Slashdot reader boudie2 shares this article from BBC Future, which notes that astronomers spent decades looking for objects from outside our solar system — until two arrived at once. “‘Oumuamua has not yet been definitively classified as a comet or an asteroid — it might be something else entirely,” the article points out. For one thing, ‘Oumuamua didn’t have a comet-like tail:
Two things in particular fixated scientists. The first was its mysterious acceleration away from the Sun, which was hard to reconcile with many ideas about what it might have been made of. The second was its peculiar shape — by some estimates, it was 10 times as long as it was wide. Before ‘Oumuamua, the most elongated known space objects were three times longer than they were wide… [F]inally, earlier this year Jackson and his colleague Steven Desch came up with an explanation that seems to explain ‘Oumuamua’s quirky features, without the need for any alien technology… “We just realised that nitrogen ice could supply exactly the amount of push it needs — and it’s observed on Pluto,” he says. To corroborate the idea, they calculated how shiny the surface of ‘Oumuamua was and compared it to the reflectivity of nitrogen ice — and found that the two were more or less exact matches.
The team concluded that the object was likely to be a chunk of nitrogen ice, which was chipped off the surface of a Pluto-like exoplanet around a young star. Based on the evolution of our own solar system, which started out with thousands of similar planets in the icy neighbourhood of the Kuiper belt, they suggested that the fragment may have broken off around half a billion years ago…
Though the object would have finally reached the very outermost edge of the Solar System many years ago, it would have taken a long time to travel to the balmy, central region where it was first discovered — and been gradually worn down into a pancake as it approached. This explains its unusual shape and its acceleration in one go, because the evaporating nitrogen would have left an invisible tail that propelled it forwards. “Our atmosphere is mostly nitrogen and you can see though it,” says Jackson. “Nitrogen gas is difficult to detect.”
Again, not everyone is happy with this suggestion.
Luckily, the second interstellar object, 2I/Borisov “has turned out to be emphatically less difficult to decipher than its cosmic companion. It’s been recognised as the first interstellar comet ever found.”
Read more of this story at Slashdot.