y the time humanity noticed the object, it was already leaving the solar system. 19 October 2017. Astronomers at the University of Hawaii notice an odd shape tumbling away from Earth, a bright speck hurtling through the deep dark. Informally, they name it ‘Oumuamua, from the Hawaiian for “scout”, and classify it an interstellar asteroid, the first known to visit our solar system. Really, nobody could be sure what it was. Asteroids are rocky and dull and commonly round, but ‘Oumuamua was shiny and elongated. Astronomers had first thought it a comet, but comets have bright gassy tails, and here there wasn’t one. The more data was collected, the more mysterious the object seemed. “Time after time it looked unusual,” says the astrophysicist Avi Loeb, over Zoom. “At some point it crossed a threshold for me. And at that point you say, ‘OK, come on!’” Loeb is the Frank B Baird Jr Professor of Science at Harvard and, until recently, the longest-serving chair of Harvard’s department of astronomy. When we speak, he is in his home office – big old fireplace, books about the cosmos, a remarkable quantity of dark wood – preparing to discuss his new book, Extraterrestrial, in which he argues an exotic hypothesis: that ‘Oumuamua was “designed, built and launched by an extraterrestrial intelligence”. Loeb is 59, but energised like a child. “I should tell you,” he warns, gently teasing, a few days after the US Capitol is stormed. “Today I’m supposed to be interviewed by Fox News. Some people said, ‘Avi, don’t do it. How could you do that?’ And I said, ‘Look, science doesn’t have a political agenda – we should speak to everyone!’” In Extraterrestrial, Loeb outlines an eventful three or so years that has involved a lot of talking. In the summer of 2018, almost a year after ‘Oumuamua was spotted, he began working on a scientific paper that attempted to explain its various peculiarities: its weird geometry, its luminosity, its lack of a cometary tail, the strange fact that it appeared to have accelerated away from the sun in a straight line, out of its orbit, somehow propelled. He’d already written a controversial article for Scientific American. “I said we should consider the possibility of this being technological debris from other civilisations.” But now he more closely analysed the available evidence. “I approached this just like any other scientific anomaly,” he says. “I rule out possibilities and whatever looks the most plausible is what I put out. That’s the way science is done. You just collect more evidence.” Soon, Loeb and a Harvard postdoc, Shmuel Bialy, developed a theory: ‘Oumuamua wasn’t an asteroid or a comet or any other naturally occurring object; it was a huge, pancake-shaped sail, less than a millimetre thick, propelled by the sun’s radiation. Loeb knew nature was incapable of producing such a thing, which thrilled him. “The implication was obvious,” he writes in Extraterrestrial, “something or someone” had manufactured it. As he and Bialy finished their paper, they wrote that ‘Oumuamua “may be a fully operational probe sent intentionally to Earth’s vicinity by an alien civilisation,” though they acknowledged we’ll never know for sure. “It’s too late to image ‘Oumuamua with existing telescopes or chase it with chemical rockets,” they added. Remarkably, nobody had been able to photograph the object while it remained within range. And now it was gone. Loeb grew up on a farm near Tel Aviv, and as a kid he was drawn to life’s big questions. Every weekend he would “grab a work of philosophy” – Nietzsche, Sartre, spurred on by his mother, who was known locally as an excellent baker – and “drive our tractor to a quiet spot in the hills and read for hours”. Later he realised that physics, rather than philosophy, might give him a shot at understanding the world’s truths. He has investigated dark matter and black holes; he has tried to understand when and why the universe formed. In ‘Oumuamua, he is convinced he’s discovered a plausible answer to perhaps the biggest question of all: is there anybody else out there? In October 2018, Loeb and Bialy submitted their hypothesis to a journal, The Astrophysical Journal Letters, which accepted it within three days. When a couple of science bloggers later broke the news – “Harvard Scientist Believes in Aliens!” – the story went viral. Suddenly there were reporters in Loeb’s office, on the Harvard campus, at his home. Shortly after publication, Loeb was due to speak at a conference in Berlin, and as he left for the airport he was confronted by a television crew. “They said, ‘We have to ask: Do you think we’re alone in the universe?’” Loeb recalls, laughing. In a hurry, he managed to reply: “A quarter of all stars host a planet the size and surface area of the Earth,” before adding, “It would be arrogant to think we are alone.” By the time Loeb reached Germany, he’d become an international sensation. On the conference’s opening night there was a dinner for the speakers. He recalls, “All these scientists who work in completely different areas of study, unrelated to what I do, they say: ‘We know who you are! You were in the news!” Truly, he wasn’t expecting the attention, he says. He’d hoped a few scientists might read his paper; he never thought it would reach a significant portion of the entire planet. “There were back-to-back interviews,” he says, of the weeks following publication. He describes the reaction as intense and overwhelming – a frenzy. “At one point they had to put 10 reporters in a room, just to save time.” He remembers looking out at a “forest of outstretched microphones”, repeating his hypothesis to sceptical strangers. What Loeb expected less was the backlash he received from the science community. In Extraterrestrial, he writes that his theory “put me at odds with most of the scientific establishment”, even though, as a tenured Harvard professor on various academic boards, he worked at the core of it. When his paper was published, colleagues turned their noses up. Some thought it was ridiculous, others damaging to the community. Whenever he shared his theory, “Ninty-nine per cent of the time, I’d get this silence,” he says. On Twitter, one scientist described the hypothesis as insulting. Another said: “Next time there’s another unusual object, let’s not tell Avi!” – a petty swipe, Loeb’s theory reduced to a punchline. “That made me upset,” he says. “It’s like kindergarten. Let’s just talk about the science!” The reactions still bother him. “If someone comes to me and says, ‘For these scientific reasons, I have a scenario that makes much more sense than yours,’ then I’d rip that paper up and accept it,” he says. “But most of the people who attacked, they hadn’t even looked at my paper, or read the issues, or referred to the items we discussed.” It’s now widely believed that ‘Oumuamua was pancake-shaped. But there are other hypotheses. Some scientists think the object was a giant hunk of frozen hydrogen. (Hydrogen burns transparently, which would explain the force propelling the object and why nobody could find a cometary tail.) Others think it was a huge cloud of loosely bound particles, what Loeb describes, not kindly, as “a dust bunny”, slowly floating through the cosmos. Like Loeb’s, neither theory can be proved beyond doubt. In Extraterrestrial, he writes, “Nature has shown no propensity to produce pure-hydrogen comets or fluffy clouds of materials that are… structurally cohesive.” So he argues: if these theories can be taken seriously, why can’t mine? In some ways, Loeb sees the argument around ‘Oumuamua as a proxy for a larger debate about the scientific process. Of his colleagues, he thinks: where are the progressive, exciting ideas? Where are the scientists making bold hypotheses without worrying they might damage their careers? He is convinced conservatism is ruining science, to the point where a hypothesis can now be dismissed outright just because it seems silly or outlandish or unfashionable, even when it is as theoretically plausible as any other theory available. Of ‘Oumuamua, he says: “The only reason I was courageous enough to come out was because people privately told me, ‘Yes, this object is something quite unusual.’ They say it privately because they’re afraid to make a public statement. But I’m not afraid. What should I be afraid of?” Once, Loeb went to a seminar on ‘Oumuamua at Harvard. As he left, he got chatting to an astronomer who’d spent his entire career studying objects in the solar system. “He tells me: ‘This object looks so weird, I wish it never existed,’” Loeb recalls, disapprovingly. To him the comment was scandalous. “As scientists we should accept, with pleasure, whatever nature gives us. Science is a dialogue with nature, it’s not a monologue. And what people don’t realise is, nature isn’t supposed to make us happy, or satisfied, or proud of ourselves. Nature is whatever it is.” He goes on, “I find those instances when the data gives us some uneasiness, when the evidence doesn’t line up with what we expect, I feel these are the most exciting moments. Nature is telling you, ‘Your thinking on this is wrong.’ That’s what I’m here for, to learn something new. I’m not in it to feel good about myself, to get likes on Twitter, for the prizes. I’m in it to understand. So a colleague telling me, ‘I wish it never…’” He shakes his head. “You know, I’ve noticed a chilling effect on some people who have worked with me,” he says. “The moment there is backlash from the scientific community, they stop.” I ask why. “Because people at this stage – students and postdocs – they worry about their careers.” Loeb is convinced that, every now and then, a collaborator of his will be told that working with him could damage their hunt for a faculty position, as though it were an ugly blotch on an otherwise stellar CV. “I think that’s the part that is unhealthy here,” he says. “Science is supposed to be without prejudice, open to discussion. Not the bullying.” “Why don’t scientists take this topic seriously?” I ask. “There is a taboo on the subject,” he says. “It frightens credible scientists from discussing it.” He thinks that science fiction, which enables the community to laugh the topic off as unserious, doesn’t help. “Then you have the lunatic fringe. ‘Oh, I saw something in my back yard, a UFO.’ I get those emails now.” All of this dogs Loeb. “My point is, how dare scientists shy away from this question, when they have the technology to address it, and when the public is extremely interested – while at the same time you have theoretical physicists talking about extra dimensions, string theory, about the multiverse? The multiverse is extremely popular in the mainstream. You ask yourself, how can that be part of the conservative mainstream” – but not the search for extraterrestrial life? In his book, Loeb writes that throughout his career he has worked hard to approach problems with childlike wonder, often in defiance of conventional thinking. “If you speak to friends of mine, people from my childhood, they’ll tell you I haven’t changed much,” he says. “That’s on purpose. You might think of me as naive. But when people say, ‘As you get older, you need to abandon risk taking, become more rigid,’ I don’t accept that!” Loeb enjoys working with ideas nobody else has thought to, or actively avoid. These ideas just “bubble up,” he says. A lot of the time they come to him in the shower. (On learning this, a Dutch TV crew filming a programme about Loeb once insisted on recording footage of his bathroom, as if the space were sacred.) “These things are all completely straightforward to me,” he says, of his theories. “But somehow people around me regard them as unusual.” While investigating dark matter, he produced far more speculative ideas, he says. “And yet this went viral.” A broadcaster once asked him why he thought the public is so interested in this topic particularly. “I said, ‘Look, people find the situation on Earth to be so depressing that they’re looking for exciting news from the sky.” He shakes his head. “The situation is so dire here.” Then he searches his mind for a name. “Oscar Wilde! Oscar Wilde said, ‘We’re all in the gutter, but some of us are looking at the stars.’ I think that captures it.” I ask, “Do you have a better idea of what ‘Oumuamua was now?” He shrugs. “It could just have been the outer layer of a spacecraft that was ripped apart. Just some foil – not very sophisticated. It could have been space junk. It could have been, you know, a probe, sent to a habitable region of the solar system a long time ago – I find that less likely, but it’s a possibility. Maybe it’s a member of a grid, an array of such things, used for communications, or as road posts for navigation.” These are all suggestions he puts forward in his book. I say, “But we won’t ever know exactly.” “Right,” he says. “But the idea is to collect data on objects like it, and then figure out if any of them look artificial. My point is, if you aren’t looking for interesting things – wonderful things – you’re never going to discover them!” I nod. “I’m not arguing we know for sure it was an artificial object. I’m saying it’s a reasonable plausibility based on the evidence. And since we can’t chase this one, we should do the next best thing, which is to find similar objects.” In Extraterrestrial, Loeb writes: “The most egregious error we can make is not to take this possibility seriously enough.” Part of the reason he wrote the book, he says, was to inspire emerging scientists to remain open-minded, to take the possibility of extraterrestrial life seriously enough. Because who knows what we’ll miss if we never look? Extraterrestrial: The First Sign of Intelligent Life Beyond Earth by Avi Loeb is published on 4 February by John Murray at £20. Order it for £17.40 from guardianbookshop.com • This article was amended on 31 January 2021 to change a reference to Oumuamua leaving the “galaxy” to it leaving the solar system.