Book Review – Extraterrestrial (Avi Loeb)

Avi Loeb believes aliens are real, and they’ve visited our Solar System. But this man is no fringe conspiracy theorist, dressed in ratty flannels and a tinfoil hat, broadcasting rants from a pirate radio station in the wilderness—he’s a Harvard University professor, who has done world-renowned research on cosmology and astrophysics. This is a serious scientist we’re talking about! In Loeb’s book Extraterrestrial, published in 2021, he argues that we’ve already witnessed credible evidence of an alien civilization, even if the scientific community refuses to recognize it as such. Let’s take a look.

The coming of ‘Oumuamua

Extraterrestrial revolves around an object called ‘Oumuamua, which passed near Earth in the fall of 2017. You may recall headlines from around that time. It was the subject of considerable fanfare, hailed as the first interstellar visitor to enter our Solar System. Its closest approach was just 37 times further than the Moon. Telescopes only caught it on its outbound flight, when it was heading away from the Sun, and they tracked it over a period of 11 days, from October 19 to October 30. After that, it was too far away and too dim for astronomers to distinguish it from background stars.

‘Oumuamua is an Indigenous Hawaiian word, roughly translating to “first scout” or “first messenger.” Observations by the Pan-STARRS telescope showed it was quite small, less than a kilometer across, perhaps no more than 100 meters. Enormous variations in its brightness suggested it was either cigar-shaped or pancake-shaped. Within months, astronomical circles had produced a dominant model of ‘Oumuamua: an extremely long, thin chunk of ice, perhaps with some rock or metal mixed in. Unusual, but not artificial. Just another piece of space debris, drifting into our neighborhood from who knows where, probably ejected long ago from another star’s Oort cloud. In any case it was long gone, so why fret?

Avi Loeb would disagree.

Something strange in the neighborhood

Loeb devotes the first stretch of Extraterrestrial to explaining why ‘Oumuamua was like nothing we’ve ever seen before. You see, a normal asteroid would have traveled on a fairly simple ballistic path, shaped only by the Sun’s gravitational pull. ‘Oumuamua deviated from this path. Somehow, it accelerated away from the Sun. This might have been explained by outgassing, similar to what we see in comets, with sunlight causing ice to boil and turn ‘Oumuamua into a kind of all-natural rocket; the problem is, Earth-based telescopes didn’t spot any such gases. Significant amounts of water, methane, or dust would have been picked up and logged, if they existed. Something else had to be pushing it.

Loeb mentions some attempts by mainstream scientists to explain this phenomenon—and picks them apart. One such theory is that ‘Oumuamua was a “hydrogen iceberg,” made from frozen molecular hydrogen, which would not have been detectable when it boiled off. This, according to Loeb, is far-fetched. I agree. We have never observed any natural object made from frozen hydrogen, which has to be unbelievably cold to enter a solid state; its melting point is just 14 degrees above absolute zero. Even the light from passing stars, in interstellar space, would likely be warm enough to boil such an object into vapor.

There are other oddities. Statistically, the odds of a naturally forming interstellar asteroid just happening to pass by Earth are… small. Extrapolating backwards from the observed event, and assuming ‘Oumuamua was not literally one-in-a-billion, stars across the galaxy would have to eject several orders of magnitude more material than they could reasonably produce. And the way in which ‘Oumuamua accelerated looked less like the result of cometary outgassing, and more like solar pressure pushing it along. For a cigar- or pancake-shaped asteroid to move that much from sunlight alone, it would need to be ludicrously lightweight, more of a debris field than a solid object.

Avi Loeb rejects such explanations to suggest that ‘Oumuamua is something else entirely: a technological artifact. A gossamer-thin light sail, launched by some unknown alien civilization, drifting through the galaxy for who knows how long. This, he argues, fits best with the observed evidence. Sunlight could easily have given such a craft the acceleration we observed. A reflective sail would match the light curve just as well as a pancake or cigar shape. It’s a radical hypothesis, to be sure, but why not consider it?

Worlds beyond

Avi Loeb would know something about light sails. Alongside discussions of ‘Oumuamua, the book Extraterrestrial weaves in extensive details about Loeb’s life and career, and prominent among these is his involvement with the organization Breakthrough Starshot, on which he serves as the lead scientific advisor. The goal of Breakthrough Starshot is to send computer-chip-sized spacecraft to Proxima Centauri within a human lifetime; the chips, attached to extremely thin, lightweight sails, would be accelerated to a large fraction of light-speed using a network of powerful lasers¹.

Laser-propelled interstellar sails are nearly within humanity’s grasp, Loeb says. So why shouldn’t others have already launched such vessels? They’re (relatively) easy, they’re cheap, and they could be used to scout out much of the galaxy within the lifespan of a technological civilization. ‘Oumuamua may just be the practical execution of what we, ourselves, have merely theorized.

The author spends a lot of time pushing back against scientists who have dismissed his theories out of hand. I didn’t get the impression of a man who hates his colleagues, but he does think they’re closed-minded—he repeatedly references Galileo, who was persecuted by the Catholic Church for observing that it is the Earth which revolves around the Sun. Likewise, he believes that caution and conservatism within mainstream science have blinded it to evidence which might challenge its assumptions.

This is why SETI (the Search for Extraterrestrial Intelligence) has long been viewed as a laughing stock by more “serious” researchers; it’s why other scientists have blasted Loeb’s claims about ‘Oumuamua, even as their own hypotheses (e.g. hydrogen icebergs) seem no less implausible. Anything to do with non-human intelligence gets sorted into the same bin as crackpot Ufologists and lurid sci-fi novels. When confronted by genuine evidence of civilizations other than our own, dogmatically stating “It’s not aliens” may be little better than the crazy guy on the History Channel who says “It’s definitely aliens.”

Questions, more than answers

I admit, I’m a lot more receptive to Loeb’s claims than his colleagues are. I’m a sci-fi novelist who never internalized their notions of “serious” scientific inquiry. I grew up with tales of strange new worlds, of aliens hidden in vaults at Area 51, and, for me, it’s not that far-fetched to imagine that extraterrestrial intelligence might really be out there. Just judging by what I read in this book, the hypothesis of ‘Oumuamua as a light sail seems to fit the data better than other alternatives.

How is Extraterrestrial as a book? It’s an easy, breezy read. Scarcely 200 pages. It helps that Avi Loeb is a fantastic writer, in addition to his talents as a scientist—he handles the prose with agility and imagination. He talks about much more than ‘Oumuamua itself; over the course of the book he takes the reader on far-flung voyages to the beginning of the universe, to the edges of the galaxy, and to the Israeli village where he began his scientific career. There’s a deeply personal touch, all throughout.

Dr. Loeb urges us to seriously consider the prospect of alien intelligence as something we can discover in the near term. To this end he repurposes the classic Pascal’s wager into “’Oumuamua’s wager,” where the benefits of taking a chance on ‘Oumuamua being alien technology—a revolutionary new understanding of our place in the universe, long-term thinking as a species—far exceed any costs. Now, I don’t quite think this idea is fully cooked. His name for it is very cheesy. I like the gist, though.

Above all, Extraterrestrial is a plea to keep our imaginations open, dreaming big dreams and following scientific inquiry wherever it may lead us. You can feel a palpable sense of wonder, dripping off the page. It’s hard not to share the author’s awe at the vastness of our universe. That, I think, is the key takeaway from this book, regardless of whether ‘Oumuamua really is an alien laser sail, or something more mundane. We’re called to curiosity, and an optimistic spirit. We need a lot more of both in today’s world.

Rating: 9/10. Strongly recommended.

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1. For more on laser sails: I have discussed their merits here, here, and in a short story. ↩︎