Was it an asteroid, comet, or even an alien spaceship? For years, astronomers have been perplexed by ‘Oumuamua, a mysterious object up to 400 meters long that entered the Solar System in 2017. No such object from beyond our Sun’s reaches had visited us before, with this interloper moving so fast it could not be bound to the Sun. ‘Oumuamua, as scientists christened it, was also odd in that it looked like an asteroid but behaved like a comet.
Now, a team of researchers says ‘Oumuamua was definitely a comet, albeit one with an unusual makeup. “We can explain a lot of the strange behavior,” says Jennifer Bergner, a chemist at the University of California, Berkeley, who led the work, published today in Nature.
The study is “the most convincing model so far” for ‘Oumuamua, says Marco Micheli, an astronomer at the European Space Agency in Italy who was not involved with the work. The alien visitor, he says, was actually not so different from Solar System comets.
‘Oumuamua was first spotted on 19 October 2017. A telescope in Hawaii spied it as it made its way past the Sun, reaching a top speed of 87 kilometers per second—too fast to have originated in the Solar System. Astronomers named the object 1I/2017 U1 (‘Oumuamua), Hawaiian for “a messenger from afar arriving first.”
NASA’s Hubble and Spitzer telescopes found that ‘Oumuamua had an oddly elongated, cigarlike shape between 100 meters and 400 meters long. It also sped up slightly as it left the Solar System. That can happen with comets as they recoil from the material they emit, explains Micheli, who led the initial work on ‘Oumuamua’s acceleration in 2018. ‘Oumuamua, however, showed no such ejecta. There was no visible coma of dust and gas around the object, nor any tail, both of which would be expected from a comet.
Bergner and her colleague Darryl Seligman, an astronomer at Cornell University, think they can now explain what happened. Their modeling shows ‘Oumuamua could have begun life as a regular water-rich comet around a nearby star, before being ejected. They found that high-energy cosmic rays that pervade the Galaxy, emitted by supernovae and other energetic events, could have turned up to 30% of the comet’s water ice into hydrogen, which could have become trapped in ‘Oumuamua’s ice as it journeyed through interstellar space.
As ‘Oumuamua approached the heat of the Sun, it would have released the trapped hydrogen, giving the object its observed speed boost. But molecular hydrogen, being much less massive than the carbon monoxide or carbon dioxide ejected in typical comets, would not have had the momentum to pull much dust with it, explaining the lack of a coma or tail. “We don’t need to invoke anything superexotic to explain this behavior,” Bergner says.
Previously, researchers suggested ‘Oumuamua might be an asteroid or even a shard of hydrogen ice formed in a cold interstellar cloud of dust and gas. Seligman, who proposed that idea in 2020, now favors the comet explanation. “This theory explains every single weird aspect of ‘Oumuamua,” he says. “It’s exactly what you’d expect an interstellar comet to be.
There were also widely debunked claims that ‘Oumuamua could be an alien probe firing out a thruster as it flew past our Sun. Can we now rule that out? “I would say so,” Bergner says.
The work provides a good explanation for what ‘Oumuamua actually was, says Alan Fitzsimmons, an astronomer at Queen’s University Belfast. “It pulls everything together.”
Yet Karen Meech, an astronomer at the University of Hawaii, Manoa, doesn’t rule out a simpler explanation. “I’m still feeling a normal comet model would work,” she says, noting that the object could simply have had a low amount of dust. “But I think this is a creative model. It might be right.”
No telescope can study ‘Oumuamua anymore. The object is now past Neptune’s orbit on its way out of the Solar System. “We’ll never truly know the identity of this object,” Bergner says.
But more objects like it are expected. An upcoming European mission called Comet Interceptor will park itself in orbit beyond the moon in the hopes of spotting a future interstellar object when and if one whizzes through the Solar System. “Hopefully we can be ready” for the next one, Bergner says.
Source: Science Mag