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The word “interstellar" found physical meaning and validation, if you like, not so much with the 2014 film of that name. Instead, it was 2017, because that was when we first observed an object inside our solar system that came from outside it. Often likened to a cigar, it was later named ‘Oumuamua (see my “Cigar from Outer Space",https://bit.ly/3JUldaM). When the astronomer Robert Weryk first discovered ‘Oumuamua, it was actually on its way out of the solar system. It had soared past Mercury and the Sun, then Venus and our Earth. It was about 30 million km beyond the Earth when it was detected, in images taken at the Pan-STARRS observatory in Hawaii.

Now comets make their way around the Sun, so wasn’t this just another comet? It didn’t look like other comets, though. It did not sport that characteristic long tail. A few hundred metres long at most, it was relatively small for a celestial object. What’s more, even if it was that far away, astronomers were able to deduce its shape: long and thin, almost cylindrical. Like a cigar.

In short, ‘Oumuamua was unlike any object we’ve encountered in our solar system. And its peculiar shape and size got plenty of people wondering: was this something more interesting than just another rock? Could it be a piece of technology from some interstellar civilization? Some astronomers took this possibility seriously indeed. Even as it raced away from us at a phenomenal speed, close to 90 km per second as it swung past the Sun, they focused instruments on ‘Oumuamua, listening for radio signals it might emit. At least one widely-admired and respected professor of astronomy, Avi Loeb, at Harvard University, wrote a book ("Extraterrestrial: The First Sign of Intelligent Life Beyond Earth") laying out exactly why he believed ‘Oumuamua was the product of some unknown civilization out there.

It’s actually a pretty compelling book, even though many scientists dismiss the notion of extraterrestrial civilizations. That’s partly because it is such a strange, alien (pun intended) idea. But it’s also partly because the probability of a random object hurtling through space being something other than a rock is vanishingly small: after all, every other such we’ve ever known has been a rock.

And yet there are other ways of looking at other relevant probabilities. Consider all the stars and galaxies in our universe. Make a reasonable estimate of the fraction of them that have planets, and what fraction of those can support life, at least as we know it. Allow for the passage of time comparable to how long it’s been on Earth between the first signs of life and the civilization we have today. These factors figure in what astronomers know as the Drake Equation (about which, another time, though look it up). The sheer number of stars itself says something about the probability that we’re the only civilization, only form of life, in our universe: it’s pretty small, too.

So what, then, is ‘Oumuamua? If Loeb made a case that it was a spaceship of some kind, there’s recent research that offers another explanation.

One thing about ‘Oumuamua that astronomers had to explain was its unusual acceleration. Making its way through our solar system, it sped up noticeably. In a recent paper, two astronomers remark on this: “Although typical cometary activity tracers were not detected"—no tail, of course—“‘Oumuamua showed a notable non-gravitational acceleration." ("Acceleration of 1I/‘Oumuamua from radiolytically produced H2 in H2O ice", Jennifer B Bergner and Darryl Z Seligman, Nature, 22 March 2023, https://www.nature.com/articles/s41586-022-05687-w.epdf)

“Non-gravitational acceleration": it cannot be explained by its flight path, slingshot-style, past the Sun and other planets. So, what caused the acceleration? If ‘Oumuamua was indeed a spaceship, for example, we might have speculated that it had engines that fired, driving the craft faster and faster.

But this paper proposes another, and perhaps simpler, mechanism. As it passed through our solar system, ‘Oumuamua released “entrapped molecular hydrogen". This release acted like a jet - or indeed a spacecraft engine. That’s why the cigar sped up.

This idea actually gives us clues to ‘Oumuamua’s origins and history. The researchers suggest that it was formed perhaps 100 million years ago—in cosmic terms, that’s very young—as an “icy planetesimal". As this ice-covered little rock travelled through space, it was bombarded by “cosmic rays and high energy photons." That process produced hydrogen, which was trapped beneath the icy surface. And when ‘Oumuamua neared our Sun, in effect the sunlight melted some of the ice and released the trapped hydrogen.

Of course, this process can happen with more routine comets, too. So why don’t they accelerate like ‘Oumuamua did? In a news report, Bergner explained that more routine comets are far larger than ‘Oumuamua —as much as “several kilometers across". Their emission of hydrogen, then, would not have a “detectable effect", certainly not a definite acceleration. But ‘Oumuamua is not several kilometers across. Because it is so small, Bergner thinks the emission of hydrogen “produced sufficient force to power this acceleration."

In fact, she and her colleague actually calculated the number of hydrogen molecules a shell of ice on a cylindrical body like ‘Oumuamua’s could contain. They compared this to the number of hydrogen molecules needed to produce the observed acceleration in a comet of this size. If the first number is greater than the second, this model is a feasible explanation of ‘Oumuamua’s behaviour. It turned out that the explanation held up in a wide range of theoretical models of ‘Oumuamua’s composition and surroundings. So a hydrogen jet it is—or certainly can be— that is powering this strange object as it streaks away from our solar system.

So if ‘Oumuamua is just a comet, Professor Loeb is wrong in his hypothesis of an alien civilization, right? Not necessarily. It’s just that the icy planetesimal explanation may be the simpler and perhaps more complete one.

Yet here’s a question to chew on. Is ‘Oumuamua any less fascinating for being “just" an icy planetesimal?

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

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Updated: 30 Mar 2023, 09:32 PM IST
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