If you are a scientist looking to indulge in a bit of speculation, or if you are a journalist looking for a sensational claim to attribute to a scientist, your best bet would be to set your sights on the penultimate paragraph of a scientific paper. It is here that scientists often challenge their ideas, present alternate hypotheses, or propose future research before reaffirming the most stripped down summation of the study’s conclusion.
Unfortunately, presenting this portion of a study as a headline -- while great for clicks -- rarely elucidates the actual scientific debate at hand and often muddies the factual information presented by the paper.
A prime example of this phenomenon hit the viral news machine in early November 2018 with headlines such as “Mysterious Interstellar Object Floating in Space Might Be Alien, Say Harvard Researchers” running in USA Today:
It's been dubbed a comet, an asteroid, and a new class of interstellar object. Now, a paper from Harvard astronomers suggests one more possibility into the mysterious object nicknamed "Oumuamua": Alien probe.
It is true that a draft of a study posted online on 1 November 2018 (set to be published in Astrophysical Journal Letters) written by two Harvard astronomers, Professor Shmuel Bialy and Department Chair Avi Loeb, suggested the possibility that a recently observed object of interstellar origin named ‘Oumuamua could actually be a device created by an alien civilization. (The character preceding “Oumuamua” is a Hawaiian symbol known as an ʻokina, which denotes a glottal stop sound.)
Their paper tested the plausibility that the object's apparent acceleration could be explained if the object were a thin, pancake-shaped sheet pushed by the force of solar radiation. Their speculation for why such a shape might exist is what grabbed the headlines. “A more exotic scenario is that ‘Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization,” the authors added toward the end of the paper. A myopic focus on this one sentence, while perhaps good for clicks, provided an incomplete view of what was on its own a fascinating scientific mystery.
What Is ‘Oumuamua?
On 19 October 2017, a ground-based telescope in Hawaii known as PAN-STARRS detected a rapidly moving object about quarter-mile in diameter with a wildly abnormal trajectory. In a paper published in Nature titled “A Brief Visit from a Red and Extremely Elongated Interstellar Asteroid,” a team of researchers concluded the object was an interstellar object detected passing through our solar system -- a first:
Designated as 1I/2017 U1, this object is clearly from outside our solar system, and as such is the first detected interstellar object. Given its discovery and follow-up observations from multiple Hawai‘i observatories, 1I/2017 U1 has been named ‘Oumuamua, which in Hawaiian reflects the way this object is like a scout or messenger sent from the distant past to reach out to us.
‘Oumuamua, which is moving too fast to be captured by the gravity of our solar system, has a dark red surface “consistent with the organic-rich surfaces of comets” and is tumbling and/or rotating rapidly.
What About ‘Oumuamua Were The Harvard Researchers Investigating?
On 27 June 2018, a group of researchers published another paper in Nature about ‘Oumuamua, this one exploring the observation that ‘Oumuamua was accelerating as a result of a force other than gravitational pull from nearby objects, suggesting that could be the result of cometary outgassing:
We report the detection ... of non-gravitational acceleration in the motion of ‘Oumuamua ... After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects..., we find comet-like outgassing to be a physically viable explanation, provided that ‘Oumuamua has thermal properties similar to comets.
Many researchers, including the authors of the headline-grabbing study, found the comet propulsion explanation lacking, as observations indicate the object is not a comet, and may more accurately be described as an asteroid. “Despite its close Solar approach,” the authors wrote, “‘Oumuamua shows no signs of any cometary activity, no cometary tail, nor gas emission/absorption lines.”
In their paper, Bialy and Loeb presented an alternate hypothesis to the comet propulsion explanation: “The possibility of ‘Oumuamua being a thin object accelerated by Solar radiation pressure, which would naturally result in an excess acceleration.”
What Did The Harvard Researchers Find?
Such an object, when created by humans, is known as a light sail (or solar sail), and for them to be effective they would generally need to be wide and very thin. The researchers at Harvard modeled what the object would have to be shaped like for the solar radiation explanation to work and investigated that theory’s overall plausibility based on those requirements:
For radiation pressure to be effective, the mass-to-area ratio must be very small. In [section two] we derive the required mass-to-area ratio, and find [it would have to be shaped like a sheet about 0.3 − 0.9 mm wide]. We explore the ability of such an unusually thin object to survive interstellar travel, considering collisions with interstellar dust and gas [in section three], as well as to withstand the tensile stresses caused by rotation and tidal forces [in section four]). Finally, [in section five] we discuss the possible implications of the unusual requirements on the shape of ‘Oumuamua.
While the exact geometry of ‘Oumuamua is unknown, the authors suggested, based on their calculations, that it could be possible for an object that thin to travel through interstellar space, and that what little is known about the object is consistent with a flat geometry:
Our inferred thin geometry is consistent with studies of its tumbling motion. In particular, Belton et al. (2018) inferred that ‘Oumuamua is likely to be an extremely oblate spheroid (pancake) assuming that it is excited by external torques to its highest energy state.
Pancake objects like this don’t exist in our Solar System, as far as we know. This, the authors pointed out, opens up their theory to a world of speculation about how such an object could exist in the first place:
While our scenario may naturally explain the peculiar acceleration of ‘Oumuamua, it opens up the question what kind of object might have such a small mass-to-area ratio? ... Known Solar System objects, like asteroids and comets have mass-to-area ratios orders of magnitude larger than our estimate for ‘Oumuamua. If radiation pressure is the accelerating force, then ‘Oumuamua represents a new class of thin interstellar material, either produced naturally, through a yet unknown process in the ISM or in proto-planetary disks, or of an artificial origin.
That speculation, as discussed earlier, is what is found in the near-final paragraphs of the paper authored by Bialy and Loeb:
One possibility is that ‘Oumuamua is a lightsail, floating in interstellar space as a debris from an advanced technological equipment ... Alternatively, a more exotic scenario is that ‘Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization.
Unfortunately, the authors conceded, it is unlikely we will ever know what ‘Oumuamua’s shape is, as it’s only getting farther away from us with every passing second. “Since it is too late to image ‘Oumuamua with existing telescopes or chase it with chemical rockets, its likely origin and mechanical properties could only be deciphered by searching for other objects of its type in the future,” they concluded.
What Do Other Researchers Think?
In an interview with The Verge, Avi Loeb, one of the Harvard researchers, made it clear he thinks an alien lightsail explanation is entirely plausible, if not the most likely explanation:
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.
Other researchers, however, feel as though the cometary outgassing explanations should not be discounted, given the fact that there were very few observations of `Oumuamua in the first place. At issue is the presence, or absence, of dust. Seeing gas exhaust from a ground-based telescope is challenging, but seeing the dust pushed off a comet as a result of outgassing is easier. Michele Bannister, an astronomer who has also studied `Oumuamua but is uninvolved in the Harvard research, told The Verge that telescopes may have simply missed the dust or that it may have less dust than expected:
Because of weather and what weather occurred with which telescopes on the planet, we weren’t able to potentially see the dust ... You can fit this [orbital path] with a straightforward comet-like object ... [`Oumuamua] just has to not emit as much dust as normal comets do.
Alan Fitzsimmons, an astronomer at Queen’s University Belfast who has argued that `Oumuamua may be a comet, told The Verge that some rare comets in our own solar system do not contain much or any dust. “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,” he said. “As a scientist, I can’t sit here and say I have 100 percent evidence this was a natural object ... It’s just that all the observations can be matched with a natural object."
Sadly for these debates, humanity discovered this mysterious object only after it had made its closest approach to the Sun. ‘Oumuamua is now traveling away from the sun at 27 miles per second in the general direction of the constellation Pegasus, and it is (almost certainly) not coming back.