The mysteries surrounding ‘Oumuamua, our solar system’s first known interstellar visitor, are many: How did it get here, and from where? What gave rise to its extremely elongated (circa 250 meters long), potentially cigar-like shape, which so starkly distinguishes it from any natural object ever seen orbiting the sun? And most of all, what caused it to “hit the gas” after it swooped by our star, accelerating away like a passenger-filled car that accidentally entered a bad neighborhood?
The most obvious explanation for ‘Oumuamua’s properties and behavior—particularly its anomalous acceleration—is that it is a comet from another star system, albeit a decidedly weird one. In this scenario, ‘Oumuamua would have been ejected from its home system by a gravitational interaction with a large planet, perhaps gaining its shape from the associated wrenching forces and subsequent eons of exposure to cosmic radiation. Its speedy departure from our inner solar system, then, would be due to its briefly spouting plumes of gas from its icy, light-warmed surface after its close passage by our sun. This is the explanation preferred by European Space Agency scientist Marco Micheli, University of Hawaii astronomer Karen Meech and their colleagues, who first reported ‘Oumuamua’s anomalous acceleration.
But observations of the object with the Spitzer Space Telescope failed to detect any signs of a comet-like tail that would be expected from such outgassing, and provisional estimates for the frequency of interstellar comets had earlier suggested we would be exceedingly lucky to ever witness an object as small as ‘Oumuamua passing through our system. Roman Rafikov, an astrophysicist at the University of Cambridge, dealt the idea another blow in a peer-reviewed study published last year. Rafikov calculated that essentially any outgassing sufficient to account for ‘Oumuamua’s observed acceleration would also dramatically increase the object’s spin—something difficult to reconcile with data that suggested the object has an approximately four-hour rotational period.
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Against the standard “comet” hypothesis, theorists have devised a host of bizarre possibilities: Perhaps ‘Oumuamua was a fluffy, fractal-like clump of dust, or a threadbare “comet skeleton” stripped of its weighty ices—either of which could be accelerated solely by the subtle pressure of starlight itself. Suffice to say, such low-density objects have never before been seen in the solar system. Most controversially, as postulated by the prominent Harvard University astrophysicist Avi Loeb, perhaps it was an alien artifact—a gossamer-thin sheet-like spacecraft engineered to coast on starlight through the interstellar depths. Such “star sails” are a crucial aspect of the Breakthrough Initiatives, a private effort to develop interstellar missions for which Loeb serves as chief scientific advisor. If we are contemplating building star sails now to visit nearby stars, Loeb reasons, perhaps other galactic civilizations have already constructed them as well to visit us.
Now, a new study set to appear in Astrophysical Journal Letters by Darryl Seligman and Greg Laughlin of Yale along with Konstantin Batygin of Caltech is seeking to resurrect the comet hypothesis. After revisiting all the available observations and constructing new models of ‘Oumuamua, they argue that the object’s acceleration can be explained if it is indeed a roughly cigar-shaped comet that outgassed perhaps 10 percent of its total mass as a nozzle-like jet of relatively pure water vapor that migrated across the surface in lockstep with the warmth of the sun. Such a jet would have been invisible in the Spitzer telescope’s observations, and the jet’s movement, they argue, would stabilize ‘Oumuamua against the wild spinning predicted by Rafikov’s models. This would imply, among other things, that ‘Oumuamua has not passed close to any other stars since leaving its home system, since otherwise it would have likely evaporated long before arriving here.
A numerical simulation of a nozzle-like jet migrating across ‘Oumuamua’s notional surface, following the sun’s warmth. This model may explain ‘Oumuamua’s mysterious acceleration as it left the inner solar system.
“We found that if the cometary jet continuously tracks the sun's direction, ‘Oumuamua rocks back and forth like a pendulum instead of spinning up,” says Seligman. That motion, in turn, would independently peg ‘Oumuamua at some 250 meters long—consistent with earlier size estimates derived from the object’s brightness. “I cannot say that we have ruled out a more sensationalist claim,” Seligman says, noting even so that their “more physically motivated outgassing model” would eliminate the “need to rely on less likely explanations.”
That comes as a relief for some astronomers, such as Micheli and Meech, who balk at invoking “aliens” for any astronomical curiosity. “The [study’s] overall reasoning seems sounds, and the results are a really good match to the observed characteristics of ‘Oumuamua,” Micheli says.
Others are less accepting. Implicit in the “comet” hypothesis, says Loeb, is the assumption that ‘Oumuamua should be a fairly typical object—or that our solar system and its retinue of comets should somehow be an outlier. Yet Seligman, Laughlin and Batygin’s model reinforces ‘Oumumua’s distinctiveness from native objects around our sun. “It does not look like at least 99.999 percent of the solar system’s comets,” Loeb says, suggesting that our a priori chances of having detected it in the first place with present-day telescopes are no better than a million to one. “Note that one in a million is a small probability,” he adds. “When I thought my wife is that special, I married her.” All of which could mean that either we are extremely lucky to have discovered ‘Oumuamua—or that there are deep flaws in our interpretation of it as a comet.
Rafikov, the author of the study that initially dismissed cometary outgassing as a plausible explanation for ‘Oumuamua’s acceleration, still stands by his result. “In a nutshell, [the authors’] explanation is that ‘Oumuamua is a rare unicorn, which is not new,” he says. “If ‘Oumuamua is not an ideally symmetric object, then the claimed non-gravitational force will cause its rapid spin-up…. I just don’t believe in ideally shaped comets; we have to consider the implications of deviations from perfect assumptions.” (Seligman and his co-authors insist their models have accounted for deviations from the ideal, which do not meaningfully change their results).
Ed Turner, an astrophysicist at Princeton University who along with Loeb in 2009 made among the first predictions for the size and prevalence of interstellar comets, says that the latest iteration of the “comet” hypothesis unavoidably relies on “special pleading to explain away some of [‘Oumuamua’s] puzzles.” Then again, he adds, the problem with pleading instead to “aliens” is that such wide-open speculations can easily explain everything while predicting very little. “I think,” he concludes, “that ‘Oumuamua is wonderful fun…. It is amusing how some authors seem determined to make the explanation as prosaic as possible while others are determined to have an exotic one.”
Alas, ‘Oumuamua itself is unlikely to ever reveal anything more about its mysterious nature: the object has already exceeded the orbit of Saturn, too faint and far away for further observations by even our best telescopes. And while it is not projected to breach the farthest reaches of our solar system for another 20,000 years or so, its outbound speed of nearly 50 kilometers per second still places it beyond the reach of any conceivable reconnaissance mission. The true test of all the competing explanations will only come when astronomers find additional interstellar interlopers—something that a next-generation observatory such as the Large Synoptic Survey Telescope may achieve as frequently as once per month when it comes online in the 2020s.
“We will definitely learn something new by studying more interstellar objects of [‘Oumuamua’s] size,” Loeb says. “Unless ‘Oumuamua is unique—in which case, it would appear even weirder than we thought.”