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The “interstellar asteroid” is not an asteroid after all. A detailed study of its path as it hurtled through our solar system indicates that it might actually be a comet, made mostly of ice instead of solid rock.

In October 2017, astronomers spotted an object, later named ‘Oumuamua, flying through our solar system at an unusual angle. Analyses of its trajectory showed that it wasn’t circling our sun, but rather simply passing through. It was the first-ever seen interloper from another star, hurled in our direction from somewhere in the southern sky.

At first, researchers thought it might be a comet, since they often have weird, elongated orbits. We also expected the first interstellar object we’d see to be a comet, because they are brighter and easier to spot than asteroids and more common in our own solar system.

Interstellar mystery

But when most comets pass near the sun, ice from their surfaces heats up and turns into gas creating their distinctive tails and halos. While ‘Oumuamua did have a bright surface like a comet, it showed no signs of gas or a tail.

Now, Marco Micheli at the European Space Agency and his colleagues have found a telltale sign that ices actually are coming off ‘Oumuamua. They modelled the path it should be taking as it gets pulled around by the gravity of the sun, the planets, and other large bodies in the solar system, then compared this to the real path as observed by the Hubble Space Telescope and several ground-based telescopes.

They found that its trajectory could not be explained by momentum and gravity alone – there had to be some kind of thruster pushing ‘Oumuamua faster from the direction of the sun.

Probably not aliens

Before you get too excited, that doesn’t mean ‘Oumuamua is an alien spaceship equipped with rocket boosters. As ‘Oumuamua moved away from the sun, the thrust weakened. The researchers compared this acceleration pattern to models of several possible explanations, but found that the only one that matched was ice heating up from beneath a dusty or rocky surface layer and turning into gas that pushed the interstellar visitor forward.

A more purposeful acceleration from, say, alien thrusters, would likely remain constant as ‘Oumuamua moved through our solar system. “We can explain the behaviour of the object with cometary outgassing, so we don’t need to invoke any more exotic explanation,” says Micheli.

That ice would make it more like a comet than an asteroid. “This just means that it’s not an asteroid. It’s still definitely interstellar,” says Micheli. That indicates that comets in other stellar systems are not radically different from the ones here, he says.

“Just like different bakeries specialize in different sorts of baked goods, different planetary systems might make different sorts of planetesimals,” says Michele Bannister at Queen’s University Belfast. “The fact that this object is fairly similar to comets in our own solar system is an indication that we have a pretty good understanding of how planetary systems form and evolve.”