On October 19th, 2017, astronomers from the Haleakala Observatory in Hawaii announced that the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS-1) telescope had detected a strange object that had flown past Earth about twelve days earlier (on Oct. 7th). It was quickly realized that this visitor was an interstellar object, the first-ever to be observed passing through our solar system.
As is tradition, the object was named in honor of the institution which first spotted it, in this case, the Haleakala Observatory in Hawaii. Hence, the International Astronomical Union (IAU) named it 'Oumuamua (pronounced O-moo-a-moo-a), which is Hawaiian for "Scout" (or more precisely, "messenger from afar arriving first.")
For the next eleven days, observatories from all over the world (and instruments in space) conducted observations of 'Oumuamua as it sped away towards the edge of our Solar System and in the direction of the Pegasus constellation. What they observed left many in the scientific community scratching their heads and looking for possible explanations.
For starters, there was a great deal of confusion concerning 'Oumuamua's classification. Initially, it was thought to be a comet, then an asteroid, then something else entirely. But between optical, spectral, and infrared data that suggested odd dimensions and were inconsistent with any known celestial object, no one could say for certain what 'Oumuamua was.
Then there was its odd behavior, like how it failed to develop a tail as it made its closest pass to our Sun, but then accelerated as it left our Solar System. Only two things could be said with certainty: one, the object came from interstellar space; two, it was something astronomers had never before seen.
It was at this point, almost a year after 'Oumuamua came and went from our Solar System, that Prof. Abraham Loeb — the Frank B. Baird Jr. Professor of Science at Harvard University — offered a controversial explanation. If it was not consistent with any known natural object, could it possibly be an artificial object — for example, an interstellar spacecraft?
Since then, Prof. Loeb has explained and built on this hypothesis in a number of studies, articles, essays, and (most recently) a book titled Extraterrestrial: The First Sign of Intelligent Life Beyond Earth. His book hit the shelves on January 26th and has drawn more attention to the debate surrounding 'Oumuamua.
Far from being simply a matter of scientific interest or theory, Prof. Loeb shares stories from his personal life and how these influenced him to propose this theory. He also takes the time to explain the significance of this "interstellar visitor" and what it could mean for the future.
If 'Oumuamua was actually interstellar spacecraft, the implications for humanity and the search for extraterrestrial intelligence (SETI) are immeasurable. But to truly understand, one has to look past the controversy and sensationalism so they can examine the evidence.
A Truly "Exotic" Object
The confusing nature of 'Oumuamua was illustrated in the way that the IAU changed the object's designation three times between October and mid-November. In accordance with modern IAU naming conventions, an object's designation is an alphanumeric code consisting of the following:
- A prefix, alluding to the type of object:
- P/ for a periodic comet
- C/ for a non-periodic comet
- X/ for a comet for which a meaningful orbit cannot be computed
- D/ for a periodic comet that no longer exists or is deemed to have disappeared
- I/ for all interstellar objects, whether comets or asteroids
- The year of discovery
- A letter identifying the half-month of observation
- A number representing the order of discovery within that half-month
Initially, the IAU designated it C/2017 U1, which reflected the prevailing theory that it was a comet (C). They then switched it to A/2017 U1 based on data that said it was more consistent with an asteroid (A). Finally, by mid-November, they switched it for the last time to 1I/2017 U1, indicating that it was interstellar in origin (I).
As Professor Loeb explained in an article published in Scientific American (Nov. 20th, 2018), 'Oumuamua presented six major anomalies that scientists were hard-pressed to find a natural explanation for.
To summarize them briefly, during the 11 days that it was visible to our instruments, data gathered on 'Oumuamua indicated that it was either extremely elongated (like a cigar) or more likely disc-shaped. It was also highly-reflective and deviated from a path dictated by our Sun's gravity, which was a result of its acceleration.
The kinematics of 'Oumuamua indicated that it must have originated in a very special frame of reference (consistent with 1 in 500 stars in our galaxy) and that it would have taken a very powerful kick to knock it out of its parent star system.
Last but not least, the very fact that it was detected implied that there is a huge population of interstellar objects, which would be possible if every star in our galaxy ejected one quadrillion (1015) objects in their lifetime.
While not impossible, Loeb cites previous and subsequent studies where the researchers attempted to place constraints on the likely number of interstellar objects in our galaxy. As he indicates, these produced estimates that fell short of 1015 by several orders of magnitude.
Any one of these anomalies could be explained in terms of a 1 in 10, 1 in 100, or 1 in 1000 chance of probability. But considered together, the odds of such an exotic object showing up in our Solar System and behaving as it did were about 1 in a million!
An "Exotic" Theory
What was particularly interesting to Prof. Loeb was the way 'Oumuamua accelerated as it left the Solar System, apparently in response to radiation pressure from our Sun. This was consistent with how comets behave, which experience outgassing as a result of exposure to solar radiation.
However, scientists had already ruled out the possibility that 'Oumuamua was a comet, which was further indicated in how the increase in acceleration was not accompanied by an increase in the tumble rate. And since scientists could not attribute this to any gravitational forces acting on it, solar radiation pressure was the only explanation.
To this, Professor Loeb and one of his postdoctoral students (Dr. Shmuel Bialy) began to consider an alternative explanation. What if 'Oumuamua wasn't a natural object at all, but an interstellar spacecraft — or possibly a derelict piece of one?
At the time, Prof. Loeb was fresh from developing a concept for a lightsail spacecraft for Breakthrough Starshot, a program launched by Breakthrough Initiatives in 2016 to create an interstellar spacecraft that could reach Alpha Centauri (4.37 light-years away) in our lifetimes.
Similar in concept to a solar sail, the Starshot design would incorporate a sail composed of highly-reflective material that is accelerated by a 100-gigawatt laser. With a smartphone-sized spacecraft (Starchip) to gather data, Starshot would be capable of achieving speeds of up to 20% the speed of light (134 million mph; 216 million km/h) and make it to Alpha Centauri in about 20 years.
In addition to Prof. Loeb, Breakthrough Starshot has also relied on the expertise of Prof. Philip Lubin of UC Santa Barabara. For many years, Prof. Lubin has conducted research with support from NASA into directed-energy (laser) applications for space travel and planetary defense.
Beyond Starshot, Prof. Loeb and Prof. Lubin have collaborated extensively on a number of studies, which included seminal papers on how extraterrestrials could be detected by looking for specific signs of technological activity (aka. "technosignatures").
In 2016, Prof. Lubin published a paper that explored the possibility of looking for signs of directed energy systems as a possible technosignature. By 2018, spillover from directed-energy systems (i.e., flashes of laser light) was included in NASA's Technosignature Catalog.
With all of this in mind, Prof. Loeb began to investigate the possibility of a lightsail to see if this explanation could account for 'Oumuamua's anomalous behavior. As the founder of the Institute for Theory and Computation (ITC) at Harvard University, Loeb is well-versed in testing theories and crunching numbers.
What they revealed was rather interesting, in a mind-blowing sort of way! Not only was 'Oumuamua's sudden acceleration consistent with a lightsail, but its other oddities — a disc-like shape, a low mass-to-area ratio, and even its trajectory — were all consistent with a probe that drifted into our star system.
Another "more exotic" possibility, said Loeb, was that it might have been sent intentionally! After all, its trajectory had allowed it to swing around our Sun and make a quick pass by Earth, which are consistent with a gravity assist and flyby maneuver.
These possibilities certainly seemed farfetched at the time. But what was especially confounding was the way the lightsail explanation fit the facts in a way that alternative (natural) explanations could not. As Prof. Loeb explained via email:
"The lightsail hypothesis explains the excess acceleration of `'Oumuamua as a result of the reflection of sunlight. The many anomalies exhibited by '`Oumuamua forced all-natural interpretations of it to invoke object types that we have never seen before – all with major drawbacks, like a hydrogen iceberg - which will likely evaporate by absorbing starlight during its journey, a "dust bunny," a hundred times more rarefied than air - which might not have the material strength to withstand heating to hundreds of degrees by the Sun or a tidal disruption relic – which would not possess the pancake-like shape inferred for '`Oumuamua."
These arguments were raised in a paper titled “Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?” that appeared online in October of 2018. The study was peer-reviewed, accepted for publication, and then appeared in the Nov. 8th issue of The Astrophysical Journal Letters.
To be fair, Prof. Loeb was not alone in thinking that 'Oumuamua might be an extraterrestrial spacecraft. In fact, the possibility had already been considered by Prof. Stephen Hawking and his colleagues at Breakthrough Listen. This project, which was also launched by Breakthrough Initiatives, is the most massive-SETI project ever conducted.
Using the initiative's network — which includes the Green Bank Telescope, the Parkes Telescope, and the Automated Planet Finder — Hawking and the Listen team monitored 'Oumuamua during the 11-day observational window, for signs of radio signals. None were detected, but as far as entertaining the possibility of ETI was concerned, the cat was out of the bag!
Since then, astrophysicists have attempted to offer other explanations. These include the possibility that 'Oumuamua was a fragment from a comet that shattered as it neared the Sun, a slab of frozen molecular hydrogen (a dark hydrogen iceberg), or a cometary fractal aggregate (or interstellar "dust-bunny) made up of pieces of shattered comets.
However, none of these theories survive scrutiny for long. As Loeb argues, they either suffer from a fatal flaw, involve an extremely unusual object (the likes of which has never seen before), and/or extremely unusual circumstances in order to account for the many anomalies observed.
In the three years that have passed since Loeb first shared his hypothesis, the response has predictably been mixed. On the one hand, it has inspired a great deal of enthusiasm and sensation from the general public and media. Loeb said:
"The reaction from the public was extraordinarily positive. After the book was published, it became an instant bestseller in many countries worldwide. I had about 300 interviews for podcasts, radio and TV stations, and newspapers over the last 8 weeks. There were 24 filmmakers and producers who showed interest in the book."
At the same time, Loeb was rather disheartened by the reaction it garnered from the scientific community. With the release of the paper (and even more so with the release of the book), many in the astronomical and even SETI community have been critical of his theory.
Much of the consternation appears to be due to the subject matter — i.e., extraterrestrial intelligence — which Loeb attributes to the misperception of it being a non-scientific matter:
"[T]he response from the scientific community was mostly hostile, except for two dozen colleagues who emailed me to say that they agree with my statements but cannot express their admiration publicly because of the fear that it might damage their career opportunities. I was hoping that the scientific discussion would focus on explaining the anomalies exhibited by '`Oumuamua because that constitutes the natural detective process of sorting out whether `'Oumuamua was natural or artificial in origin."
Require Extraordinary Evidence!
Throughout the book Extraterrestrial, Loeb reiterates many times how his hypothesis came down to the available evidence and that it was arrived at in an entirely scientific manner. What was discovered those 11 days in October 2017 left the scientific community baffled — and for a good reason!
What natural object could account for a bright object that was flat, highly reflective (but cool), varied considerably in brightness as a result of its spin, and was able to deviate from a path dictated by the Sun's gravitational pull?
As Loeb stressed, all of the natural explanations for 'Oumuamua as a natural object have invoked objects that have never been seen before and are statistically less likely than a spacecraft. He argued that, if 'Oumuamua was a comet, it would have formed a tail. If it were an asteroid, it would not have accelerated as it moved away from our Sun.
If it were a hydrogen iceberg, it would have evaporated as it made its closest pass to the Sun. If it were an extremely rare assemblage of comet fragments (aka. interstellar "dust bunny"), it would have burned up around our Sun or been pulled apart. None of these explanations fit, so why are they considered more likely than an interstellar lightsail?
As Loeb and Bialy wrote in their 2017 paper, "‘Oumuamua represents a new class of thin interstellar material, either produced naturally, through a yet unknown process in the [interstellar medium] or in proto-planetary disks, or of an artificial origin." In short, if it were natural, then it was the type of exotic object (and the result of processes) that has never been detected before.
Just for fun, let's assume that it was possible to know what 'Oumuamua was with certainty. What then? If it were a natural object, the likes of which we have never seen before, it would mean that there are exotic objects and processes in our Universe that we've also never seen before. The resulting potential for research would be astronomical (no exaggeration!).
But if it were actually an interstellar lightsail, the implications could be far, FAR greater! When asked to comment on what these would be, Loeb described them in the following way:
"This would have a huge impact on the way we perceive our place in the universe, our aspirations for space, and our philosophical and theological beliefs. The psychological shock would be similar to the one encountered by my daughters on their first day of kindergarten. Before that day, my daughters thought that they were unique and special, that the world centers on them, and there is nobody smarter than them."
He's not kidding! For the first time, humanity would have hard evidence that it was not alone in the Universe. Fermi's Paradox would be resolved for all time, and we would know what to be on the lookout for in the future.
We might also be a little nervous that someone had buzzed our system without calling. We would have straightened the place out if we knew they were coming! Many of us would be clutching our pearls and hoping that whoever sent the probe wasn't planning on showing up later to kick in the door!
Could it Be?
Ever since Prof. Loeb presented his controversial theory in 2018, there has been no shortage of people issuing the oft-quoted line from Carl Sagan, including myself (scroll up, you'll see!). Specifically, it's the one that was inspired by the late scientist's SETI research:
"Extraordinary claims require extraordinary evidence."
Prof. Loeb himself cites this quote in his book, which manages to encapsulate the approach he and his colleagues took with regards to 'Oumuamua. Far from hastily concluding that 'Oumaumua had to be an alien spacecraft, he lays out how his initial curiosity led him to consider the evidence presented before offering an explanation.
Similarly, Loeb cites another famous quote in his book that describes the process that led him to his controversial hypothesis. In this case, it was the words of Sir Arthur Conan Doyle, which were spoken through his famous literary creation, Sherlock Holmes:
"When you eliminate the impossible, whatever remains, however improbable, must be the truth."
This quote effectively summarizes the deductive reasoning approach, which was popularized by famed French polymath René Descartes. If we exhaust all natural explanations for 'Oumuamua, what remains? Although it seems outlandish to think so, the only remaining explanation is an unnatural one.
But that's beside the point. In the end, we will never know for certain what 'Oumuamua was, as no further data can be gathered at this point. What Loeb is advocating with his book Extraterrestrial (and the controversial theory that inspired it) is for people to take the possibility seriously.
In this respect, he is hardly alone. In addition to Dr. Stephen Hawking and Breakthrough Listen's efforts to detect signals from 'Oumuamua, the great Carl Sagan was also known for encouraging imagination, in addition to skepticism, when dealing with the unknown.
In 1966, Sagan and Iosif Shklovsky released Intelligent Life in the Universe, which was the English-language version of an earlier work by Shklovksy (Universe, Life, Intelligence, published in the USSR in 1962). In Chapter Thirty-Three, "Possible Consequences of Direct Contact," they claim that scientists should take the possibility that extraterrestrials have visited Earth in the past seriously.
They further suggested that evidence of past contact could be preserved in the folklore and mythological traditions of various cultures. As an example, Sagan and Shklovsky point to an account of the contact made between the Tlingit people of the Pacific Northwest and the La Perouse expedition French expedition in 1786.
This event was not recorded until a century later by anthropologist G.T. Emmons. While it was told in the context of Tlingit mythology and contained supernatural elements, the descriptions left no doubt that it was a faithful retelling of an actual event. Sagan and Shlovksy held that the same could be true of extraterrestrials that visited in the past.
Sagan and Shklovsky were sure to emphasize the need for skepticism, in large part because of "Ancient Astronaut" theories that were popular by the late 1970s. Nevertheless, they urged that the scientific community should consider the possibility before passing judgment.
In the same vein, Prof. Loeb urges the scientific community to do the same with 'Oumuamua. It's not just a question of the scientific method or evidence for him, though. As he explained, it's also about inspiring people to look beyond the comforting and familiar and consider the greater possibilities:
"Initially, I told my publisher that I would be satisfied if one person around the world would decide to become a scientist after reading my book. Last week, a woman from Malawi in Africa wrote to me that she finds my book to be so great that it inspired her to become an astronomer. I told her the story about my initial conversation with the publisher and asked: "Are you that person who will become a scientist after reading my book?" and she replied: "Either me or my young girl." I invited her to visit me at Harvard."
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Fan's of Sagan and Shklovsky's seminal book, Intelligent Life in the Universe, be advised! Prof. Loeb and Dr. Manasvi Lingam — Assistant Professor at Florida Tech and former postdoc of Loeb's — have written a modern version, titled Life in the Cosmos: from Biosignatures to Technosignatures. The book is available for preorder now and is expected to be released on June 29th, 2021.