloud of dust and gas around ‘Oumuamua? The dust could have been stripped from the comet as it flew through space, or maybe astronomers just missed it. And gas is actually hard to detect, Meech says. “You need a bright comet, or a really big telescope. And this was a very faint comet. So people tried, but the data were very noisy.”

“To be able to study that in any kind of detail is a truly magnificent endeavor.”

It’s also possible that astronomers were looking for the wrong gases. One gas that researchers would expect to see rising off of ‘Oumuama was cyanide, Meech says. “It emits light very strongly in blue colors, and no one saw it. If the chemistry was the same, we should have seen some,” Meech says. That means the space rock’s chemical makeup could be different from comets that originate closer to home — which is why this interstellar visit is so exciting, Meech says. “It gives us a glimpse of the process of building planets elsewhere,” she says.

Of course, the real proof would come from sending a spacecraft to get an up-close look at ‘Oumuamua’s surface, but that’s unlikely to happen. The space rock is too far away, and traveling too quickly. So Meech’s focus is shifting to the next one: “If, in the future, you could have a spacecraft ready and waiting to go, then that would be a much easier mission.”

Fitzsimmons, too, is looking forward to the discoveries that might fly into our Solar System with the next interstellar visitor. “How cool is that — we’ve been talking in recent years about trying to design a spaceship that might get to a nearby star,” he says. “Stars are actually sending us material from their own planetary systems. And to be able to study that in any kind of detail is a truly magnificent endeavor.”