The interstellar asteroid ‘Oumuamua is almost definitely not an alien spaceship. A review of all the data we have on the strange object, which came hurtling through our solar system in 2017, has concluded that everything about it can be explained by natural processes. Astronomers spotted ‘Oumuamua in October 2017, three days after it made its closest approach to Earth and more than a month after it flew past the sun. The asteroid’s trajectory made it clear that it must just be passing through from beyond our solar system, rather than originating here. It is a weird object: it’s likely cigar-shaped, at least six times as long as it is wide, and tumbling end over end every eight hours. Its shape and spin may be related to its origin. Researchers have suggested that it could be a shard of a planet that got too close to its star and was ripped apart, the shreds tossed out of the planetary system. Alternatively, interstellar dust may have ground it down into its unusual shape over the course the journey to our solar system. The strangest thing about ‘Oumuamua is its acceleration. As it moved past the sun, it started to speed up, more than could be accounted for by gravitational forces alone. The simplest explanation was that, like a comet, it was releasing dust and gas as the sun heated it up, which would act as a sort of thruster to push it forward. But observations showed that there was no small-grained dust coming off ‘Oumuamua, and none of the gases that we looked for showed up. That led some researchers to speculate that it might be large and flat, like a solar sail, and the sun’s light alone was pushing it to speed up – a few even pondered whether it could actually be a solar sail constructed and sent here by aliens. That’s not the case, says Matthew Knight at the University of Maryland, part of the team that put together this new report. “The argument for the solar sail is that it has to be aligned with our sun to give it the acceleration that we see,” he says. “You can’t have it be working as a solar sail and also spinning.” Instead, he says, ‘Oumuamua could be outgassing water vapour, just at a lower level than we were able to detect with observations. “There are plenty of ways to form it that don’t involve aliens, and I don’t think there’s any reason to invoke something that extraordinary unless we rule everything else out,” Knight says. Now, ‘Oumuamua is too far away to to any more observations, so the data we have is all we’re going to get until we spot another interstellar object. “The next time an interstellar object is discovered we’ll be better prepared with the things we want to know about it,” Knight says. Maybe next time we’ll be able to rule out an alien craft for certain.
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