Is Oumuamua an asteroid? An explanation weirder than aliens

Harvard researchers grabbed headlines with the notion that the famed interstellar object may have been an alien spaceship, but here are the other top theories.

Eric Mack Contributing Editor
Eric Mack has been a CNET contributor since 2011. Eric and his family live 100% energy and water independent on his off-grid compound in the New Mexico desert. Eric uses his passion for writing about energy, renewables, science and climate to bring educational content to life on topics around the solar panel and deregulated energy industries. Eric helps consumers by demystifying solar, battery, renewable energy, energy choice concepts, and also reviews solar installers. Previously, Eric covered space, science, climate change and all things futuristic. His encrypted email for tips is ericcmack@protonmail.com.
Expertise Solar, solar storage, space, science, climate change, deregulated energy, DIY solar panels, DIY off-grid life projects, and CNET's "Living off the Grid" series Credentials
  • Finalist for the Nesta Tipping Point prize and a degree in broadcast journalism from the University of Missouri-Columbia.
Eric Mack
5 min read
uropean Southern Observatory / M. Kornmesser

The space object named Oumuamua (Hawaiian for "scout" or "messenger") got lots of overheated attention this week after a widely respected Harvard scientist co-authored a paper suggesting the interstellar visitor could have been an alien spacecraft powered by solar radiation pressure

In fact, the new paper by Abraham Loeb and post-doctoral researcher Shmuel Bialy, which is still working its way through the peer review and publication process, seems to have garnered more attention than the original 2017 discovery of the first object to pass through our solar system from beyond.

But the truth is no-one knows exactly what Oumuamua is and the idea that it might be of alien origin isn't even necessarily the strangest theory put forward. 

Earlier, research teams suggested Oumuamua might be a probe and used radio telescopes to scan it for signs of artificial signals. Those observations all came back negative. Sadly we'll probably never be able to investigate Loeb and Bialy's hypothesis fully as Oumuamua has been traveling away from Earth at very high speeds for over a year now. 

Astrophysicist and cosmologist Katie Mack (no relation) suspects the fact it's difficult to disprove the "it might be aliens" theory might be part of the calculation behind publishing the paper.

"If you come up with something in the category of "not *obviously* wrong and also HUGE IF TRUE," the chance that publishing it will backfire is small, and the low-probability high-reward payoff might be tempting enough to make it worth facing the eyerolls of your colleagues," Mack tweeted about the paper and the huge response it's received. 

In other words, anytime something mysterious happens that's difficult or impossible to perform follow-up studies on, you just can't completely rule out aliens as a plausible explanation. Furthermore, anytime aliens could be a plausible explanation for something, someone is sure to step in and fill that vacuum. This time a big-deal Harvard astrophysicist and cosmologist filled that void, which caused the internet to convulse. 

Best places in space to search for alien life

See all photos

There's no hard evidence Oumuamua is an alien spacecraft -- Loeb just happened to notice it moved in a manner similar to a so-called "light sail" craft, like the one the Breakthrough Starshot initiative is working on. (Loeb also happens to chair Breakthrough Starshot's advisory committee.)

Loeb and Bialy's paper is just one of literally dozens about Oumuamua out there. It's not the first to propose the object could be artificial and it's not even the weirdest proposed origin story for the big interstellar cylinder or cigar or whatever you think it looks like.

Here's a brief rundown of the other theories for where Oumuamua came from. 

The invisible universe made visible?

One of the earlier and more far-out explanations proposed Oumuamua could actually be a big hunk of "macroscopic dark matter." Dark matter is the unseen material thought to make up much of the universe. 

"Contrary to widely held misconceptions, dark matter need not be in the form of weakly interacting elementary particles, but might instead be found in much larger pieces," reads the very brief paper by scientists at Case Western Reserve University, Canada's Perimeter Institute and Stanford. 

The researchers posit that if their hypothesis were true, Oumuamua's passage could have altered the orbits of Mercury, Earth and the moon. No one has yet confirmed any changes to those planetary paths. 

Crumbs from another solar system

One of the most popular explanations for Oumuamua's origin in the literature is the idea that it's left over from the process of planetary formation around another distant star. Basically an interstellar asteroid from across the cosmos.

It's thought the early days of any solar system is turbulent and chaotic. With pieces of debris all over the place some might even get knocked out of the system altogether. 

Watch this: NASA's hunting for exoplanets, and it's got its eye on the Goldilocks zone

One recent study used new data to try to narrow down exactly which star systems the vagabond object may have been exiled from. 

Another theory suggests Oumuamua may come not from the scraps of planetary formation, but from the leftovers of a planet's destruction. 

"I conclude that the origin of Oumuamua as a fragment from a planet that was tidally disrupted and then ejected by a dense member of a binary system could explain its peculiarities," the SETI Institute's Matija Cuk writes in an article in the Astrophysical Journal Letters

The idea here: a run-in with a dense red dwarf star may have ripped a planet apart, flinging at least one cigar-shaped piece in our direction. 

A comet out of a coma

Another early explanation was that Oumuamua was some sort of weird comet from another side of the galaxy shaped like no comet we've ever seen and lacking an obvious tail. It did, however accelerate on its way out of the solar system like a comet might as it gets a boost from heated ice and water aboard on its pass by the sun.

Various researchers have suggested perhaps it was a dead comet nucleus, a comet that was fragmented in a manner similar to the aforementioned planet fragment explanation or just a comet-like uh... thing.

Not that alien after all

There have also been a few suggestions that Oumuamua might not even be that alien. Some of the most recent research explores the idea it may have come from the edges of our own solar system. One paper went so far as to suggest its odd behavior and trajectory might be explained by having been "scattered" by a "yet unknown" planet in our solar system.

Yes, that's a reference to what's sometimes called Planet 9 or Planet X, another oft-confused and conflated concept that tends to drive the internet wild. 

A follow-up paper by noted astronomer Jason Wright from Penn State throws cold water on the idea that an unseen planet might have flung Oumuamua at us, however. 

As long as it stays on its current path, the mystery of humanity's first interstellar fly-by will remain. Well, unless 'Oumuamua suddenly makes a U-turn.

A 23rd-century tourist guide to the galaxy

See all photos

NASA turns 60: The space agency has taken humanity farther than anyone else, and it has plans to go further.

Taking It to Extremes: Mix insane situations -- erupting volcanoes, nuclear meltdowns, 30-foot waves -- with everyday tech. Here's what happens.