New Statesman story (behind paywall), thus linked via Outline.
If you could fly two billion miles in the direction of the Pegasus constellation, and knew where to look, you would find a thin, flat object, about the size of a football field and up to ten times more reflective than the average comet. If you watched it for a while, you would notice that it is tumbling as it moves away from the sun, turning end over end roughly every seven hours.
This object passed the Earth in October 2017. As it began its return to interstellar space, the Canadian astronomer Robert Weryk identified it among the images from what was then the world’s most powerful camera, a telescope in Hawaii called Pan-STARRS1. The astronomers in Hawaii called it ‘Oumuamua, a Hawaiian word meaning “first scout from a distant place”.
‘Oumuamua was the subject of great excitement. It was the first object humans have observed travelling through the solar system from interstellar space. But it also became controversial: its shape, the way in which it approached us, and the way it moved away are not consistent with the behaviour of an asteroid or comet. For 11 days, the world’s telescopes searched for meaning from this strange visitor.
A year later, the debate about ‘Oumuamua intensified when one of the world’s foremost astronomers, Avi Loeb, submitted a paper to the Astrophysical Journal Letters. In it, Loeb and his colleague, Shmuel Bailey, argued that ‘Oumuamua’s strange properties indicated that it was “a new class of thin interstellar material, either produced naturally, through a yet unknown process […] or of an artificial origin”. Since then, Loeb has maintained that the most rational, conservative explanation is that ‘Oumuamua was produced by an alien civilisation.
HT: Kimball Corson.
Alien object speculation
On 26 October 2018, Avi Loeb and his postdoc Shmuel Bialy submitted a paper exploring the possibility of ʻOumuamua being an artificial thin solar sail accelerated by solar radiation pressure in an effort to help explain the object’s non-gravitational acceleration. Other scientists have stated that the available evidence is insufficient to consider such a premise, and that a tumbling solar sail would not be able to accelerate. In response, Loeb wrote an article detailing six anomalous properties of ʻOumuamua that make it unusual, unlike any comets or asteroids seen before. A subsequent report on observations by the Spitzer Space Telescope set a tight limit on cometary outgassing of any carbon-based molecules and indicated that ʻOumuamua is at least ten times more shiny than a typical comet. A detailed podcast produced by Rob Reid provides the full details about the differences between ʻOumuamua and known comets. In January 2021, Loeb is releasing a book, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, which describes the possibility that ‘Oumauamua may be alien technology, although such an explanation is considered very unlikely by most scientists.
Loeb’s Six Anomalous Properties in Scientific American.
1. Assuming that other planetary systems resemble the solar system, Pan-STARRS should not have discovered this or any other interstellar rock in the first place. In a paper published a decade ago, we predicted an abundance of interstellar asteroids that is smaller by many (two to eight) orders of magnitude than needed to explain the discovery of ‘Oumuamua, assuming it’s a member of a random population of objects. Put another way, ‘Oumuamua implies that the population of interstellar objects is far greater than expected. Each star in the Milky Way needs to eject 1015 such objects during its lifetime to account for a population as large as ‘Oumuamua implies. Thus, the nurseries of ‘Oumuamua-like objects must be different from what we know based on our own solar system.
2.‘Oumuamua originated from a very special frame of reference, the so-called local standard of rest (LSR), which is defined by averaging the random motions of all the stars in the vicinity of the sun. Only one star in 500 is moving as slowly as ‘Oumuamua in that frame. The LSR is the ideal frame for camouflage, namely for hiding the origins of an object and avoiding its association with any particular star. The relative motion between ‘Oumuamua and the sun reflects the motion of the sun relative to the LSR. ‘Oumuamua is like a buoy sitting at rest on the surface of the ocean, with the solar system running into it like a fast ship. Could there be an array of buoys that serves as a network of relay stations or road posts, defining the average galactic frame of reference in interstellar space?
3. Most interstellar asteroids are expected to be ripped away from their parent star when they lie in the outskirts of their birth planetary system (such as our solar system’s Oort cloud, which extends to 100,000 times the Earth-sun separation), where they are most loosely bound to the star’s gravity. At these outskirts, they can be removed with a small velocity nudge of less than a kilometer per second, in which case they will maintain the speed of their host star relative to the LSR. If ‘Oumuamua came from a typical star, it must have been ejected with an unusually large velocity kick. To make things more unusual, its kick should have been equal and opposite to the velocity of its parent star relative to the LSR, which is about 20 kilometers per second for a typical star like the sun. The dynamical origin of ‘Oumuamua is extremely rare no matter how you look at it. This is surprising, since the first foreign guest to a dinner party should be statistically common (especially given the larger than usual population inferred in the first point above).
4. We do not have a photo of ‘Oumuamua, but its brightness owing to reflected sunlight varied by a factor of 10 as it rotated periodically every eight hours. This implies that ‘Oumuamua has an extreme elongated shape with its length at least five to 10 times larger than its projected width. Moreover, an analysis of its tumbling motion concluded that it would be at the highest excitation state expected from its tumultuous journey, if it has a pancake-like geometry. The inferred shape is more extreme than for all asteroids previously seen in the solar system, which have an length-to-width ratio of at most three.
5. The Spitzer Space Telescope did not detect any heat in the form of infrared radiation from ‘Oumuamua. Given the surface temperature dictated by ‘Oumuamua’s trajectory near the sun, this sets an upper limit on its size of hundreds of meters. Based on this size limit, ‘Oumuamua must be unusually shiny, with a reflectance that is at least 10 times higher than exhibited by solar system asteroids.
6. The trajectory of ‘Oumuamua deviated from that expected based on the sun’s gravity alone. The deviation is small (a tenth of a percent) but highly statistically significant. Comets exhibit such a behavior when ices on their surface heat up from solar illumination and evaporate, generating thrust through the rocket effect. The extra push for ‘Oumuamua could have originated by cometary outgassing if at least a tenth of its mass evaporated. But such massive evaporation would have naturally led to the appearance of a cometary tail, and none was seen. The Spitzer telescope observations also place tight limits on any carbon-based molecules or dust around ‘Oumuamua and rule out the possibility that normal cometary outgassing is at play (unless it is composed of pure water). Moreover, cometary outgassing would have changed the rotation period of ‘Oumuamua, and no such change was observed. Altogether, ‘Oumuamua does not appear to be a typical comet nor a typical asteroid, even as it represents a population that is far more abundant than expected.