Scientists say they've solved the mystery of cigar-shaped comet 'Oumuamua

From the moment scientists named 'Oumuamua – which roughly translates to "scout" in Hawaiian – the first interstellar object to enter the solar system, Earth has buzzed with speculation about its nature and origin.

The cigar-shaped oddity from outer space was detected passing through the solar system 2017 and is, by now, on its way to the Pegasus constellation.

'Oumuamua moved in a comet-like way but lacked the tail of vapour that would normally be seen trailing a comet. It was also small compared to comets, which can be several kilometres wide.

Its elongated shape and other unusual characteristics led to theories it was everything from a "cosmic dust bunny," to an alien probe, to a chunk of a distant planet.

Scientists now say they know what it is, and the answer is less sensational than some previous theories have suggested.

According to a study published in the scientific journal Nature on Wednesday, 'Oumuamua is most likely an interstellar comet.

Until 'Oumuamua, scientists had only ever observed comets that originated within the solar system and had similar characteristics. NASA describes these comets as "frozen leftovers from the formation of the solar system composed of dust, rock, and ices."

As solar system comets orbit the sun, solar radiation causes them to heat up and spew jets of vapourized dust and gas that are visible from Earth: their signature tails.

The authors of this latest study, Jennifer Bergner and Darryl Seligman, have figured out that 'Oumuamua probably does have a cometary tail. It's just invisible. Because 'Oumuamua is so much smaller than the comets scientists typically observe, Bergner and Seligman wrote that it could have generated a hydrogen gas jet that was just too small and thin to be detectible to telescopes.

The way it moved through the solar system supports the idea, too.

According to NASA, cometary jets are powerful enough to give comets a speed boost at certain points in their orbit, independent of the sun's gravitational pull. 'Oumuamua accelerated as it passed through Earth's inner solar system in a manner similar to a comet, but the lack of a visible tail suggested it wasn't a comet, leaving scientists confused about the source of its acceleration.

If 'Oumuamua's tail was too small to be visible, but just strong enough to give it a boost, Bergner and Seligman said that would explain its comet-like, but confusing, characteristics.