'Oumuamua is one fascinating rock. The 400-meter-long (1,300-ft) cigar-shaped object is the first interstellar visitor ever detected passing through our Solar System, and while it was originally classed as an asteroid after its discovery, astronomers have now updated the label. 'Oumuamua, it turns out, is a comet after all, but it's unlike any other known comet.

'Oumuamua (then known as A/2017 U1) was first spotted on October 19, 2017 by the Pan-STARRS 1 telescope in Hawaii, and it was initially assumed to be a run-of-the-mill asteroid. It wasn't until its trajectory was plotted that astronomers realized it was on a hyperbolic path, meaning it was just popping in for a single lap around the Sun before slingshotting back out of our neighborhood forever.

Scientists from around the world excitedly studied the object during its short visit, and we're still learning new things about it. Once its interstellar origins were established, 'Oumuamua was believed to be a comet, since those are more likely to be whizzing around in deep space between star systems. But on closer inspection, no signs of dust or gas were detected around it, suggesting it was an asteroid. It was even scanned for signs of radio signals, just to make sure it wasn't some kind of alien probe.

A team of astronomers continued to study the object with Hubble until January 2018, when it became too faint to pick up. Using that data, the researchers now say that 'Oumuamua is a comet after all.

The revised label comes from measurements of its velocity and position. The team calculated the trajectory it was expected to take on its way out of the Solar System, based purely on gravitational influences of the Sun and planets like Jupiter. But 'Oumuamua's observed trajectory differed slightly from that path.

"Unexpectedly, we found that 'Oumuamua was not slowing down as fast as it should have under gravitational forces alone," says Marco Micheli, lead author of the study.

Clearly, some other force was at play. The team studied a range of possibilities, including radiation, heat or "solar wind" from the Sun, a collision with another object, or the off-chance that it's in fact two separate closely-knit objects, but none of these fit.

That leaves just one real explanation: 'Oumuamua is a comet. Small jets of dust and gas disturbing its orbit would neatly explain the observed path. Although astronomers haven't been able to directly detect any of these signatures emanating from the object, the team says that this could be because only a very small amount of dust is being released, or perhaps it's mostly pure gas.

"We tested many possible alternatives and the most plausible one is that 'Oumuamua must be a comet, and that gasses emanating from its surface were causing the tiny variations in its trajectory," says Davide Farnocchia, co-author of the study.

Although 'Oumuamua is currently speeding away from us at 114,000 km/h (71,000 mph), astronomers now have a better idea of what to look for and how to study these objects the next time one wanders in from interstellar space. You never know what kind of clues they could be bringing to us from their home star system.

The research was published in the journal Nature.

Source: ESA

Update (July 2, 2018): This article originally stated that 'Oumuamua was on a "parabolic path" when it should have been a "hyperbolic path". Our sincere apologies for the error which has now been corrected and thanks to the readers who pointed it out.

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