As ‘Oumuamua barreled away, scientists pored over their data. The object was unlike any asteroids and comets they knew. It was rusty red, shaped like a cigar, and tumbling uncontrollably. The asteroids and comets in our solar system formed when the system was young, alongside the planets. Asteroids are mostly rocky because they formed closer to the sun, where the heat melted away any ice. Comets are mostly made of ice because they formed farther out, safe from the glare.

Astronomers had predicted an interstellar object would someday stumble into our view, but they had expected it to be a comet. Since our comets reside toward the edge of the solar system, significant disturbances—like the violent movements of young planets swirling into shape out of gas and dust—could send them flying out into the universe. Here at the edge, it’s much easier to escape the gravity of the sun. Perhaps ‘Oumuamua was a relic of a young solar system jostling into arrangements. Perhaps some comets that once resided in our young solar system are now hurtling through others.

But ‘Oumuamua appeared to be rocky, and it lacked signs of a distinctly cometary phenomenon. When comets get close to a star, the heat boils some of their ice. The reaction causes dust to fly off the comet and trail behind it, producing a glowing, white tail called a coma. ‘Oumuamua didn’t have a coma. So astronomers classified the mysterious space rock as an asteroid.

But then they started to see something strange in their data, particularly from the Hubble Space Telescope, which continued to track ‘Oumuamua after it faded from view from even the most powerful ground-based telescopes. As seen in the GIF at the top of this story, the sun’s power gravity bent ‘Oumuamua’s trajectory, acting like a slingshot. But “the path was not behaving as it would if it were just merely controlled by the sun’s gravity,” says Karen Meech, an astronomer at the University of Hawaii Institute for Astronomy, and one of the discoverers of ‘Oumuamua. Something else was giving ‘Oumuamua an extra push.

Astronomers used computer modeling to explore possible explanations for the mysterious acceleration, including potential effects of solar radiation, drag-like forces, and interaction with solar wind, the charged particles emitted by the sun. None of them fit. The best explanation, they found, was a process called outgassing. Outgassing occurs when an icy object gets too close to the warmth of a star and starts releasing gases that propel it forward, scattering dust off its surface as it goes.

This phenomenon occurs in comets. So if ‘Oumuamua got pushed around like this, then it had enough ice to get the interaction started. Which would make the mysterious space rock a comet, and not an asteroid.

The results, led by Marco Micheli, an astronomer with the European Space Agency’s Near-Earth Object Coordination Center, were published Wednesday in Nature.