A Review of “Extraterrestrial” by Prof. Avi Loeb

On October 19th, 2017, astronomers from the Haleakala Observatory in Hawaii announced the first-ever detection of an interstellar object in our Solar System. In honor of the observatory that first spotted it, this object (designated 1I/2017 U1) was officially named ‘Oumuamua by the IAU – a Hawaiian term loosely translated as “Scout” (or, “a messenger from afar arriving first.”)

Multiple follow-up observations were made as ‘Oumuamua left our Solar System and countless research studies resulted. For the most part, these studies addressed the mystery of what ‘Oumuamua truly was: a comet, an asteroid, or something else entirely? Into this debate, Dr. Shmuel Bialy and Prof. Avi Loeb of the Harvard Institute for Theory and Computation (ITC) argued that ‘Oumuamua could have been an extraterrestrial probe!

Having spent the past few years presenting this controversial theory before the scientific and astronomical community, Prof. Loeb has since shared the story of how he came to it in his new book, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth. The book is a seminal read, addresses the mystery of ‘Oumuamua, and (most importantly) urges readers to take seriously the possibility that an extraterrestrial encounter took place

Credit: Houghton Mifflin Harcourt

To break it down succinctly, Prof. Loeb expands on an argument he made roughly three years ago about the first interstellar object ever to be observed in our Solar System. Beyond reiterating the evidence, however, Loeb also takes the time to share personal stories and relate how his upbringing and experiences over the years led him down the path that culminated in this controversial theory.

These include his formative years in Israel, his lifelong love of philosophy and the big questions – i.e., “Why are we here?” “How did it all begin?” “How will it end?” and “are we alone in the Universe?” – and how an aptitude for math and science eventually led brought him to astrophysics. But as he relates, it was the way astrophysics is also concerned with the big questions in life (but also attempts to answer them) that to become a leading figure in the field.

After looking at all the evidence ‘Oumuamua provided during the 11 days we were able to observe it, Loeb (and co-author Dr. Bialy) concluded that the possibility that happened to fit all of the facts (limited though they were) was the same one scientists were not likely to take seriously (at least at first). In the end, Loeb presents his case for an interstellar messenger in a way that is simple, poignant, and not difficult at all to follow.

While some technical knowledge is certainly helpful to appreciate this book, it is by no means a prerequisite. Like any science communicator worth his salt, Loeb is able to relate scientific findings in a way that is accessible. But like any truly great science communicator, in the mold of Carl Sagan, he manages to convey it all in a way that is both inspired and inspiring.

Oumuamua as it appeared using the William Herschel Telescope on the night of October 29. Queen’s University Belfast/William Herschel Telescope

Interstellar Messenger

To recap, the story of ‘Oumuamua’s visit began when the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1), located at the Haleakala Observatory in Hawaii, announced the first-ever detection of an interstellar object passing by Earth. Based on the available data, astronomers concluded that the object entered the orbital plane of the Solar System on Sept. 6th and made its closest pass to the Sun (perihelion) on Sept. 9th.

On October 7th, it flew past Earth on its way out of the outer Solar System towards the Pegasus constellation. The object was only visible to Earth-based instruments for a total of 11 days, but what the observations revealed in this amount of time was quite fascinating – not to mention confounding. For starters, the images captured by Pan-STARRS-1 implied that ‘Oumuamua was either a highly elongated body, or pancake-shaped.

‘Oumuamua also appeared to be spinning rapidly and have a high density, which is indicative of a rocky and metallic composition (i.e., an asteroid). But when it made its closest pass to the Sun, it did not form a tail (as is customary with comets). Nevertheless, spectral and thermal imaging showed that ‘Oumuamua was rather bright and reflective, which consistent with ice.

As ‘Oumuamua began to leave the Solar System, the Hubble Space Telescope snapped some final images that showed the object increasing in velocity. The most obvious explanation for this was that ‘Oumuamua was venting material from its surface due to solar heating (aka. outgassing), which is again consistent with a comet. However, as Bialy and Loeb pointed out, there would be evidence of this that would be impossible to ignore.

Artist’s impression of the first interstellar asteroid/comet, “Oumuamua”. This unique object was discovered on 19 October 2017 by the Pan-STARRS 1 telescope in Hawaii. Credit: ESO/M. Kornmesser

For starters, there was the fact that it had formed no tail when it made its closest pass to our Sun. In addition, the sudden acceleration could not be attributed to gravitational forces since these should have been slowing ‘Oumuamua down at the time. And last, outgassing would have caused a rapid change in ‘Oumuamua’s spin (causing it to become more violent) which was also not observed.

According to Loeb and Bialy, the only thing that could be confirmed was that radiation pressure was the likeliest cause for the increase in velocity. But if ‘Oumuamua was neither a comet nor an asteroid, what could it be? Moreover, what could account for all of its strange behavior? Here too, scientists could only say with confidence that it was unlike any object we’ve ever seen before.

A Bold Theory

As we addressed at the time, Dr. Shmuel Bialy and Prof. Avi Loeb originally proposed their theory in October 2018 in a study titled “Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?” The study was peer-reviewed and accepted for publication in The Astrophysical Journal Letters shortly thereafter, and was met with a mix of reactions.

Citing a number of lines of evidence, the two ventured that ‘Oumuamua’s anomalous nature could be explained in the following way: its pancake-like profile and highly-reflective nature were both consistent with a light sail. Its sudden acceleration and deviation from its expected orbit were consistent with radiation pressure interacting with a light sail, and the way it entered our Solar System was what one might expect of an interstellar explorer.

Diagram showing the trajectory of ‘Oumuamua from early August to late October of 2018. Credit: SETI Institute

Naturally, this claim generated quite a bit of controversy. On the one side, there was the predictable response where various media claimed that Prof. Loeb and his colleague were claiming, “IT WAS ALIENS!” On the other side, there were those who scoffed at the very notion that a highly-accredited scientist with Loeb’s reputation would argue something so “unscientific.”

And yet, Loeb not only maintained this argument over time, but he has since released a book that addressed his controversial theory and the path that led him to it. Titled, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, Loeb makes a more concerted and human appeal to the scientific community and general public, urging them to seriously consider the possibility that ‘Oumuamua was an alien messenger.


As Loeb indicates in the book, this theory was partly inspired by his work with Project Starshot, a program for interstellar exploration sponsored by Breakthrough Initiatives. Beginning in 2015, Loeb and his colleagues at the Harvard Smithsonian Center for Astrophysics (CfA) worked to design a spacecraft capable of reaching the Alpha Centauri system in our lifetimes and report on what is there.

After much consideration, they realized that a lightsail pushed by a 100 gigawatt-laser was the best way to achieve relativistic speeds (a significant fraction of the speed of light). They further concluded that the spacecraft would have to be about the size and mass of a mobile phone. Given all of the advancements in recent years in terms of electronics and digital technology, they determined this was entirely feasible.

Artist’s impression of the interstellar object, `Oumuamua, experiencing outgassing as it leaves our Solar System. Credit: ESA/Hubble/NASA/ESO, M. Kornmesser

With the help of Prof. Philip Lubin of UCSB, an expert in directed-energy propulsion and applications, the concept came together to yield Starshot – a lightsail and a Starchip spacecraft. Another result of this project was the way it inspired Loeb and other scientists to suggest that SETI researchers should look for optical signals (lasers), which could be signs of alien communications or propulsion systems.

In this respect, says Loeb, ‘Oumuamua arrived at a time when Starshot was still fresh in his mind and (between its peculiar characteristics and behavior) the pieces fell into place. It was here, Loeb writes, that he asked Dr. Bialy to join him in testing the hypothesis:

“To be clear, my attitude at the time was simply That might work. The astronomical world had been presented with an exciting discovery, an interstellar object, about which we had collected a trove of confounding data. We confronted facts that were hard to match to a hypothesis that accounted for all of them. When I proposed that Bialy and I explain ‘Oumuamua’s deviation by way of sunlight, I was following the same scientific tenet I had always followed – a hypothesis that satisfied all the data ought to be considered.”

It was only after they checked the numbers, and confirmed that they worked, that the interstellar probe theory really began to form. The next step was to compute what an interstellar probe would look like in terms of size and composition, thickness, and reflectivity. In the end, all of the evidence proved to be consistent. As Loeb explained it, it all rested on the notion that ‘Oumuamua wasn’t natural in origin:

“The lightsail inference may seem outlandish,” Loeb summarized, “but getting to it did not require any wild leaps. Shmuel and I went down a logical path. We followed the evidence, and, in the grand tradition of the
detective work of science, we hewed closely to a maxim of Sherlock
Holmes: “When you have eliminated the impossible, whatever remains,
however improbable, must be the truth.” Hence our hypothesis:
‘Oumuamua was artificial.”

Artist’s concept for the Breakthrough Starshot spacecraft. Credit: Breakthrough Initiatives

Another important aspect of Loeb’s hypothesis involves a concept known as the Copernican Principle, which takes its name from Nicolaus Copernicus – the famed 16th-century Polish astronomer who developed the first complete heliocentric theory of the Universe. This principle essentially argues that Earth (and humanity, by extension) is not in a unique and privileged position to view the Universe.

Extended to the cosmological realm (where it’s also referred to as the Cosmological Principle), it asserts that habitable planets like Earth are likely to be representative of the norm, and are therefore not special. While this principle sounds more like philosophy than science, it provides SETI researchers for establishing parameters (much like the Drake Equation).

This also raises the issue of the Fermi Paradox, which expresses the disparity between the (assumed) statistical likelihood of extraterrestrial life and the apparent lack of evidence for it. Given the expanse of the cosmos, the number of stars and planets, and the fact that the ingredients for life are present everywhere in abundance, is it really so hard to consider that an advanced species has sent out probes to answer the big question?

And is it really so farfetched, when no alternative explanation is able to account for all the evidence, that ‘Oumuamua was one such probe? Like the possibility that it was an extraterrestrial lightsail, these questions cannot be answered just yet. Maybe someday, but in the meantime, they are vital food for thought.

An artist’s illustration of a light-sail powered by a radio beam (red) generated on the surface of a planet. Credit: M. Weiss/CfA

A Grand Tradition

“Extraordinary claims require extraordinary evidence.” These words were made famous by famed astronomer and science communicator Carl Sagan. In truth, he was paraphrasing Laplace’s principle, which states “the weight of evidence for an extraordinary claim must be proportioned to its strangeness.” You have to admit, Sagan’s version sounds better!

And there’s also the famous axiom of Sir Arthur Conan Doyle, which I need not repeat since Prof. Loeb beat me to it (well played, sir!). These words, which emphasize the need to eliminate the answers that don’t work in order to find the one that does, manages to capture the essence of deductive reasoning – a concept articulated by Renee Descartes.

Readers of Prof. Loeb’s work can certainly be forgiven for treating his arguments with skepticism, and he would surely be the first to recommend they do. But to reject this hypothesis without considering the arguments presented – or to label it as wistful, unscientific, or “the stuff of fantasy” – would represent a failure of both the empirical tradition and imagination.

Through his examination of the limited evidence we have on ‘Oumuamua and the application of the Copernican Principle, Loeb manages to make the case for a possible interstellar lightsail quite effectively. While we will never know for certain if ‘Oumuamua was an interstellar probe or not, it’s certainly worth considering what the implications of that would be.

One implication, as Prof. Loeb and Dr. Manasavi Lingham (also a researcher with the ITC) demonstrated in a subsequent study, is that our Solar System has likely captured thousands of interstellar objects over its lifetime. In yet another study, Prof. Loeb and Dr. Bialy surveyed known objects in the Solar System and identified several possible captured objects for future missions to study.

These and other hypotheses were validated less than two years after ‘Oumuamua was encountered when astronomers detected another “visitor” to our Solar System – the interstellar comet known as C/2019 Q4 (Borisov). Because of this, there are several proposals under consideration for sending a mission to rendezvous with an interstellar object in the future, like the ESA’s Comet Interceptor, Project Lyra, and others.

And as Prof. Loeb and Harvard astrophysicist Amir Siraj showed with more than one paper on the subject, we stand a much better chance of detecting these objects in the future. With the Vera C. Rubin Observatory, formerly known as the Large Synoptic Survey Telescope (LSST), astronomers will be able to detect a handful of interstellar objects a month – as part of the observatory’s Legacy Survey of Space and Time (LSST).

So while Fermi’s Paradox (i.e. “Where is everybody?”) has not been resolved (yet), it can certainly be argued that we are closed to it than ever before. No one can ever say with certainty how and when a first contact situation will occur (assuming it hasn’t already). But we can confidently say that if an extraterrestrial probe comes through our system in the future, we stand a far better chance of spotting and identifying it!

To learn more about Prof. Loeb’s work, to check out his book, or to have a gander at the many papers and works he has published over the years, check out his CfA webpage here! And be sure to check out our series that takes a look at the unresolved questions Search for Extraterrestrial Intelligence (SETI) titled, Beyond Fermi Paradox as well!

Matt Williams

Matt Williams is a space journalist and science communicator for Universe Today and Interesting Engineering. He's also a science fiction author, podcaster (Stories from Space), and Taekwon-Do instructor who lives on Vancouver Island with his wife and family.

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