Why we shouldn’t cry ‘aliens’ about that interstellar space rock just yet

The head of Harvard’s department of astronomy thinks that there’s a possibility that a strange object that visited our Solar System from interstellar space may be an alien probe sent from a distant civilization. He and a colleague outlined their idea in a paper published this week analyzing what the mysterious space object might be, setting off a media frenzy.

But let’s take a breath before we jubilantly cry “aliens.” A single idea about what this object could be doesn’t make it the only explanation, and many scientists still argue that a natural explanation is more plausible. To add a bit of context, one of the scientists making this “exotic” claim is currently working on an initiative to look for extraterrestrial life in deep space, by sending probes from Earth to other star systems.

The paper that captured everyone’s attention is written by Harvard astrophysicists Avi Loeb and Shmuel Bialy, who tried to describe some weird behavior exhibited by a space rock called `Oumuamua. Spotted last October, `Oumuamua is a mysterious object that is passing through our Solar System, coming from some unknown deep-space origin. Objects like this one are thought to pass through our Solar System all the time, but this is the first exo-comet — or a comet from outside our cosmic neighborhood — that we’ve ever detected.

In addition to being a rare find, `Oumuamua is a bit bizarre. Astronomers expected a visitor of this kind to be an icy comet, surrounded by a trail of gas and dust as it passed close to the Sun. But `Oumuamua seems to lack this kind of cloud, making it look more like an asteroid, which is mostly made of rock and metal. So no one was quite sure what this thing was — a comet, an asteroid, or something totally new. Then after analyzing `Oumuamua’s orbit, scientists from the European Space Agency noticed that the object was accelerating, more so than it should be if it was just interacting with the gravity of the planets and Sun in our Solar System. They concluded that `Oumuamua must be a comet; the Sun is likely heating up ice within the object, creating gas that provides an extra boost of speed.

A graphic showing `Oumuamua’s path through the Solar System.
ESO/K. Meech et al.

However, Loeb and Bialy are skeptical about this “outgassing” claim, mostly because no one was able to observe gas and dust coming from `Oumuamua. They also point to recent research from another lab, which is still under review by other scientists, that indicates that if gas were coming from this object, it would change how the rock is rotating — something that hasn’t been observed. “This rules out the possibility that it’s a comet,” Loeb tells The Verge.

So they decided to look at another possible explanation for the acceleration: could `Oumuamua be getting faster thanks to light from the Sun? Our Sun’s light can actually exert force onto objects, giving them a slight push. Perhaps this phenomenon is enough to explain why `Oumuamua is getting faster. However, if this mechanism — known as solar radiation pressure — is to blame for the extra speed, then `Oumuamua would have to be extremely light and super thin, just one millimeter thick.

That gave Loeb the idea that `Oumuamua could be what’s known as a light sail — a thin artificial sail that rides on sunlight. And this light sail may have actually been sent here on purpose. “A more exotic scenario is that `Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization,” he and Bialy write in their paper.

Loeb has been looking into light sails for years now. He is the chair of the advisory board for Breakthrough Starshot, an initiative that calls for sending a super-thin light sail spacecraft into interstellar space, propelled by a giant laser. Loeb admits that his work with Starshot gave him the idea that `Oumuamua could be an alien light sail. “Our imagination is limited to what we know about,” he says. “And the fact that I’m involved in a project that uses the light sail allowed me, or encouraged me, to think about it.”

Loeb says he welcomes other explanations that don’t involve aliens, but he’s fairly certain that his idea is correct. “I cannot think of another explanation for the peculiar acceleration of `Oumuamua,” he says.

But other scientists argue that a natural explanation can still apply here. Just because we haven’t seen gas and dust coming from `Oumuamua doesn’t mean this material isn’t there. Scientists only had about two weeks to observe this object back in late October, before it got too far from Earth and became incredibly faint to see with ground-based telescopes. The Hubble Space Telescope was then the only tool we had to observe `Oumuamua until December, and the observatory mostly tracked its orbit around the Sun.

So it’s entirely possible that the telescopes we used to observe `Oumuamua just weren’t able to see the materials flowing off the object. That may be because we didn’t observe the object in the right kind of light, or certain crucial telescopes weren’t available to view this peculiar space rock. “Because of weather and what weather occurred with which telescopes on the planet, we weren’t able to potentially see the dust,” Michele Bannister, an astronomer who studied `Oumuamua at Queen’s University Belfast, but was not involved with this research, tells The Verge.

The comet ISON and its tail of gas and dust, as seen by the Hubble Space Telescope
Image: NASA

It’s also possible that `Oumuamua just doesn’t emit a lot of dust, which would make this outgassing effect harder to observe. “You can fit this with a straightforward comet-like object,” Bannister says. “It just has to not emit as much dust as normal comets do.” Comets in our Solar System usually release countless microscopic dust particles that reflect sunlight, which can then be seen from Earth. If `Oumuamua doesn’t have a lot of dust on its surface, then it may only be releasing gas, which is easier to miss. And there are examples of these kinds of objects in our own Solar System. “We have comets we know of — rare comets that must be said — in our Solar System, that emit so little dust that you have to look for the gas to actually see the outcome,” Alan Fitzsimmons, an astronomer at Queen’s University Belfast, who wrote a paper explaining how `Oumuamua could be a comet, tells The Verge.

Typically, astronomers find it to be good practice to exhaust all of the possible natural explanations for an observed phenomenon before resorting to the alien argument. There’s a quote, made popular by astronomer Carl Sagan, that many astronomers resort to when aliens are propped up as an explanation: “Extraordinary claims require extraordinary evidence.” Most agree there is no extraordinary evidence to back up the light sail claim right now. “Every observation we have on this object, comes with a huge uncertainty because we have very little data on it,” Katie Mack, a theoretical astrophysicist at North Carolina State University, tells The Verge. If anything, this alien artifact would seem to be out of control, as it’s chaotically tumbling through our Solar System. “If it was an alien spacecraft, it was the Brexit of alien spacecraft. It was a complete muck-up,” says Fitzsimmons.

However, sometimes astronomers will write up wild theories like this, so that those in the community can dissect the claim and pick it apart. “Sometimes you write a paper about something that you don’t believe to be true at all, just for the purpose of putting out there,” says Mack. Loeb, on the other hand, doesn’t want people to discredit his idea just because it might be inflammatory. “We should not dismiss this possibility just because some people don’t like to hear it, he says. “The point is we should not have a prejudice in science. We should base our conclusions on evidence, on data, and not on prejudice.”

But whenever aliens are invoked, that’s usually the explanation that gets the most attention. A similar situation occurred in 2015, when astronomers proposed the idea that a distant star’s weird behavior might be explained by alien megastructures in the star’s orbit. The theory, which many have been skeptical about, became so ubiquitous that the star eventually became referred to as the “alien megastructure” star.

Of course, the possibility exists. But aliens are a very bold claim to make when natural explanations are still on the table. “I can understand the excitement, and as a scientist, I can’t sit here and say I have 100 percent evidence this was a natural object,” Fitzsimmons says. “It’s just that all the observations can be matched with a natural object.”

And that could be a problem when we actually do find signs of alien life one day. Astronomers are finding new planets outside our Solar System all the time, and we’re working on more sophisticated technology to peer into the atmospheres of these worlds. One day, we may find solid evidence that life exists in deep space, but it may be hard for the public to swallow if they think aliens have already been discovered. “I don’t want people to think we already saw that when it actually happens,” says Mack. “I want people not to be super cynical about claims about aliens by the time we actually have something that is really solid evidence.”

Mary Beth Griggs contributed to this report.

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