by Avi Loeb
A billiard ball sits on a pool table, unmoving, even as fourteen other balls careen around it; it remains unmoving until another ball strikes it.
A solitary billiard ball sits on a pool table, unmoving, until a pool cue strikes it.
A billiard ball sits, stationary, on a pool table until someone lifts up one end of the table.
A billiard ball sits on a pool table, not moving, until a conical depression suddenly appears in the middle of the table.
In either of the last two cases, gravity takes hold and the ball starts to move. Once moving, it will do so in a line dictated by the force that acted on it and will continue traveling along that line until another force acts on it.
Trajectory of ‘Oumuamua through the solar system, showing the location (inset) of the object and the planets on October 19, 2017, the date that ‘Oumuamua was discovered by PanSTARRS. Unlike all asteroids and comets observed before, this object was not bound by the Sun’s gravity. ‘Oumuamua originated from interstellar space and returned there with a velocity boost as a result of its passage near the Sun.
Image by Mapping Specialists, Ltd. adapted from European Southern Observatory/K. Meech et al. (CC BY 4.0)
‘Oumuamua entered our solar system on a trajectory that was roughly perpendicular to the orbital plane of Earth and the other planets. Just as the Sun exerts its gravitational force on those eight planets and everything else orbiting it, the Sun exerted its gravitational force on ‘Oumuamua. On September 9, 2017, ‘Oumuamua sped around the Sun at almost 200,000 miles per hour, gaining momentum from the Sun’s gravitation, and then kicked in a different direction of motion. Thereafter it continued its journey through and beyond the solar system.
The universal laws of physics allow us to predict with certainty what a given object’s trajectory should be as it speeds around the Sun. But ‘Oumuamua didn’t behave as we expected.
In June 2018, researchers reported that ‘Oumuamua’s trajectory deviated slightly, but to a highly statistically significant extent, from a path shaped by the Sun’s gravity alone. This is because it accelerated away from the Sun, being pushed by an additional force that declined roughly as the square of the distance from the Sun. What repulsive force, which opposes the attractive gravitational force, can be exerted by the Sun?
Comets from the solar system show a deviation similar to ‘Oumuamua’s, but they are accompanied by a cometary tail of dust and water vapor from ice heated by sunlight.
If you have been lucky, you’ve seen a comet from your backyard. You’ve certainly seen photographs of comets or artists’ renderings of comets, their centers, or nuclei, fuzzily aglow and illuminated tails stretching out behind them. The glow and tail are due to the fact that comets are icy rocks of varying size. Their ice is composed mostly of water, but reflecting the random distribution of materials throughout the universe, that ice often includes other substances—ammonia, methane, and carbon, for example. Whatever the ice’s composition, it usually evaporates into gas and dust that scatter sunlight as the comet passes close to the Sun. This is what causes the comet’s coma, the enveloping atmosphere of evaporating ice and debris that gives a comet its glow and produces its distinctive tail.
If that tail reminds you of fuel coming out the back of a rocket, it should. The comet’s evaporating ice acts like a jet that pushes the comet. Because of that rocket effect, an outgassing comet can deviate from a path shaped by the Sun’s gravitation alone. Indeed, when astronomers observe such a comet, we can be precise. When we see an outgassing comet and measure the extent of its deviation, we can calculate how much of the comet’s mass was used up in giving it this extra push.
If the extra push that propelled ‘Oumuamua was from the rocket effect, as it is for comets, then our interstellar object should have lost a tenth of its mass in order for it to be propelled as much as it was. This is not a negligible amount of outgassing that could easily have been missed by our telescopes. But deep observations of the space around ‘Oumuamua did not reveal any trace of water, carbon-based gases, or dust, ruling out the possibility that it was being pushed by cometary vapor or visible dust particles. Moreover, it did not change its spin rate as it should have if one-sided jets were pushing it sideways, as they often do in comets. Then, too, such a massive evaporation would have changed the tumbling period of ‘Oumuamua, a phenomenon that is seen in solar system comets. No such change in the spin rate was recorded.
Ultimately, all of these mysteries can be traced back to one: ‘Oumuamua’s deviation from its expected path. All hypotheses as to what ‘Oumuamua is have to account for that deviation, and that means explaining the force that acted on it while respecting the fact that if there was any cometary tail of gas and dust behind it, that tail was slight enough to go undetected by our equipment.
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At the time of this writing, the scientific community has coalesced around the hypothesis that ‘Oumuamua was a comet, albeit a peculiar one. A virtue of this hypothesis is its familiarity. We have observed many comets whose trajectories deviated from paths shaped by the Sun’s gravity alone. We also know why that happens: in all cases, it is due to outgassing.
But as I have just explained, ‘Oumuamua showed no outgassing. And yet it deviated.
We know that ‘Oumuamua showed no outgassing visible to the infrared camera aboard the Spitzer Space Telescope. After its launch in 2003, Spitzer spent almost two decades circling about a hundred and fifty-five million miles above us while gathering an extraordinarily detailed body of information about our universe. And while its store of liquid helium, used to cool certain of its instruments to make them operable, was exhausted in 2009, its infrared array camera (IRAC) remained operational until January 2020, when it was finally taken offline.
The infrared camera on the Spitzer Space Telescope was ideal for surveying how much carbon dioxide comets produced. To an infrared camera, sufficient carbon dioxide is plainly visible. Because carbon is routinely part of comets’ icy mix, and carbon dioxide is routinely the by-product of the evaporation of that mix when it is put under heat and stress, we frequently used Spitzer to observe comets’ passing.
IRAC was trained on ‘Oumuamua for thirty hours as our interstellar visitor sped past the Sun. Had there been even trace carbon dioxide in ‘Oumuamua’s outgassing, the camera should have been able to observe it. But IRAC saw nothing—not a trail of gas behind the object and certainly not the object itself. (Interestingly, the Spitzer Space Telescope did not detect any heat being emitted from ‘Oumuamua either, implying that it must be shinier than a typical comet or asteroid; that is the only way it could have reflected as much sunlight as it did while still being small enough not to produce much heat.)
In a paper summing up their findings, the scientists who studied the IRAC data acknowledged that they “did not detect the object.” However, they went on to state that “ ‘Oumuamua’s trajectory shows non-gravitational accelerations that are sensitive to size and mass and presumably caused by gas emissions.”
Presumably. Having inserted that question mark in the middle of the sentence, the authors accurately concluded their article’s abstract with the statement “Our results extend the mystery of ‘Oumuamua’s origins and evolution.”
Other scientists using state-of-the-art equipment recorded results similar to the IRAC data. In 2019, astronomers reviewed images collected by the Solar and Heliospheric Observatory (SOHO) and the Solar Terrestrial Relations Observatory (STEREO) taken in early 2017 when ‘Oumuamua was near perihelion (closest to the Sun). Built to observe the Sun, STEREO and SOHO were not intended to be comet finders (although after the latter identified its three thousandth comet, NASA declared it “the greatest comet-finder of all time”). Just like Spitzer, SOHO and STEREO did not detect anything in the area; to these instruments, ‘Oumuamua was invisible. This can only mean that ‘Oumuamua had a “water production rate” that was “smaller than any of the previously reported limits by at least an order of magnitude.”
Invis
ible to Spitzer’s IRAC, to SOHO, and to STEREO—and yet ‘Oumuamua deviated.
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To explain ‘Oumuamua’s trajectory and retain the assumption that it was a comet, scientists have strained to the breaking point their theories about its physical size and composition. For example, some scientists have hypothesized that ‘Oumuamua’s ice was entirely made of hydrogen, and this extreme composition explains why IRAC did not see it. (Outgassing containing carbon is visible to IRAC’s infrared camera, but outgassing of pure hydrogen would not be.) In a detailed paper, my Korean collaborator Thiem Hoang and I calculated that a hydrogen iceberg traveling through interstellar space would evaporate long before it reached our solar system. As the lightest element in nature, hydrogen easily boils off an icy surface that is warmed up by interstellar radiation, gas and dust particles, and energetic cosmic rays. In fact, the periphery of the solar system is populated by numerous icy comets that are exposed to the same harsh environment (and the solar wind is unable to shield them since it is capped by the pressure of the interstellar medium much closer to the Sun). But a comet with ice composed of pure hydrogen—or, for that matter, pure anything—would be wildly exotic. We have never seen anything remotely like it before.
Or, rather, we know of nothing like it that is naturally occurring. To be sure, we have built such things; for instance, spacefaring rockets, for which pure hydrogen is the preferred fuel.
There is yet another difficulty with the outgassing-comet hypothesis, regardless of whether ‘Oumuamua outgassed pure hydrogen or not. Its acceleration during deviation was smooth and steady. Comets are ungainly rocks; their rough and irregular surfaces retain unevenly distributed ice. As the Sun melts the ice and the outgassing produces propulsion, it does so across that rough and pitted surface. The result is what you would expect—a herky-jerky acceleration. But that is not what we saw ‘Oumuamua do. In fact, it did the very opposite of that.
The odds of a naturally occurring comet composed of 100 percent hydrogen ice that outgasses from one location producing smooth acceleration? About the same as the odds of natural geological processes producing a space shuttle.
Moreover, to account for ‘Oumuamua’s degree of deviation, a statistically significant portion of its total mass would have had to be outgassed. The nongravitational push was substantial enough—about 0.1 percent of the Sun’s gravitational acceleration—that cometary outgassing could be responsible for the deviation only if the process had expended at least 10 percent of ‘Oumuamua’s mass. That’s a lot, and of course, this percentage represents a greater and greater amount of material the larger we hypothesize ‘Oumuamua was; 10 percent of a thousand meters of stuff is more than 10 percent of a hundred meters of stuff.
Then, too, the more material we have to imagine ‘Oumuamua invisibly outgassed, the less likely it becomes we would have failed to observe it. And the smaller we have to imagine ‘Oumuamua was in order to explain why we did not see the material it outgassed, the odder its luminosity and width-to-length ratio becomes—and the shinier it would have had to be.
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Outgassing isn’t the only explanation for why an object would deviate from a path shaped by the Sun’s gravity alone. Another explanation has to do with the object’s disintegration.
If an object fractures, breaks up, and becomes smaller objects surrounded by dust and particles, the smaller objects follow a new trajectory. Thus, if ‘Oumuamua began to break up around the time it reached the perihelion, that disintegration might have caused the object to deviate from the path dictated by the Sun’s gravity.
The problem with using this explanation in the case of ‘Oumuamua is that, just as with outgassing, our telescopes should have been able to register something—in this case, the relic fragments and dust from such a disintegration. It is unlikely that ice would have no carbon and even more unlikely that a disintegrating rock would contain no carbon. Further, one must wonder whether a collection of smaller objects would appear as a single body. ‘Oumuamua, the evidence shows, continued to tumble every eight hours like a solid object with a persistent extreme shape.
The object’s smooth acceleration also defies the hypothesis that ‘Oumuamua fractured around perihelion, breaking up and losing enough of its mass to explain its deviated path. Our instruments observed no debris indicating such a fracturing and disintegration; in fact, we saw evidence of the opposite: a smooth, steady acceleration. Had ‘Oumuamua started to break apart, the odds of it doing so while retaining smooth acceleration is, again, infinitesimal. Imagine a snowball thrown into the air that suddenly explodes into pieces but without any shift in the trajectory of the pieces.
For the disintegration hypothesis to hold, we are forced to make ever more exotic assumptions about ‘Oumuamua’s composition to explain why we would not notice the vapor of the fragmented debris. Fragmentation should have increased what our instruments detected. After all, many small pieces of a disintegrating rock would increase the total surface area available, producing even more cometary gases and heat than the parent object alone.
And then there is the evidence that the extra force acting on ‘Oumuamua, the force that was causing it to deviate, declined in inverse proportion to the square of ‘Oumuamua’s distance from the Sun. If the extra force were the result of outgassing, we would expect a faster deceleration of an object as it rapidly distances itself from the Sun. Evaporation of ice and water halts due to insufficient heating by sunlight, which ends the rocket effect. A rocket exhausts itself, and the extra force it was providing an object abruptly ceases; whatever path the object was on when that occurred is the path it thereafter follows. That is not what we saw ‘Oumuamua do. Again, the force acting on it declined in inverse proportion to the square of ‘Oumuamua’s distance from the Sun.
What else could push ‘Oumuamua in this smooth power-law form? One possibility is the momentum delivered to ‘Oumuamua’s surface by reflected sunlight. But for that to be effective, the surface-to-volume ratio needs to be unusually large. This follows from the fact that the solar push acts on the surface of the object, whereas the mass of an object (with some particular density of material) scales as its volume. Hence, the acceleration exhibited by the object increases in proportion to an increasing surface-to-volume ratio, which is maximized for an extremely thin geometry.
When I read reports that the extra force on ‘Oumuamua declined inversely with the distance from the Sun squared, I wondered what could be pushing it if not outgassing or disintegration. The only explanation that came to mind was the sunlight bouncing off its surface like wind off a thin sail.
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Other scientists were busily crafting their own explanations. In pursuit of a theory that made sense of all the evidence, one scientist at the NASA Jet Propulsion Laboratory offered a new hypothesis that built on findings about the propensity for diminutive comets in nearly parabolic orbits to disintegrate right ahead of perihelion. Perhaps, he suggested, this was ‘Oumuamua’s fate. By the time it deviated from a trajectory determined by the gravity of the Sun, it had become a fluffy cloud of dust. Or, in his more precise language, ‘Oumuamua became “a devolatilized aggregate of loosely-bound dust grains that may have exotic shape, peculiar rotational properties, and extremely high porosity, all acquired in the course of the disintegration event.”
However loosely bound that cloud, this hypothesis still requires a devolatilized ‘Oumuamua to be bound to some extent. After all, whatever remained had enough structural integrity that it was observed speeding away. Devolatilization means an object—say, a hunk of coal—is put under conditions, perhaps high heat, during which one element is removed. The example of devolatilization we are all familiar with is when a hunk of coal is heated to the degree that it becomes char.
This hypothesis holds that a comet not composed of carbon devolatilized into a highly porous bound exotic shape that was able to deviate to the statistically significant degree we observed ‘Oumuamua deviate. And for that, it requires one more step.
This structurally loosely bound dust cloud deviated without visible outgassing or debris from the “effects of solar radiation pressure.”
A similar concept of an icy porous aggregate was advanced a few months later by a researcher at the Space Telescope Science Institute. A decade earlier, this same scientist and I had collaborated to make the first prediction for the expected abundance of interstellar objects based on data for our solar system. (This prediction turned out to be orders of magnitude smaller than needed to account for ‘Oumuamua, another implied anomaly.) Now my colleague wanted to explain the object’s anomalous motion. In order for sunlight to produce the needed push, she calculated that the mean density of a porous ‘Oumuamua had to be extraordinarily low, a hundred times more rarefied than air.
Just imagine an elongated cigar or pancake the size of a football field, sturdy enough to tumble every eight hours but so fluffy that it is a hundred times lighter than a cloud. This hypothesis strains plausibility, to put it mildly, not least because imagination is all we have to base it on, and we have never observed anything like it. Of course, the same is true of a naturally occurring cigar-shaped object or a naturally occurring pancaked-shaped object. We haven’t seen such shaped objects, fluffy or not, at the extremes of ‘Oumuamua.
Briefly ignore what the object is composed of and let’s consider more carefully its shape. No one at a breakfast table would ever confuse a cigar for a pancake. They are dramatically different. So are we really left to choose between these two outlier shapes when we envision ‘Oumuamua tumbling through space?
