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


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:

Oh hi guys … lolhttps://t.co/TX10I0Ecrn

— Elon Musk (@elonmusk) November 6, 2018

Love Oumuamua jokes

— Elon Musk (@elonmusk) November 6, 2018

So if you see a headline saying that “scientists claim…” and it’s one paper and it’s an EXTREMELY BIG CLAIM, please keep in mind that the only thing you can conclude is that the authors of the paper (probably) didn’t see a reason the idea had to be 100% false.

— Katie Mack (@AstroKatie) November 6, 2018

I realize it is appealing to believe that the solar-sailing aliens will save us all but unfortunately our current situation is that we have to actually go out and #vote.

— Katie Mack (@AstroKatie) November 6, 2018

There have been Qs about the difference in 'Oumuamua's observed outbound trajectory vs its predicted path (see below.) This is most likely a result of outgassing, even if the particles were too large to detect. Confirmed? No. But still likelier than ALIENS https://t.co/qzzEosT56x

— Jason Major (@JPMajor) November 6, 2018

My publicist asked me for a quote on the 'Oumuamua story making the rounds. Here it is:

"No, 'Oumuamua is not an alien spaceship, and the authors of the paper insult honest scientific inquiry to even suggest it."

Feel free to use that, @fcain, @tariqjmalik!

— Paul M. Sutter (@PaulMattSutter) November 6, 2018

If "it's never aliens" is the thing I'm most remembered for in my career… that's fine.

— Miriam Kramer (@mirikramer) November 6, 2018

In a little detail, for those who have asked me: Shmeul and Avi's paper propose that an alien lightsail is pretty much the only explanation they can come up with for 'Oumuamua. But this depends on assumptions:

— Jonathan McDowell (@planet4589) November 6, 2018

(A) modelling the observations indicate an orbit with non-gravitational acceleration (seems secure, but maybe there's something weird with the observations that is throwing us off…)

— Jonathan McDowell (@planet4589) November 6, 2018

(B) The Rafikov paper cited by Shmeul and Avi looks at the most obvious non-grav explanation, gas boiling off the asteroid acting like a rocket and pushing it – 'outgassing'. Rafikov claims to rule this out, given fairly reasonable (but not absolutely certain?) assumptions

— Jonathan McDowell (@planet4589) November 6, 2018

(C) the next most common kind of non-gravitational acceleration for orbiting objects is radiation pressure acting on big flat sail-shaped objects. We don't know of any natural objects of the right size which would have that high an A/m ratio. (Doesn't mean there aren't any)

— Jonathan McDowell (@planet4589) November 6, 2018

(D) If it's not gravity, outgassing or light pressure, they can't think of any other force that could be causing the acceleration. (Doesn't mean there can't be one we haven't thought of).

— Jonathan McDowell (@planet4589) November 6, 2018

So, all of A,B,C,D mean 'Oumuamua's still a bit of a mystery. But our Bayesian prior probability for it being aliens is quite low (zero if you are @mirikramer) . The probability that one of A,B,C,D is wrong is low, but not as low. So it's (much) more likely that it's not aliens.

— Jonathan McDowell (@planet4589) November 6, 2018

Something to note about the paper though: Avi is the lead science person on Project Starshot, trying to figure out how to send exactly such a lightsail to Alpha Cen. So it's reasonable for him to be thinking about other species having done it first and what it would look like

— Jonathan McDowell (@planet4589) November 6, 2018

The way to read this paper is that here's the lightsail guy going "Hey, if you are into lightsails, note that 'Oumuamua looks just like a lightsail would look". Which is very very very very different from "it's probably a lightsail".

— Jonathan McDowell (@planet4589) November 6, 2018

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