A few months later, another collaboration found that ’Oumuamua wasn’t just being pulled by the sun’s gravity. Instead, it was being slightly accelerated by an unseen force, which they argued could only be attributed to comet “outgassing” acting like a thruster. With this additional information, the case appeared to be closed. “Interstellar asteroid is really a comet,” read the headline of a press release put out by the European Space Agency.

The explanation seemed to fit with what we know about our own solar system. In the distant reaches beyond Neptune, countless comets orbit our sun. Anytime one of these comets gets too close to a planet, it could be ejected out into the galaxy. In contrast, there are far fewer asteroids in the asteroid belt, and they orbit closer to the sun, where they’re harder to knock into interstellar space. “There are more comets, and it’s easier to fling them away from a planetary system,” said Ann-Marie Madigan, an astrophysicist at the University of Colorado, Boulder. “For the first interstellar traveler that we see in our solar system, for that to be an asteroid, would be shocking.”

Yet comets have tails. And ’Oumuamua, if it was indeed made out of icy rock and propelled by jets of gas as it passed by the sun, should have displayed a tail that would settle the question of its origin. Yet no tail was ever found.

Now in a new study that is currently under peer review, Roman Rafikov, an astrophysicist at the University of Cambridge, argues that the same forces that appeared to have accelerated ’Oumuamua — the same forces that should have also produced a tail — would have also affected its spin. In particular, the acceleration would have torqued ’Oumuamua to such a degree that it would have spun apart, breaking up into smaller pieces. If ’Oumuamua were a comet, he argues, it would not have survived.

“There’s very strong and unequivocal evidence on both sides,” said Rafikov. “If it’s an asteroid, then it’s really unusual, with exotic scenarios for its formation.” He proposed such a scenario earlier this year, whereby an ordinary star dies, forming a white dwarf, and in the process rips apart a planet and launches the shards clear across the galaxy. ’Oumuamua is one of those shards. “Basically, it’s a messenger from a dead star,” he said.

In part to help resolve the impasse, researchers have tried to identify the star system where ’Oumuamua originated by combing through the newly released data troves of the Gaia space telescope. Perhaps it came from a binary star system, or a system with a giant planet, either of which could have launched the object into interstellar space.