In September 2017, a very strange object streaked fast across the solar system and passed close to the sun before heading away. Shiny, oblong and potentially hundreds of feet in length, the object was unlike anything scientists had ever seen. Not exactly an asteroid. Not exactly a comet.
Five years later, scientists are still arguing over the object, which they’ve named ‘Oumuamua. That’s Hawaiian for “scout.” It’s a debate that could shake up whole fields of science.
On one side is a camp led by iconoclast Harvard physicist and noted alien-hunter Avi Loeb, who contends that we should at least consider the possibility that ‘Oumuamua is an alien spacecraft. On the other side is a loose confederation of scientists who argue for more pedestrian explanations for ‘Oumuamua’s mysteries.
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“What’s at stake in the debate over ‘Oumuamua is whether the scientific community can hold open every plausible hypothesis about the object's origins, without fear or prejudice, until there’s more evidence available—or until we think of better questions to ask about the object,” Wade Roush, a popular science lecturer and author of the nonfiction book Extraterrestrials, told The Daily Beast.
Loeb has blasted the skeptics for what he describes, in essence, as scientific laziness—and a failure to appreciate the stakes of the ‘Oumuamua debate. “If we find a partner in interstellar space, it will give a meaning to our cosmic existence,” Loeb told The Daily Beast.
Meanwhile, at least one of the skeptics has defended the opposite position by citing an ages-old logical principle: Ockham’s Razor, named for 14th-century English philosopher William of Ockham. The simplest explanation is usually the best one. Even if the more complex answer is more cosmically satisfying.
The debate over ‘Oumuamua currently centers on the object’s speed—more specifically, the nature of its acceleration as it zipped across the solar system, piling on velocity until it was traveling at nearly 200,000 miles per hour. Even for an asteroid or comet, that’s fast. NASA described it as “blistering.”
It’s conceivable that an asteroid might travel that fast, if it started its journey really, really far away—say, several star systems distant—and had plenty of time to pick up speed from the gravity of nearby stars before it arrived in our solar system, where our own sun grabbed ahold of it for another speed-inducing gravitational tug.
What doesn’t make sense is that, as astronomers tracked ‘Oumuamua through various telescopes, they detected what NASA described as “non-gravitational acceleration in the motion of 'Oumuamua.” In other words, acceleration that scientists can’t attribute to gravity alone.
Something inside ‘Oumuamua was adding speed to the object. To Loeb, that’s yet another clue that maybe, just maybe, ‘Oumuamua is an alien craft—one with some kind of propulsion system.
But in a paper that appeared in Nature on March 22, Cornell University astrophysicist Darryl Seligman and Jennifer Bergner, an astrochemist at the University of California Berkeley, challenged that conjecture.
Seligman and Bergner reminded readers that comets can produce their own acceleration when their ice sublimates into gas and shoots out into space, kind of like a jet engine. “We report that the acceleration of ‘Oumuamua is due to the release of entrapped molecular hydrogen,” Seligman and Bergner wrote.
That is to say, ‘Oumuamua isn’t a spacecraft. It’s just a very weird comet.
That explanation is consistent with Ockham’s Razor, Seth Shostak, an astronomer with the California-based SETI Institute, told The Daily Beast. “One should choose the simplest among the possible understandings,” Shostak said. “And while it’s certainly possible that ‘Oumuamua is actually an alien artifact, the simpler explanation is that it’s an asteroid or comet—possibly a fragment of one or the other.”
Shostak has a point. Scientists have observed lots of asteroids and comets—some quite strange—in our centuries of peering into the night sky. But we’ve never actually seen anything we know for sure is an alien craft. “Comet” is the simpler explanation for the strange object that visited our corner of the galaxy five years ago.
Soon after the Nature study was published, Loeb shot back: Seligman and Bergner’s explanation might be simple—but it’s wrong. For their theory to work, he argued, ‘Oumuamua would have to be hot enough to break apart the frozen water on its surface and send the leftover hydrogen shooting into space.
But in calculating ‘Oumuamua’s temperature, the Berkeley and Cornell scientists failed to take into account the cooling effect from any hydrogen that evaporated from the object’s surface as it traveled past the sun, Loeb said. “As a result of the decrease in surface temperature, the thermal speed of outgassing hydrogen is reduced by a factor of three,” the Harvard physicist wrote on a blog posted to Medium on March 23.
After cutting the jet-like effect of sublimating ice by a third, it’s no longer believable that ‘Oumuamua accelerated as a result of natural phenomena, Loeb argued. “When the emperor has no clothes, we better admit it,” he wrote. What he meant was, the scientist community should stay open to the possibility that ‘Oumuamua might be something new—an alien spacecraft—rather than just another asteroid or comet.
Bergen said Loeb’s counterpoint doesn’t change her team’s findings. “We have identified several reasons why the inclusion of effects such as [hydrogen] evaporative cooling… are unlikely to alter our conclusions,” she told The Daily Beast. But she declined to specify what those reasons are, saying she prefers to let the scientific peer-review process play out for any further formal studies Loeb might publish on the subject of ‘Oumuamua.
In a discussion streamed online and sponsored by the Lexington Observer news website on Tuesday, Loeb tried to explain why skeptics are so reluctant to entertain the idea that ‘Oumuamua might be a ship. Some scientists can’t seem to think beyond their own experience, he said. “People who have been studying rocks for decades—when they see something in the sky, they say it’s a rock.”
It’s a failure of imagination, Loeb said. An inability to see a new discovery for what it is. “Nature has better imagination than my colleagues,” he quipped.
Bergner objected to that characterization. She told The Daily Beast that scientists from many different fields are doing their best to understand ‘Oumuamua. “A lot of very novel and creative ideas have been proposed to try to explain its behavior.”
Roush conceded that the scientific establishment does have a “phobia of talking about extraterrestrials.” That phobia could work for skeptics such as Seligman and Bergner and against freethinkers such as Loeb. The former might be more likely to be taken seriously, even if their studies include basic flaws, like Loeb claimed.
“Personally I think Avi is more likely to be wrong than right about ‘Oumuamua,” Roush said. “But I will defend to the death any scientist's right to have their ideas heard respectfully and judged on the merits, even if they come with perceived cultural stigma.”
Brian Keating, a cosmologist at the University of California, San Diego, tried to stake out a middle ground between Loeb and the skeptics. They’re both doing important work, and in good faith.
Whichever side ultimately turns out to be correct—Loeb with his aliens hypothesis, the skeptics positing their odd comet—humanity stands to learn something new, Keating stressed. “If the object is a comet, it’s not as interesting,” he said, “but it still would be of note for astronomers because it isn’t like anything we’ve ever seen before.”
