Scientists spotted an oddly shaped visitor in our solar system in 2017. And it was accelerating in an unexplainable way.

The icy object, called ‘Oumuamua, which roughly translates to "messenger from afar" or "scout" in Hawaiian, was unlike any object ever seen in the solar system. It looked unusually long and skinny, like a cigar; had a reddish hue; and created no telltale tail. With more information, planetary scientists now say the interstellar object is "pancake" or dish-shaped.

Unsurprisingly, it's actually an icy comet, not an alien spaceship. And researchers now have a good idea of why it's been speeding up as it traverses our cosmic neighborhood. The explanation is simple, they say:

As the sun heated the comet, the icy object released a particular gas, accelerating and changing ‘Oumuamua's expected trajectory through the solar system.

Researchers Jennifer Bergner and Darryl Seligman recently published their findings in the science journal Nature. The explanation was originally Bergner's idea.

"What’s beautiful about Jenny’s idea is that it’s exactly what should happen to interstellar comets,” Seligman said in a statement. “We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it’s just the most generic explanation."

A strange, speeding comet

‘Oumuamua wasn't just a curious object because of its unusual shape. It was also the first ever observed visitor from outside our solar system, giving it the unique title of "interstellar." And, crucially, it didn't behave like other comets, which are heavily composed of ice. When other comets veer relatively close to the sun, the heat causes them to release a trail of water vapor, gas, and dust, leaving behind an iconic coma or tail, which can move or propel the object. But small ‘Oumuamua, at just some 115 meters long, couldn't possibly release that much water, and astronomers couldn't see any tail.

So what happened?

Bergner, a chemist at UC Berkeley who researches icy space objects, and Seligman, who researches planetary science at Cornell University, propose that the comet shot out the gas hydrogen into space. Ultimately, this gave the comet an accelerating kick.

• The most mysterious asteroids in the solar system
• If a scary asteroid will actually strike Earth, here's how you'll know
• NASA discovers asteroid is a dead ringer for the Empire State Building
• NASA's Hubble confirms largest comet ever seen
• That interstellar visitor from another solar system wasn't an asteroid after all

Space is teeming with radiation. And when this radiation hits water ice, it produces hydrogen, which then became trapped on or just beneath the comet's surface. Ultimately, when sunlight hit 'Oumuamua, it released the gas.

"For a comet several kilometers across, the outgassing would be from a really thin shell relative to the bulk of the object, so both compositionally and in terms of any acceleration, you wouldn’t necessarily expect that to be a detectable effect," Bergner said. "But because ‘Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration."

Want more scienceand tech news delivered straight to your inbox? Sign up for Mashable's Top Stories newslettertoday.

Other solar systems certainly have speeding icy objects, too. After all, the universe is flush with water. With ever-larger and more advanced telescopes, which are coming online this decade, astronomers will likely spot many more of these foreign interstellar visitors. No, it's not aliens, and it may never be aliens. But they are fascinating travelers from distant realms in our galaxy.