Get the Mach newsletter.
By Corey S. Powell

Astronomers have cataloged eight planets, 6,500 comets and more than 525,000 asteroids, but Oumuamua is one of a kind. The elongated space object, which was discovered speeding past the sun in 2017, is the only celestial body known to have visited our solar system from another.

Oumuamua is believed to be an exotic type of comet or asteroid, but it's such an oddity that some astronomers have speculated it could be an alien spacecraft.

How was Oumuamua discovered?

Robert Weryck, a postdoctoral researcher at the University of Hawaii’s Institute for Astronomy, found Oumuamua by accident. On Oct. 19, 2017, he was using the Pan-STARRS telescope on Maui to scan the skies for Earth-approaching asteroids. At first he thought he had found one. “But then I was able to locate it in two images from the previous night,” he said, “and when I combined them together the orbit didn’t make sense.”

After collecting additional observations, Weryck and his colleague Marco Micheli concluded that the object’s trajectory indicated that it had originated “from outside our solar system.”

Astronomers had been seeking such a visitor for decades. “What's most surprising is that we've never seen interstellar objects pass through before,” Karen Meech, another astronomer at the institute, told NASA a week after the discovery.

What’s with that name?

The object was officially cataloged as 1I/2017 U1 (“1” for first and “I” for interstellar), but such a historic find clearly needed something more memorable.

“Everyone agreed that we wanted it to have a Hawaiian name,” Weryck said. “We contacted the Hawaiian studies group at the University of Hawaii. We told them about the object, how we came about finding it, and they proposed Oumuamua.” The name, which means “first scout from a distant place,” is pronounced “oh MOO ah MOO ah.”

What does Oumuamua look like?

Picture a cigar floating through space, and you’ve got the right idea.

Oumuamua passed too far from Earth to appear as anything more than a dot through even the biggest telescopes. But the way its light brightened and then dimmed indicated a stretched-out shape — at least seven times as long as it is wide, Meech estimated — and it's small, no more than 3,000 feet by 400 feet. The object tumbles as it goes, completing one full flip about every eight hours.

Despite the many realistic-looking illustrations you might have seen of Oumuamua, astronomers have no idea about what it looks like up close — although evidence suggests it has a reddish surface.

What is Oumuamua made of?

No one knows that for sure, though its color is similar to that of some comets, which are made of rock and ice. Oumuamua sped up after it passed the sun, as if pushed by an “outgassing” of frozen material from its surface, which seemed to confirm its identity as a comet. But observations by NASA’s Spitzer Space Telescope revealed no cometlike tail of gas or dust, perplexing astronomers.

“It has to be cometary to explain the acceleration,” Meech said, adding that it might look different from the comets we know because it was it born around another star. In addition, Oumuamua would have been blasted by radiation and dust during its long journey through deep space, which could have formed a crust that trapped most of its frozen gases inside. Such a rough journey might also account for its long, skinny shape.

Where did Oumuamua come from? Where’s it going?

Oumuamua came from the direction of the constellation Lyra and is now heading toward the constellation Pegasus. Its path and motion don’t obviously match up with those of any nearby star. But by backtracking its movements, one team of astronomers determined that Oumuamua would have been close to a small red star named HIP 3757 about a million years ago, suggesting that as its possible point of origin.

The sun’s gravity is slowing Oumuamua, but not enough to hold onto it. It will eventually settle into its cruising velocity of 59,000 mph (16 miles per second), bobbling along between the stars of the Milky Way.

Could it really be an alien ship?

Harvard astronomer Avi Loeb gave this controversial idea a big boost with a scientific paper arguing that Oumuamua’s unusual shape and its surprising acceleration suggest that it could be a lightsail — a spacecraft driven by sunshine — made by aliens. Loeb doesn’t claim that it's an alien craft, only that scientists should consider the possibility.

The paper was dismissed by many astronomers, but it inspired some researchers to listen (futilely) for radio transmissions from Oumuamua. That suggests that the only way to know what it is for sure would be to study it up close.

With that in mind, a group of space engineers drew up Project Lyra, a proposal for an ultrafast spacecraft that could catch up to Oumuamua. The authors propose that such a mission would be possible by sending a large rocket on a slingshot trajectory around the sun.

Is Oumuama really one of a kind?

Although Oumuamua is the first object of its type ever observed, astronomers believe there could be trillions of others like it out there. It's possible that they've been passing by us all the time but are so fast and faint that we’ve missed them until now.

The Large Synoptic Survey Telescope in Chile, which will continuously scan the sky with a 27-foot light-collecting mirror, could be able to spot more such objects when it begins operation in 2022. When it's no longer one of a kind, astronomers should finally get a clearer picture of what Oumuamua really is.

Want more stories about astronomy?