How the Oumuamua mystery shook up the search for space aliens

The interstellar visitor made it respectable to talk about alien spaceships.

Oumuamua likely traveled through space for millions of years before its chance encounter with our star system.M. Kornmesser/European Southern Observatory / NASA

Last year, an enigmatic object named Oumuamua startled astronomers when it came streaking past the sun, giving humanity its first close-up look at an object from beyond our solar system. This year, the interstellar visitor did something even more remarkable: It made it respectable to talk about alien spaceships.

The turning point came in November, when Avi Loeb, the head of the astronomy department at Harvard University, co-wrote a paper saying that Oumuamua is so unusual that scientists should consider the possibility that it’s not a far-out comet or asteroid, as his colleagues assumed, but rather an artificial structure.

In other words, maybe it’s an interstellar craft built by extraterrestrials.

Some of Loeb’s colleagues were intrigued. Others were disconcerted. But suddenly mainstream scientists were talking about how to tell if Oumuamua is a natural object or — as Loeb raised as a possibility in his paper — an alien spacecraft designed to capture the force of sunlight (a so-called lightsail).

Theoretical physicist Avi Loeb co-authored a controversial paper that raised the possibility that Oumuamua is an alien spacecraft.Jemal Countess / Getty Images file

Loeb is well aware that most scientists recoil from anything that sounds like UFO craziness, but he believes an overabundance of skepticism has cut them off from out-of-the box ideas. “The point of doing science is not to have a prejudice,” he says. “A prejudice is based on the experience of the past, but if you want to allow yourself to make discoveries, then the future will not be the same as the past.”

Jason Wright, a Penn State astronomer who recently launched a graduate program in SETI (the search for extraterrestrial intelligence), shares Loeb’s desire for open discussion — and offers an upbeat assessment of the field’s growing respectability. “There’s a real culture change. SETI is becoming a serious scientific discipline,” he says.

Strange visitor from the stars

It was clear from the start that Oumuamua (pronounced oh-MOO-uh-MOO-uh) would shake up astronomy’s status quo. Shortly after its discovery, scientists realized that the object’s unusual trajectory meant it had to have come from outside the solar system — and that it could have been traveling for millions of years. They quickly dubbed the mysterious object Oumuamua, a Hawaiian word meaning “messenger from the past.”

There were more surprises. Oumuamua was too far away for astronomers to observe its shape directly, but they could tell by the extreme way its brightness shifted as it tumbled through space that it wasn’t like any space rock they had ever seen.

“It’s very elongated, with an axis ratio of at least 7 to 1,” astronomer Karen Meech of the University of Hawaii said in an email. In other words, it’s at least seven times as long as it is wide — shaped like a cigar, perhaps. Or, as Loeb proposes in his paper, maybe a flattened disk.

Astronomers’ models predict that most of the small bodies wandering in interstellar space are comets. But when Meech and others examined it, Oumuamua showed no sign of the expected comet-like tail. It’s also quite small, on the order of 1,000 feet long, and it seems to be much more reflective than the comets we know.

Scientists said Oumuamua must be a highly elongated object because it varied dramatically in brightness over every seven- to eight-hour periods.NASA/JPL-Caltech

Intrigued by its oddities, several groups of SETI researchers listened for possible radio transmissions from Oumuamua — and heard nothing.

An invisible push

The biggest puzzle about Oumuamua was the way it moved. As it zoomed away from the sun, it sped up slightly, as if given an invisible push. “It doesn’t take much to provide the little acceleration we see,” says Michele Bannister, a comet expert at Queens University Belfast, “but the effect is definitely there.”

This artist's illustration shows Oumuamua racing toward the outskirts of our solar system, with annotations showing the locations of the planetary orbits.JPL / NASA

Comets often accelerate that way when gases boil off their surface under heat from the sun. But observations by the Spitzer Space Telescope showed no such material coming off of Oumuamua.

At this point, Loeb thought it was time to consider a more radical interpretation and, with a post-doctoral student, Shmuel Bialy, wrote the provocative paper. In it, the scientists consider the possibility that Oumuamua lacks a tail because it isn’t a comet at all, and that the acceleration was caused not by boiling gases but by the pressure of sunlight against a very wide, thin lightweight structure.

“The Spitzer data are consistent with a sail about 20 meters [60 feet] across,” Loeb says. He and Bialy suggest that such an object could be technological debris or even “a fully operational probe.”

Stirring the SETI pot

Loeb anticipated a harsh reaction to his paper, and his expectation was soon fulfilled. Twitter buzzed with snide comments. “A new paper, which claims Oumuamua is an alien probe, is full of poop,” one skeptic tweeted. “The Loeb paper says WHAT IF ALIEN LIGHT SAILS? Occam says no,” tweeted another in a reference to Occam’s Razor (a philosophical principle that when there are two competing explanations for an occurrence, the simpler one is usually better).

One researcher, who asked to remain anonymous, disparaged the Bialy and Loeb paper as “irresponsible,” and said it was “just out to grab attention.”

Loeb shrugs off the reflexive dismissals, but partly embraces that last critique, saying his lofty academic position actually obligates him to be a pot-stirrer: “I can say these things other people can’t because I have tenure at Harvard. The whole idea of getting tenure is to allow you to be free in your mind. I used the opportunity of Oumuamua to make a statement.”

In his view, searching for evidence of alien artifacts is no more outrageous than physicists exploring higher dimensions or astronomers invoking dark matter to explain the motions of galaxies. Or, more to the point, no more outrageous than searching for microbes on Mars.

“All I’m doing is following the standard scientific process, looking for explanations,” Loeb says.

What’s different is that the stakes are so high. Finding evidence of an extraterrestrial civilization would be far more profound than finding extraterrestrial microbes. It would tell us that humans aren’t the only intelligent species around.

A universe of questions

Wright believes the excitement over Oumuamua, combined with the other recent developments in the field, is breathing new life into SETI research.

“There’s a popular misperception that it’s just people listening for radio signals and coming up with one null result after another,” he says of SETI. He counters that there are actually a tremendous number of ways to search for alien life once scientists broaden their horizons.

Wright raises the possibility that a now-defunct alien civilization left behind artifacts on the moon, where they could have survived even if deposited there billions of years ago. His colleague Paul Davies at Arizona State University has enlisted students to comb through archival images of the lunar surface to see if they can find evidence of any alien “technosignatures.”

Wright has participated in studies of Tabby’s Star, whose intermittent dimming had some scientists wondering if it was encased within a vast artificial structure built by aliens. While the latest results show that’s not the case, Wright is undeterred. Almost all such searches are destined for failure, he says, and all it takes is one success to change the world.

With that in mind, he advocates digging through existing astronomical data to look for other stars whose peculiar behavior could indicate the presence of alien construction projects. “There are many rich datasets sitting out there to look through,” he says.

This is exactly the sort of unfettered curiosity that Loeb says he wanted to spark with his Oumuamua hypothesis. “Why have a prejudice? Why argue that it must be natural? What do we gain, other than putting blinders on our eyes?” he asks. “We should examine each and every interstellar object entering the solar system to check if it’s natural or not. Even if most of the time it’s natural, every now and then we might be surprised.”

And what a surprise that would be.

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