The first object ever known to have visited the Earth's Solar System from outer space, the quirky comet 'Oumuamua, has been the subject of fascination since being spotted in 2017 as it was so strange that at least one leading astronomer was convinced it was an alien vessel, including its curious acceleration as it hurtled away from the sun.

Hypotheses about its unexpected behavior were floated, including fleeting speculation that it might have been an alien spacecraft. However, a new study has offered a more sober explanation – that 'Oumuamua's speedup was due to the release of hydrogen gas as the comet warmed up in the sunlight.

'Oumuamua lacks the tail of gas and dust characteristic of many comets. It was previously described as being cigar-shaped but now is thought to resemble a rocky pancake. Smaller than initially estimated, it is now pegged at approximately 115 meters (375 feet) by 111 meters, with a thickness of about 19 meters.

Researchers said it appears that 'Oumuamua was born like many other comets as a planetesimal – a small object formed in the early stages of planet formation – and was essentially a large, icy space rock.

After it was somehow ejected from its solar system of origin, the comet's chemistry changed as it was bombarded by high-energy radiation while venturing through interstellar space. This converted some of the comet's ice – frozen water – into hydrogen gas trapped within the rest of its ice.

'Oumuamua then was warmed up as it passed through our inner solar system, causing the comet's ice structure to rearrange and releasing the trapped hydrogen gas – giving 'Oumuamua a little bit of a kick as it headed away from the sun. Removing this hydrogen in an outgassing process would not cause a visible tail.

"The key finding is that 'Oumuamua may have started as a water-rich icy planetesimal broadly similar to solar system comets. This model can explain the strange behavior of 'Oumuamua without needing to resort to any exotic physics or chemistry," said the University of California, Berkeley astrochemist Jenny Bergner, lead author of the research published this week in the journal Nature.

"The simplest explanation, and exactly what we would expect for an interstellar comet, fits all of the data with no fine-tuning," said study co-author Darryl Seligman, a postdoctoral fellow in planetary science at Cornell University.

'Oumuamua, whose name refers in the Native Hawaiian language to a messenger arriving from a great distance, was first detected by the University of Hawaii's Pan-STARRS1 telescope.

"We don't know its place of origin, but it was probably traveling through interstellar space for less than 100 million years. It had a reddish color consistent with the colors of many small bodies in the solar system. It is currently past Neptune on its way out of the solar system," Bergner said.

A second interstellar object, comet 2I/Borisov, was discovered visiting our solar system in 2019. This one looked and behaved more like a typical comet.

These alien interlopers may be more common than previously known. The researchers said one to two interstellar objects may be discovered every year in our solar system once a new astronomical observatory now being built in Chile begins operations as planned next year.