What will life on other planets look like, whennot ifits found? Arik Kershenbaum, a researcher at the University of Cambridge, takes up this question in The Zoologists Guide to the Galaxy: What Animals on Earth Reveal About Aliensand Ourselves (Penguin Press). Its a popular belief that alien life is too alien to imagine, he writes. I dont agree.

Kershenbaum argues that the key to understanding cosmic zoology is natural selection. This, he maintains, is the inevitable mechanism by which life develops, and therefore its not just restricted to the planet Earth or even to carbon-based organisms. However alien biochemistry functions, natural selection will be behind it.

From this premise, Kershenbaum says, it follows that life on other planets will have evolved, if not along the same lines as life on this planet, then at least along lines that are generally recognizable. On Earth, for instance, where the atmosphere is mostly made of nitrogen and oxygen, feathers are a useful feature. On a planet where clouds are made of ammonia, feathers probably wouldnt emerge, but we should not be surprised to find the same functions (i.e. flight) that we observe here. Similarly, Kershenbaum writes, alien organisms are apt to evolve some form of land-based locomotionLife on alien planets is very likely to have legsas well as some form of reproduction analogous to sex and some way of exchanging information: Aliens in the dark will click like bats and dolphins, and aliens in the clear skies will flash their colours at each other.

Assuming that there is, in fact, alien life out there, most of it seems likely to be microscopic. We are not talking about little green men is how Stofan put it when she said we were soon going to find it. We are talking about little microbes. But Kershenbaum, who studies animal communication, jumps straight to complex organisms, which propels him pretty quickly into Loebian territory.

On Earth, many animals possess what we would broadly refer to as intelligence. Kershenbaum argues that, given the advantages that this quality confers, natural selection all across the galaxy will favor its emergence, in which case there should be loads of life-forms out there that are as smart as we are, and some that are a whole lot smarter. This, in his view, opens up quite a can of interstellar worms. Are we going to accord aliens human rights? Will they accord us whatever rights, if any, they grant their little green (or silver or blue) brethren? Such questions, Kershenbaum acknowledges, are difficult to answer in advance, without any evidence of what kind of legal system or system of ethics the aliens themselves might have.

As disconcerting as encountering intelligent aliens would be, the fact that we havent yet heard from any is, arguably, even more so. Why this is the case is a question thats become known as the Fermi paradox.

One day in 1950, while lunching at Los Alamos National Laboratory, the physicist Enrico Fermi turned to some colleagues and asked, Where are they? (At least, this is how one version of the story goes; according to another version, he asked, But where is everybody?) This was decades before Pan-STARRS1 and the Kepler mission. Still, Fermi reckoned that Earth was a fairly typical planet revolving around a fairly typical star. There ought, he reasoned, to be civilizations out there far older and more advanced than our own, some of which should have already mastered interstellar travel. Yet, strangely enough, no one had shown up.

Much human intelligence has since been devoted to grappling with Fermis question. In the nineteen-sixties, an astronomer named Frank Drake came up with the eponymous Drake equation, which offers a way to estimateor, if you prefer, guesstimatehow many alien cultures exist with which we might hope to communicate. Key terms in the equation include: how many potentially habitable planets are out there, what fraction of life-hosting planets will develop sophisticated technology, and how long technologically sophisticated civilizations endure. As the list of potentially habitable planets has grown, the Where are they? mystery has only deepened. At a workshop on the subject held in Paris in 2019, a French researcher named Jean-Pierre Rospars proposed that aliens havent reached out to us because theyre keeping Earth under a galactic quarantine. They realize, he said, that it would be culturally disruptive for us to learn about them.

Loeb proposes that Fermi may be the answer to his own paradox. Humanity has been capable of communicating with other planets, via radio wave, for only the past hundred years or so. Seventy-five years ago, Fermi and his colleagues on the Manhattan Project invented the atomic bomb, and a few years after that Edward Teller, one of Fermis companions at the lunch table at Los Alamos, came up with the design for a hydrogen bomb. Thus, not long after humanity became capable of signalling to other planets, it also became capable of wiping itself out. Since the invention of nuclear weapons, weve continued to come up with new ways to do ourselves in; these include unchecked climate change and manufactured microbes.

It is quite conceivable that if we are not careful, our civilizations next few centuries will be its last, Loeb warns. Alien civilizations with the technological prowess to explore the universe are, he infers, similarly vulnerable to annihilation by self-inflicted wounds. Perhaps the reason no one has shown up is that theres no one left to make the trip. This would mean that Oumuamua was the cosmic equivalent of a potsherdthe product of a culture now dead.

A message an earthling might take from this (admittedly highly speculative) train of thought is: be wary of new technologies. Loeb, for his part, draws the opposite conclusion. He thinks humanity ought to be working to produce precisely the kind of photon-powered vessel that he imagines Oumuamua to be. To this end, hes an adviser on a project called the Breakthrough Starshot Initiative, whose stated aim is to demonstrate proof of concept for ultra-fast light-driven nanocrafts. In the longer term, the group hopes to lay the foundations for a launch to Alpha Centauri, the star system closest to Earth, which is about twenty-five trillion miles away. (The initiative has funding from Yuri Milner, a Russian-Israeli billionaire, and counts among its board members Mark Zuckerberg.)

Loeb also looks forward to the day when well be able to produce synthetic life in our laboratories. From there, he imagines Gutenberg DNA printers that could be distributed to make copies of the human genome out of raw materials on the surface of other planets. By seeding the galaxy with our genetic material, we could, he suggests, hedge our bets against annihilation. We could also run a great evolutionary experiment, one that might lead to outcomes far more wondrous than seen so far. There is no reason to expect that terrestrial life, which emerged under random circumstances on Earth, was optimal, Loeb writes.

When I was a kid, one of my favorite books was Chariots of the Gods?, by Erich von Dniken. The premise of the book, which was spun off into the TV documentary In Search of Ancient Astronauts, narrated by Rod Serling, was that Fermis question had long ago been answered. They had already been here. Von Dniken, a Swiss hotel manager turned author who for some reason in the documentary was described as a German professor, argued that aliens had landed on Earth sometime in the misty past. Traces of their visits were recorded in legends and also in artifacts like the Nazca Lines, in southern Peru. Why had people created these oversized images if not to signal to beings in the air?

I figured that von Dniken would be interested in the first official interstellar object, and so I got in touch with him. Now eighty-five, he lives near Interlaken, not far from a theme park he designed, which was originally called Mystery Park and then later, after a series of financial mishaps, rebranded as Jungfrau Park. The park boasts seven pavilions, one shaped like a pyramid, another like an Aztec temple.

Von Dniken told me that he had, indeed, been following the controversy over Oumuamua. He tended to side with Loeb, who, he thought, was very brave.

He needs courage and obviously he had courage, he said. No scientist wants to be ridiculed, and whenever they deal with U.F.O.s or extraterrestrials, they are ridiculed by the media. But, he predicted, the situation will change.

Its often said that extraordinary claims require extraordinary evidence. The phrase was popularized by the astronomer Carl Sagan, who probably did as much as any scientist has done to promote the search for extraterrestrial life. By whats sometimes referred to as the Sagan standard, Loebs claim clearly falls short; the best evidence he marshals for his theory that Oumuamua is an alien craft is that the alternative theories are unconvincing. Loeb, though, explicitly rejects the Sagan standardIt is not obvious to me why extraordinary claims require extraordinary evidence, he observesand flips its logic on its head: Extraordinary conservatism keeps us extraordinarily ignorant. So long as theres a chance that 1I/2017 U1 is an alien probe, wed be fools not to pursue the idea. If we acknowledge that Oumuamua is plausibly of extraterrestrial-technology origin, he writes, whole new vistas of exploration for evidence and discovery open before us.

In publishing his theory, Loeb has certainly risked (and suffered) ridicule. It seems a good deal more likely that Extraterrestrial will be ranked with von Dnikens work than with Galileos. Still, as Serling notes toward the end of In Search of Ancient Astronauts, its thrilling to imagine the possibilities: Look up into the sky some clear, starlit night and allow yourself the freedom to wonder.

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Have We Already Been Visited by Aliens? - The New Yorker