‘Oumuamua: Natural or Artificial?
Jason T. Wright (Penn State)
Steven Desch (Arizona State University)
Sean Raymond (Université de Bordeaux)
Discovery of the first large interstellar visitor
For decades, scientists anticipated the discovery of objects from other stars passing through the Solar System. (One of us remembers their grad school advisor — Tom Quinn at the University of Washington — talking about this in the early 2000s, at which time the subject already had a long history.) It was not a mainstream subject of study because no such `interstellar’ objects had yet been found.
It was already well-known that cosmic rays and some dust particles were interstellar, and we’ve had physical samples of those for decades. It had also been proposed that some comets in the Oort cloud (a vast reservoir of comets swarming around the sun thousands of times the Earth-Sun distance) had been captured from sibling stars in the Sun’s birthplace. What was missing was a clear sign of a macroscopic object coming from outside our Sun’s sphere of influence. We call these interstellar objects.
On October 18th 2017, the first (confirmed) interstellar streak was detected in an image taken with the PAN-STARRS2 telescope in Hawaii. The object looked like a streak rather than a point of light because it was moving so fast across the sky. The timeline of what happened next is fascinating and documented in Karen Meech’s TED talk. That object was named ‘Oumuamua, meaning ‘advance scout’ in Hawaiian.
‘Oumuamua was the subject of intense astronomical observation but its brightness faded so quickly that within two weeks of its discovery it was lost to all but the largest ground-based telescopes. The shortness of that observing window and the associated uncertainty in its physical properties were the main drivers of the controversy that followed.
The immediate impression was that ‘Oumuamua looked sort of like an asteroid or comet. It was too far away for us to resolve any surface details, but from its brightness at different wavelengths it appeared similar to those of water-rich asteroids or dormant comets. Unlike comets, though, it did not show any evidence of outgassing or cometary activity — normally when comets get warm, their ices evaporate and we see them form a coma, and see evidence of that gas escaping. Comets recoil from this escaping gas, which acts like a rocket’s thrust to alter their orbital trajectory.
Astronomers quickly scrambled to put ‘Oumuamua in context. Analysis of its brightness variations revealed two surprising facts. First, based on its changing brightness it became clear that ‘Oumuamua spun every 8 hours or so but in a not-quite-repeating pattern. Surface features rotating into and out of view often let us get a rotation period for an asteroid or comet, but in this case the period was irregular. This is an indicator of ‘tumbling’ rotation: a perturbed but not uncommon spin state for small bodies in the Solar System.
The second surprise was that at some points in its tumbling it was almost 10 times fainter than usual. This is too much to explain with surface features, which meant its shape could not be anything like spherical. There are currently two possibilities that seem most likely: a pancake-shape with axis ratios of roughly 6:6:1, or a cigar-shape with axis ratios of ~8:1:1. The pancake shape is more likely because there is a higher chance of seeing a tumbling pancake edge-on compared with seeing a cigar’s shape tip-on, and ‘Oumuamua gets very dim more often than the cigar shape would suggest. (This realization came just too late, for the popular image of ‘Oumuamua as a long, needle-like object captured the public’s imagination.)
Within a few months a dozen explanations had been proposed for ‘Oumuamua’s origins. These ranged from the relatively mundane, such as an asteroid or extinct comet ejected from its home planetary system, to more novel explanations, such as a leftover of an earlier generation of planets that was partially melted in the atmosphere of a dying massive star. Despite the debate, all models in the published literature remained within the realm of natural objects.
The thorn in the side of conventional origins models came from measurements of the orbit of ‘Oumuamua as it whizzed away from the Earth. The Hubble Space Telescope and the Very Large Telescopes in Chile measured its position up to two full months after its discovery, allowing for a precise estimate of its orbital trajectory. Analysis showed that ‘Oumuamua’s path as it left our view was a few tens of thousands of kilometers off of a purely gravitational trajectory. An additional force had pushed it away from the Sun and slightly off gravity’s course.
Suggestions ‘Oumuamua could be artificial
The idea that ‘Oumuamua might be an artificial object, and not a natural body like a comet or asteroid, came up independently in the minds of many scientists almost immediately after it was discovered. Indeed, many scientists and members of the lay public had been primed for the prospect for such a thing by the 1973 book Rendezvous with Rama by Arthur C. Clarke, the opening chapter of which tracks some of the events of the actual discovery of ‘Oumuamua in with eerie prescience. In that book, a long, cylindrical object enters the Solar System on an orbit that carries it close to the sun, then out again, upon which it is discovered by comet hunters, and is eventually determined to be an alien spacecraft.
In real life, conversations among astronomers on Twitter gleefully suggested the name Rama for our new visitor, and planetary scientists checked their data for any signs of unusual color or spectral features that might indicate artifice. Alas, the real object did not conform to Clarke’s vision beyond its rough shape and trajectory: it was considerably smaller (perhaps 100m across, instead of Rama’s 50km), had the same colors as typical solar system comets and asteroids, and was apparently tumbling in an uncontrolled manner (giving it a highly variable brightness, rather than steady one in the book). It was also discovered on its way out of the Solar System, too late for close inspection (or for human visitors, as in Clarke’s novel. That novel also anticipated rather fewer women scientists would study such objects than is the actual case today).
Nonetheless, the object’s novelty and the possibility for first contact was enough for astronomers to check for signs of artificial radio transmissions from the object using the Allen Telescope Array, operated by the SETI Institute in Mountain View, California, the Murchison Wide Field Array, and the 100m Robert C. Byrd Green Bank Telescope. The latter observations, by astronomers at the University of West Virginia and the University of California, Berkeley, used the Breakthrough Listen equipment in what was at the time the most sensitive and broadband SETI investigation ever performed on a single target. (No signs of artificial signals were detected).
The most prominent suggestion that ‘Oumuamua was artificial began with a paper by Bialy & Loeb, which analyzed the possibility that ‘Oumuamua might be a light sail, an extremely thin sheet used for spacecraft propulsion via radiation pressure. This paper was primarily an analysis of the survivability of thin objects in interstellar space, but concluded with the “exotic scenario” that ‘Oumuamua might have been such an object, perhaps a discarded piece of a launch apparatus.
In their paper, Bialy and Loeb estimate that ‘Oumuamua was around 1000 kg and about 1 mm thick, which seems to be too bulky to be an effective light sail (ours are typically thousands of times thinner than this). It is also unclear whether such a sail would not fold up, or if it would indeed continue to tumble after millions or billions of years in interstellar space. Until recently no one had performed an analysis of what the brightness variation and accelerations of a light sail might be, and so it was unclear whether the light sail hypothesis fit the data any better than a comet would.
Nonetheless, the suggestion has received widespread attention in the media on account of numerous media interviews by the second author Avi Loeb, and the subsequent publication of his trade science book Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, which debuted on the New York Times bestseller list and was favorably reviewed in the print edition of that newspaper. Dr. Loeb’s book and public discussions have been much less circumspect and equivocal than the Bialy & Loeb paper, and make strong claims about the quality of the evidence for ‘Oumuamua being artificial.
Extraordinary claims and extraordinary evidence
Much of the meta-discussion about Dr. Loeb’s claims has centered on variations of Carl Sagan’s phrasing of the old maxim that extraordinary claims require extraordinary evidence. Dr. Loeb argues in his book and elsewhere that his claim that ‘Oumuamua is alien technology is, in the end, not so extraordinary. This is, ultimately, a philosophical objection to the presumption many of us have made that the object is natural, and to subsequent analyses of the data made under that presumption. To Dr. Loeb, this might seem to be question begging: if we assume it is natural before we begin our analysis, then we cannot claim to have come to the conclusion that it is natural based on evidence.
We would counter: the scientists analyzing ‘Oumuamua in a natural context have not dismissed the alien technology hypothesis out of hand — indeed many of us were engaged in many of the earlier mentioned discussions of it being an artifact — just that it is unlikely enough and the evidence for that possibility weak enough that it has not been worth following up. After all, a tacit corollary of Sagan’s maxim in the scientific community is that the burden is on the one making any extraordinary claim to do the hard work of showing it holds up to scrutiny.
Ultimately, however, this part of the discussion comes down to one’s personal assessment of the likelihood that alien spacecraft visit the Solar System, which can only be debated philosophically: we believe the claim to be extraordinary, requiring much more evidence than exists to be seriously entertained. Dr. Loeb apparently finds the claim to be quite reasonable (indeed, he devotes much of his book to this argument) and therefore requires not much more work than the cursory analysis he has done for us already to consider it likely.
So rather than debate the merits of the idea that alien spacecraft exist in the Solar System (and we three authors have different opinions on this topic), we will here stay close to the data and discuss the merits of Dr. Loeb’s evidence, so that the reader can come to their own conclusions.
We will group evidence in two camps: properties of ‘Oumuamua that are consistent with natural objects in the solar system or our prior expectations for interstellar objects, and anomalous properties that might argue for either a new class or understanding of natural objects, or for artifice (depending on one’s priors).
Evidence for naturalness
What evidence is needed to demonstrate that ‘Oumuamua is a natural, non-artificial object? Most scientists are not used to defending the concept of naturalness. Each observation of ‘Oumuamua can be interpreted in the context of known populations of small bodies. On its way in, it was observed to be on exactly the type of orbit that was expected for non-active interstellar objects. Its colors are consistent with D-type asteroids, comets, and even Pluto. Its brightness variations are unusual but not unprecedented (see here for a compilation), and the census of small, faint asteroids and comets in the Solar System is woefully incomplete. Many small bodies are in tumbling rotation states, and many comets show non-gravitational acceleration. What is hard to explain is the combination of these different factors in a single object, rather than any individual trait.
On the other hand, there is no a priori reason to suppose that an artificial object should share any of these properties with comets and asteroids, except non-gravitational acceleration, so this constitutes evidence that ‘Oumuamua is natural, and of a kind with Solar System asteroids and comets.
Dr. Loeb’s evidence for anomalies
In Extraterrestrial, Dr. Loeb lays out three major and two minor anomalies that he believes argue for ‘Oumuamua’s artificial nature. The major pieces of evidence are its shape, its reflectivity (or, albedo), and, most significantly to Dr. Loeb, its non-gravitational acceleration (paraphrasing an apocryphal comment by Galileo before the Inquisition, Loeb repeats through his book: “and yet it deviated.”) The minor pieces of evidence are the space density of similar objects implied by its discovery, and its velocity through interstellar space.
Below, we address each piece of evidence (individually and in concert) in light of the significant amount of research that has been done before and since Dr. Loeb’s initial claims.
Major argument: Shape
Claim: ‘Oumuamua is too elongated to be a natural object, as its longest-to-shortest axis ratio is at least 5, perhaps 10, whereas the most extreme Solar System objects have axis ratios less than 3.
Truth: Model fits to the light curve data are most consistent with a pancake shape with axis ratios 6:6:1. That is not 10:1, but it is still much flatter than almost all Solar System objects, which typically have axis ratios less than 3:1, although there are a few with equally large light curve variations. Those rare cases also involve equally small (< 100 m) bodies, and almost always when seen directly opposite the Sun from us, when at ‘full moon’ phase, as opposed to at an angle, at ‘crescent moon’ phase. That matters because many planetary surfaces are extra reflective when directly opposite the Sun (an effect known as the ‘opposition surge’), and so some of the light curve variations may be due to that effect rather than an elongation shape. So it may yet be the case that all the observed bodies in the Solar System have axis ratios less than 3:1. On the other hand, little is known about the shapes of such small (< 100 m) bodies, and perhaps more elongated shapes are common among them. Interestingly, the Kuiper belt object Arrokoth, visited by New Horizons in 2019, is composed of two bodies, one of which is pancake shaped, although its axis ratios are more like 2:2:1.
Whether we have observed objects in the Solar System with axis ratios of 6:1 or not, this would not preclude the possibility that a population of natural objects somewhere exists with more extreme axis ratios. Most small bodies we have observed have axis ratios of at most 2:1 or 3:1 because they are fragments of asteroids, and collisions between asteroids tend to generate shards of these shapes. ‘Oumuamua could have also gotten its strange shape because it is a shard from a rocky planet gravitationally shredded by a close encounter from a close encounter with a star, or even from an unusual formation history from smaller bodies
Interestingly, it is well known that erosion can increase axis ratios, just as a bar of soap becomes flatter and flatter as soap is removed equally from all sides during showers. A disk-shaped shard that formed with axis ratios 3:3:1 that then lost 75% of its mass would end up with axis ratios 6:6:1. The objects observed in our Solar System — the inner Solar System at that — appear more consistent with rocky fragments that have not been eroded; but erosion is a natural process that can, in principle, happen to natural objects. Arguably it would more readily happen to icy shards in the outer Solar System.
The hypothesis of a light sail is very hard to test using light curves, because we don’t know what to expect from alien engineering here. Since ‘Oumuamua is tumbling, it would appear to be uncontrolled, and so the light curve we expect to see depends on things like whether the back and front were equally reflective, and whether it was crumpled or folded.
Major argument: Albedo
Claim: ‘Oumuamua reflected at least ten times as much light as a typical Solar System object, making it much more consistent with a mirror or reflective solar sail, rather than a natural body.
Truth: First, even if ‘Oumuamua reflected 90% of the light hitting it, giving it an albedo of 0.90, that would make it just as shiny as many icy bodies in our Solar System, including the large Kuiper Belt object Eris, which is a natural body. But the albedo of ‘Oumuamua is unknown. In fact, it is impossible to know from a single measurement — the brightness — whether ‘Oumuamua was a very large but dark body, or a small but reflective body. Just how shiny we infer a Solar System body to be depends on how big we determine it to be. Because we know their sizes, it has been determined that asteroids typically have reflectivities of 5–20% (up to 50%), the dusty nuclei of comets range from 2%-7%, and Pluto’s surface is about 60–70%.
All we know about the size of ‘Oumuamua is that it was not detected by the Spitzer Space Telescope, which would have detected the infrared radiation from it if it exceeded a certain size. Therefore ‘Oumuamua could not have been too extreme in the ‘large and dark’ category, but this only tells us that its albedo must be greater than 4%. Almost any Solar System body would fit the bill.
To test the hypothesis of a solar sail using albedo, one would need to know ‘Oumuamua’s albedo, and the albedo of a solar sail. The former is unknown, as it would be necessary to know the size of the object first. The latter is also unknown, as we are not privy to the aliens’ design specifications except that it should be reflective at the wavelength it was used at. Nevertheless, it could be argued that a solar sail would be as reflective as possible (i.e., albedo of 1.0), at many wavelengths. This is contradicted by the observation that ‘Oumuamua was red-colored, i.e., it reflected less strongly at blue wavelengths than red.
Major argument: Non-gravitational acceleration without evidence of outgassing or disintegration
Claim: ‘Oumuamua experienced an acceleration (distinct from gravity) as it left the Sun, unlike any experienced by any natural Solar System body, and therefore must not be natural.
Truth: ‘Oumuamua definitely experienced a non-gravitational acceleration, as determined by its position in the sky, especially as fixed by the final observation of ‘Oumuamua, by the Hubble Space Telescope. Important to bear in mind, ‘Oumuamua conformed almost exactly to a hyperbolic orbit around the Sun due to gravity alone. It is not as if it took a right angle turn to check out Earth, then zipped over to Mars. Loeb’s claim is not that it took such a turn, but that the fact that it deviated at all implies it is so light that even sunlight could push it around, and that this argues it is not a large massive object like a comet or asteroid, but something designed to be gently pushed by starlight, like a light sail.
But this slight deviation from a purely gravitational orbit was not that unusual. Comets routinely experience a non-gravitational acceleration that pushes them away from the Sun slightly. This acceleration depends on the amount of sunlight striking the comet and heating its surface, and so it varies as the reciprocal of the distance from the Sun squared. That’s because the acceleration depends on the “rocket effect”: sublimation of surface ices, which cause gasses to push off the sunlit side, accelerating the comet outward. The acceleration experienced by ‘Oumuamua also varied as one over r squared, strongly implicating sunlight as well (technically, measured to be rᵃ with a somewhere between -1.5 and -2).
For comets, the typical acceleration from this effect is 0.01% of the strength of the gravitational acceleration. For ‘Oumuamua, it was closer to 0.1% of the strength of gravity, so this is indeed somewhat anomalous.
But does this large acceleration rule out the rocket effect? Not necessarily, as the force from the rocket effect is proportional to the amount of surface area of the object, but the inertia of the object is proportional to its mass. Small bodies have more surface area per mass (you can see this by considering chopping up a cucumber: the amount of mass of cucumber is the same after chopping, but the surface area of all of the pieces is far greater than when only the green skin was on the outside). This means they will experience a much larger rocket effect, inversely proportional to their size.
So, the acceleration from the rocket effect is perfectly consistent with ‘Oumuamua being very small, not just being very thin.
Claim: ‘Oumuamua’s deviation from an ordinary orbit via outgassing requires it to have lost 10% of its mass.
Truth: This is true, but is not inconsistent with natural objects, especially if ‘Oumuamua was composed mostly of ices and not rock. Indeed, as with the bar of bath soap after many uses, extreme amounts of mass loss also explains its large axis ratios.
Claim: Loeb writes “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…a herky-jerky acceleration.”
Truth: There are two ways that we could measure the acceleration of ‘Oumuamua: by its deviation from a hyperbolic path through the solar system, and as its rotation period changed. The first of these we just don’t know about: our measurements of its path through the solar system are too sparse to say if the acceleration was smooth.
As for the rotation period, Loeb cites a paper by Roman Rafikov who finds that the outgassing required to explain ‘Oumuamua’s acceleration would have provided a torque, spinning it up to speeds much faster than we observed it tumbling and likely breaking it into pieces. Yet this only holds true if all of ‘Oumuamua outgassed from a single stationary jet. Subsequent studies have shown that if ‘Oumuamua’s jet instead followed the substellar point, the spin that would be induced would not lead to breakup but would instead match the observed tumbling rotation.
Claim: ‘Oumuamua showed no evidence of outgassing or disintegration, which we would expect if the rocket effect were responsible for the deviation.
Truth: This claim is based on the lack of any observed gasses such as H2O or CO around ‘Oumuamua by the Spitzer Space Telescope, or a dust coma observed in optical light, which did strike many as unusual at the time. But if ‘Oumuamua was very small, there would be very little gas and dust to see.
In addition, our measurements were only sensitive to small (< micron) dust, not large dust. Many comets in the solar system show similarly low levels of small dust and gas. Indeed, there are two classes of solar system object, manx comets and damocloids, that do not usually exhibit cometary tails. The comet 2P/Encke (the second to have its orbit computed, after Halley’s Comet) often shows no visible light coma at all! It is also possible that ‘Oumuamua experienced only episodic outgassing — that Spitzer unluckily observed it in a brief dormant state before it began outgassing again. Alternatively, a shard of pure ice that was not CO or H2O could disintegrate without generating dust or observable gas.
Finally, in February 2023 Darryl Seligman and co-authors announced the discovery of 6 asteroids with large non-gravitational accelerations but no sign of coma, tail, or other outgassing. These “dark comets,” many of which appear to be the size of or smaller than ‘Oumuamua, show that ‘Oumuamua’s behavior may actually be typical of such small objects.
Minor argument: Space density
Claim: Unless we were exceedingly lucky to have observed ‘Oumuamua within the first decade or so of surveys, ‘Oumuamua must be part of a population of objects in the Galaxy numbering about 10¹⁵ per star, which is higher than can be made by any natural process.
Truth: The detection of interstellar objects was anticipated for decades. Long-period comets have long been recognized to enter the inner Solar System from an extended “Oort cloud” of comets around the Sun, extending halfway to the nearest star. These comets originated (mostly) within our own Solar System, and acquired their large orbits (with periods of tens of thousands, to millions, of years) because of gravitational slingshots by Jupiter or Neptune. This process is recognized to be not particularly fine-tuned, and for every comet emplaced in the Oort cloud by these slingshots, perhaps four comets were ejected into interstellar space. If other stellar systems evolved like the Sun’s, the interstellar medium should be littered with comets. It is true that the numbers of comets ejected by this process were at first predicted be closer to 10¹²–10¹³ per star, not 10¹⁵, but a closer examination of what else would be ejected besides traditional comets reveals our estimates could easily be off by enough to explain the difference.
Estimates for the number of interstellar comets are based on the numbers of large, obvious comets we see, and we must extrapolate down to very small sizes to understand how many objects like ‘Oumuamua we should expect. These extrapolations are usually done as power laws in the diameter of the object. So, the number of objects (asteroids, comets, etc.) with a given diameter D scales as 1/Dᵖ, where p is a power-law exponent perhaps in the range of 1 to 2. Because the proper exponent is unknown, there is huge uncertainty in the number of objects at very small sizes, where we have very little data to go on. It is well within the range of possibility that the exponent is 2 or larger, which would yield a number of ejected objects closer to 10¹⁵. This is an area of active research, with many unknowns, but it’s not obvious at all that the number density of interstellar objects demands a non-natural explanation.
As far as the probability of encountering a solar sail goes, if one interprets ‘Oumuamua’s evident tumbling to mean it was derelict and we encountered it randomly, then there must be an equally large number of solar sails, about 10¹⁵ produced per star in the Galaxy.
Minor argument: ‘Oumuamua’s speed and trajectory
Claim: ‘Oumuamua entered the Solar System at a very improbable speed for a natural object, implying again that it must be artificial and not natural.
Truth: Before discussing ‘Oumuamua’s speed, we have to define what it is moving relative to. Astronomers call the average velocity of all the stars in the Sun’s vicinity the ‘local standard of rest’, or LSR. It’s a funny name because stars with the LSR velocity are actually orbiting the Galaxy at around 220 km/s.
We take the average because most stars are moving at 20–30 km/s in some random direction with respect to the LSR. ‘Oumuamua itself was moving about 5–10 km/s more slowly around the Galaxy than this, i.e., it had a velocity of about 5–10 km/s with respect to the LSR, in the opposite direction of the Sun’s 20 km/s velocity. This is why it seemed to enter the Solar System at about 27 km/s. Its low speed with respect to the LSR makes it unlike most stars, and so Loeb concluded it could not have been a natural object ejected from a stellar system.
However, it is not true that all stars move at 20–30 km/s with respect to the LSR, and it’s likely that the Sun itself probably moved at a much slower speed with respect to the LSR, similar to ‘Oumuamua, when it was younger. Molecular clouds, and the young stars that form in them, typically trail the LSR by about 6–9 km/s, just as ‘Oumuamua did. It typically takes about 2 billion years for a star to have received enough gravitational nudges from giant molecular clouds and other stars to be zipping around at 20–30 km/s with respect to the LSR. The Sun at age 4.6 billion years, is old enough, and most stars are older than 2 billion years old; but there are many younger stars as well. Any objects ejected from a young (< 2 billion year old) star would naturally have the same low velocity with respect to the LSR that ‘Oumuamua has. Indeed, most comets are ejected from stellar systems when they are young, so this is consistent.
Such a special frame might be chosen by design, however; Dr. Loeb suggests it might be or have been part of a “buoy” in the Galaxy, deliberately chosen to be close to the LSR. We need not guess the exact reason for such a frame to be chosen for such an object to recognize that it is specially chosen, but since this is also expected for young natural objects this is not necessarily a sign of artifice.
Interpretation of the evidence in the context of ‘Oumuamua as a natural object
Perhaps the simplest early model proposed that ‘Oumuamua represented a building block of planets around another star. To explain its shape, it was proposed that ‘Oumuamua was a fragment of comet-like object that passed too close to a Jupiter- or Neptune-like giant planet and was tidally torn apart. It then passed close to its star a few times and lost its surface volatiles before being ejected into interstellar space, presumably by the same giant planet responsible for its shredding. This model can explain most of ‘Oumuamua’s characteristics nicely, such as its resemblance to an extinct comet, but it would struggle to explain the large nongravitational acceleration.
An interesting alternative model is that ‘Oumuamua is not a solid body at all, but a “fractal aggregate” of loose material — effectively a cosmic “dust bunny.” Such an agglomeration of dust might form in a comet’s coma, or it could be composed of ice particles collected together in a planet-forming disk of a young star. These models would explain the anomalous shape (since these aggregates need not be round) and non-gravitational acceleration (because they have so little inertia). Such aggregates, however, probably would not have been able to survive such a close encounter with the Sun, because its gravity would tear them apart at the distance ‘Oumuamua approached. Even if not destroyed, one would expect to have seen a dust coma.
Another alternative model is that objects in interstellar space can gain large amounts of molecular hydrogen gas in their surfaces as water ice interacts with light. This material would immediately outgas the first time the object came near a star, but would not produce an obvious coma or molecular emission. Because the gas only accumulates near the surface of the object, it would only be noticeable as a source of rocket effect for very small objects like ‘Oumuamua, and for objects that have not been near a star in a very like time, like interstellar orbits or some Oort Cloud comets. This would explain why it had not been seen before.
A parsimonious explanation for nearly all of ‘Oumuamua’s anomalous behavior is that it is not a “dirty snowball” made of a mixture of rock and ices like most solar system comets, but is mostly ice.
Note by “ice” here we do not necessarily mean water ice: in the coldest depths of space many substances that are gasses on Earth form solids which astronomers call “ices”.
Ices are easily eroded and vaporized, so ‘Oumuamua can be expected to have lost a significant amount, perhaps most, of its mass this way, especially during its recent pass by the Sun. This would naturally explain its flat shape, like an old bar of soap, and why it lost so much more mass than a typical comet does in the solar system. This loss of ice would also be quite uniform across ‘Oumuamua’s surface, explaining why the accelerations were not “herky-jerky” like can happen with comets. Also because it is easily eroded, we might expect ‘Oumuamua to be a very young object, perhaps ‘only’ tens to hundreds of millions of years old, which would explain why its velocity through space is consistent with very young stars.
Ice also tends to be extremely reflective, which would give ‘Oumuamua a very high albedo. This would put it at the “small and shiny” extreme of objects of a given brightness — and that small size would explain how its rocket effect managed to be 0.1% of gravity, and so much larger than is typical for comets.
Finally, a chunk of ice might not contain much or any dust, and so there would be no evidence of a coma or tail in reflected light. Unless the ice was made of water, carbon dioxide, or carbon monoxide, we also would not expect the Spitzer Space Telescope to have detected any of these gasses.
The problem is that the most common ices in comets would not work. Water ice is common, but would not sublimate fast enough to provide a sufficient push; it is also excluded on the basis that Spitzer would have detected water at the needed levels. Carbon dioxide is also excluded for similar reasons. Carbon monoxide would provide a sufficient push, but also would have been detected by Spitzer. Most other ices that could sublimate fast enough, such as neon, are just not plausible. In the end, two ices have been examined that seem to fit the bill: hydrogen (H2) ice, and nitrogen (N2) ice.
If one rejects the possibility of ice, then the non-gravitational acceleration of ‘Oumuamua–which must be attributed to sunlight–would have to be due to the gentle push of starlight, implying reflection of light. An outstanding issue, though, is whether a crumpled-up, tumbling solar sail (to explain the extremes in the light curve) can satisfy this constraint, either.
Dr. Loeb’s Arguments Against the Ice Model
The proposal that ‘Omuamua is a chunk of hydrogen ice was first put forward in a paper by Darryl Seligman and Greg Laughlin. This model posits ‘Oumuamua is a new kind of astrophysical phenomenon, a piece of ice that condensed directly out of the hydrogen gas in a giant molecular cloud of the sort that forms stars.
Shortly after publication of their article, Loeb wrote a paper with Thiem Hoang disputing this model. They claimed that for thermal reasons that such icebergs could not form or survive in interstellar space. A careful followup study by Levine et al. argues the opposite, that hydrogen iceberg formation may indeed happen. Yet, even if they could form, ‘Oumuamua-sized hydrogen icebergs would have a limited lifetime in space. Due to galactic radiation, hydrogen ice will all evaporate within roughly 50 million years, putting an upper limit on its time of flight, although this would be consistent with ‘Oumuamua’s galactic dynamics and slow velocity with respect to the local standard of rest.
One of us (Steve Desch) & Alan Jackson proposed an alternate model assuming that ‘Oumuamua’s non-gravitational acceleration was due to the sublimation of nitrogen ice. Nitrogen ice (i.e. molecular nitrogen, like the dominant constituent of our atmosphere, except so cold it is solid) is known to be the predominant constituent of certain large outer Solar System objects such as Pluto. This model interprets ‘Oumuamua as a fragment from the outer layers of a Pluto-like planet that formed around another star. A specific sequence of events is needed to explain ‘Oumuamua: 1) an exo-Pluto must have formed and thermally evolved to create a crust of nitrogen ice; 2) the system itself must have undergone a dynamical instability similar to the one thought to have taken place in the Solar System, which would entail a bombardment of the planet from leftover comet-like objects, creating a large number of crustal fragments containing nitrogen ice; and 3) these fragments must have been ejected into interstellar space, presumably by a giant planet in the same system.
Loeb has objected to this model on several grounds, most of which we have addressed above but two of which are particular to nitrogen ice. First, he wonders why we have never seen such a thing before from our own Solar System. This is a good question, but it has easy answers. First, we expect such objects to slowly ablate away with time, so what very few should be left in the solar system (which is over 4 billion years old) will be quite rare. And since we have a very small sample size of such objects, we may not have seen enough of them to have found one yet. Secondly, who is to say we haven’t observed them before and simply not recognized them? Indeed, in retrospect the comet C/2016 R2 seems very similar to ‘Oumuamua, and also fits the nitrogen ice model.
Secondly, Loeb wonders how an object of pure nitrogen could form, since nitrogen, carbon, and oxygen all form together in stars, and so the elements should be well mixed. But the answer is as clear as the air you are breathing, which is almost 80% nitrogen but only 0.01% carbon. The chemical properties of nitrogen are such that, at low temperatures, it prefers to bond with itself to form N2, while oxygen and carbon prefer to bond with each other (to form CO and CO2) and with other elements (to form oxides and carbides). Indeed, Pluto is proof that nitrogen ice forms naturally, free from large amounts of carbon or oxygen impurities.
Finally, Loeb, working with Amir Siraj, argued that in order for us to have seen something like an ‘Oumuamua iceberg, there must be a huge amount of nitrogen ice floating around, implying that there is as much mass in the galaxy in pluto-like objects as there is in stars (which would seem to be an absurdity). But this back-of-the-envelope, rough calculation ignores many details and nuances that are calculated with great care in the paper by Desch & Jackson, who arrive at a mass that is in fact consistent with the mass thought to have been ejected by the Sun’s Kuiper belt.
Both the hydrogen iceberg and exo-Pluto fragment models serve as proof-of-concept scenarios to explain ‘Oumuamua. Each model has potential problems; for instance, when will we confirm the existence of hydrogen ice, or of exo-Plutos in dynamically unstable systems? Can nature really produce enough “nitrogen icebergs” that we can expect to have seen one? Yet each puts together a viable string of natural events that match ‘Oumuamua’s observed constraints, and a set of testable, quantitative predictions. Future work can and will refine or reject these scenarios.
Arguments Against the Artificial Model
As we have seen, there have been extensive attempts to model ‘Omuamua as a natural object, and these have resulted in a few plausible hypotheses that explain most or all of ‘Oumuamua’s properties, although plenty of mysteries remain.
But to argue it’s probably artificial, it’s not enough to fail to find a perfect, universally accepted natural model (that’s a pretty common situation in science). We have to look at whether the alternative model fits the data better. Does it? Unfortunately, Dr. Loeb has not presented a detailed model of a light sail that explains the observations, so it is difficult to confirm or refute his claims.
In August 2022, Wen-Han Zhou and his collaborators attempted such an analysis to see if Dr. Loeb’s claim was actually consistent with the data. They analyzed the interstellar flight path, tumbling and photometric properties, and orbital deviations expected if ‘Oumuamua were a rigid, thin sheet traveling through the Solar System. They found that such a sheet would not behave as ‘Oumuamua did.
First of all, if ‘Oumuamua had been aimed at the Solar System, it would have needed some sort of course-correction because the hydrogen gas between the stars and the magnetic fields in the Galaxy would have caused it to change course to far to have come so close to the Sun.
Secondly, they found that a tumbling light sail would not just be accelerated away from the Sun, as comets mostly are and as ‘Oumuamua was, but significantly to the side as well. This is a major discrepency between Loeb’s model and ‘Oumuamua’s behavior.
Finally, they found that if ‘Oumuamua were a thin sheet, it would have to least occasionally been observed edge-on, in which case it would have been too faint to detect. Despite intense monitoring, this never happened. Zhou computes only a 1.5% chance this could happen.
In short, ‘Oumuamua is not very likely to have been a thin, rigid, reflective sheet.
This does not mean there are not other models of light sails that might fit the data, but it is nonetheless a blow against the light sail hypothesis that there do exist detailed natural models that fit, but we have no artificial model that fits.
Final thoughts
Dr. Loeb has said “I follow the maxim of Sherlock Holmes: When you have excluded the impossible, whatever remains, however improbable, must be the truth.” But natural explanations are not impossible for ‘Oumuamua. Indeed in almost every respect ‘Omuamua falls within the known range of parameters for Solar System objects, and in particular the nitrogen ice fragment hypothesis appears to be fully consistent with all of the evidence.
Dr. Loeb has also said of his paper on ‘Oumuamua as an alien spacecraft “If someone comes to me and says, ‘For these scientific reasons, I have a scenario that makes much more sense than yours,’ then I’d rip that paper up and accept it… But most of the people who attacked, they hadn’t even looked at my paper, or read the issues, or referred to the items we discussed.” We believe we have addressed Dr. Loeb’s claims above point by point, and shown that there are many scenarios, especially the nitrogen ice hypothesis, that are consistent with the data and follow directly from prior theories of planet formation. They may not be perfectly persuasive to everyone, but they are far, far better than the lack of good detailed models for an alien spacecraft.
Science proceeds best when competing hypotheses are tested against data, and when even conventional wisdom must constantly prove itself against new observations. There is therefore an important role to be played by those who advance unpopular and outré theories, even against what seems to be overwhelming evidence. Regardless of its true nature, ‘Oumuamua is teaching us about the panoply of interstellar objects, how planets form, and, yes, how we can search for evidence of alien technology in the Solar System. We look forward to seeing what we can learn next!