Scientists May Finally Have an Explanation for Oumuamua's Bizarre Shape

Ever since scientists spotted their first interstellar asteroid in October, they've been stumped. The biggest question has been why this thing, now called Oumuamua, looks so darn weird compared to what scientists had predicted an interstellar asteroid would be like: It's long and thin like a cigar instead of a more typical lumpy shape, and it's not giving off the cloud of evaporating gas that scientists expected to see.

"People have been waiting for this kind of thing for decades," Sean Raymond, an astronomer at the Laboratoire d'Astrophysique de Bordeaux, in France, told Newsweek. "Since we only have one, we don't know if it's weird or if in our minds we were thinking wrong about what to expect." Raymond is the co-author of two new papers that try to explain what might have happened to Oumuamua to make it so odd.

That isn't just a matter of idle curiosity: Figuring out how Oumuamua came to be what it is now could help astronomers better understand how the universe works. In particular, Oumuamua could shed light on what happens as planets form.

Related: Panspermia: Mysterious asteroid Oumuamua could be a galactic bus carrying life between solar systems

That's because scientists believe Oumuamua and other objects like it—members of a class of objects astronomers call planetesimals—are what's left over from that process, the stuff that doesn't make it into planets themselves as they are clumping together. Raymond compares these planetesimals to "the things on the planet's plate that they didn't finish eating," like a messy baby, but with much worse table manners. "When they don't eat their food, they don't just throw it in the trash—they throw it out the window," he said.

And when those planetesimals get thrown out the window hard enough, they can end up swinging through other solar systems, like Oumuamua is doing now. But Raymond said that before getting kicked out, Oumuamua may have had a series of close encounters with local giants that gave it the weird look it has now. The two papers he worked on include simulations of how likely such encounters are.

First, a close pass by a large planet like Jupiter—with an extremely powerful gravitational tug—may have stretched a larger planetesimal so dramatically that it tore the object to pieces, plausibly accounting for Oumuamua's strange shape.

Then, Raymond's calculations show there's a chance that a fragment could get trapped orbiting close to its star for dozens or even hundreds of laps. If that happened to Oumuamua before it came to our solar system, it would explain why the object looks kind of like a comet but isn't producing the cloud of gas that comets usually release as they fly by a sun: It would have already used up all the material that could create that cloud during orbits in its home solar system.

According to Raymond's calculations, it's plausible that most hunks of rock floating between solar systems have been through a scenario like this. That's because that shredding may only happen to perhaps 1 percent of planetesimals, he says, but would create a huge number of individual pieces. His simulations suggest about two-thirds of those pieces could end up trapped in close orbit around their sun long enough to use up their store of easily evaporating compounds that create a comet's haze.

Oumuamua, here in an artist’s rendering, is long and thin like a cigar instead of a more typical lumpy shape, and it’s not giving off the cloud of evaporating gas that scientists expected to see. ESO/M. Kornmesser

"It fits everything we've got, so the story makes a lot of sense in explaining what [Oumuamua] looks like," Raymond said.

To determine if this theory is actually correct, scientists are going to need to wait to see several more interstellar asteroids to determine how well they match up with the scenario Raymond and his co-authors sketched out. And there's no way to know right now how long that will take, since astronomers aren't sure yet how many of these travelers are wandering through space. In fact, Raymond and his colleagues tried to tackle that question as well—but he says there are too many characteristics we don't know about them yet to calculate the number well.

"All we showed was that those numbers were not to be trusted," he said.