‘Oumuamua, oh my! Was interstellar object actually an alien solar sail? Not so fast

Alan Boyle
An artist’s conception shows what the interstellar asteroid Oumuamua might look like. Or does it actually look more an alien light sail? (ESO Illustration / M. Kornmesser)

‘Oumuamua is long gone from the inner solar system, but the mystery surrounding the interstellar interloper has been rekindled, thanks to a research paper written by two Harvard astronomers.

The paper, suggesting that the cigar-shaped object could have been an alien light sail, sparked headlines as well as skepticism from colleagues claiming that the astronomers were jumping to conclusions.

Among the skeptics is Doug Vakoch, who heads up METI, a San Francisco-based organization devoted to the study of alien contact. (The acronym stands for “Messaging Extraterrestrial Intelligence.”)

“I’d love to think that ‘Oumuamua is an extraterrestrial spacecraft that whipped past Earth, propelled by a stream of photons hitting its solar sail. But we need to be wary of conjuring up an explanation that fits the data gathered at one point in time, when we have no opportunity for follow-up observations,” Vakoch told me in an email.

The claims from Shmuel Bialy and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics are based on an analysis of ‘Oumuamua’s orbital trajectory, which brought it inward from interstellar space, around the sun, and then back outward in late 2017.

Researchers found that the object was subject to a bit of extra acceleration that couldn’t be explained by gravitational influences.

If ‘Oumuaumua (which was given a Hawaiian name meaning “a messenger from afar arriving first”) was a comet, the extra push might have been caused by the rocket-style effect of outgassing. And in fact that’s exactly what scientists surmised this summer, based on an analysis they published in the journal Nature.

Bialy and Loeb, however, say ‘Oumuamua showed no outward signs of outgassing while it was under observation. Instead, they consider whether the acceleration could have been caused by solar radiation pressure on the object. Their calculations showed that such could be the case, but only if the object was a broad sheet of material less than a millimeter thick.

“One possibility is that ‘Oumuamua is a lightsail, floating in interstellar space as a [piece of] debris from advanced technological equipment,” the authors write. Such a sheet could have survived the trip from another star system and would account for the object’s unusual dimensions, they say.

“Alternatively, a more exotic scenario is that ‘Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization,” they add.

Talk about clickbait … As you could imagine, some publications had a field day with the astronomers’ tentative claims. “Mysterious interstellar object Oumuamua ‘SENT BY ALIENS’ to survey galaxy – Harvard,” one headline read.

It’s natural for Loeb to think about alien light sails, considering that he chairs the advisory committee for the $100 million Breakthrough Starshot project. Starshot is aiming to send fleets of lightsail-propelled nanoprobes past the Alpha Centauri star system (and any planets that exist there) sometime in the next couple of decades. The research paper even refers to Starshot in its discussion of the alien lightsail hypothesis.

But if you follow the late Carl Sagan’s dictum that extraordinary claims require extraordinary evidence, then proving the hypothesis may be a lost cause. Even Bialy and Loeb acknowledge that ‘Oumuamua is now too distant to observe, either with existing telescopes or space probes. Instead, they suggest keeping watch for other oddballs like ‘Oumuamua.

“Deep wide-area surveys of the type expected with the Large Synoptic Survey Telescope will be particularly powerful in searching for additional members of ‘Oumuamua’s population of objects,” they write. “A survey for lightsails as technosignatures in the solar system is warranted, irrespective of whether ‘Oumuamua is one of them.”

Vakoch agreed. ” ‘Oumuamua is a modern-day Wow! Signal — something so freakish that it just might be from an advanced civilization, but so elusive that we’ll never know,” he told me. “SETI is an inherently conservative science, and ‘Oumuamua just doesn’t satisfy the stringent requirements for a confirmed detection of alien technology.”

At least it got Elon Musk’s attention. Here’s a roundup of tweets reflecting on the ‘Oumuamua mystery:

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