Mystery of alien comet 'Oumuamua solved

'Oumuamua, the first known interstellar object to pass through our solar system, got an unexpected boost in speed and shift in trajectory as it passed through the inner solar system in late 2017.

The shape of 'Oumuamua is very different from any known asteroids or comets in our own solar system. This highly-elongated cigar-shaped object was travelling at a speed of about 315,000 kilometres per hour (196,000 miles per hour) when it was nearest the Sun in Sept. 2017. It is now headed out of the solar system and will never return.

An astrochemist from the University of California, Berkeley and an astronomer from Cornell University have now come up with a new theory that the comet’s mysterious deviations from a hyperbolic path around the sun can be explained by a basic physical process that is probably widespread among many icy comets - outgassing of hydrogen as the interstellar comet warmed up in the sunlight.

"For a comet several kilometres 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. But because ‘Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration," said Jennifer Bergner, a UC Berkeley assistant professor of chemistry who studies the chemical reactions that occur on icy rocks in the cold vacuum of space.

As comets approach the sun, they typically release water and gases from their surface, producing a glowing gaseous coma and dust. However, in the case of 'Oumuamua, astronomers were unable to detect any coma or dust around it.

Bergner thought that outgassing of hydrogen entrapped in ice might be sufficient to accelerate ‘Oumuamua. According to the researchers, ice at the surface of a comet could emit enough gas, either in a collimated beam or fan-shaped spray, to affect the orbit of a small comet like ‘Oumuamua.

"Even if there was dust in the ice matrix, you’re not sublimating the ice, you’re just rearranging the ice and then letting H2 get released. So, the dust isn’t even going to come out," Bergner's colleague Darryl Seligman, now a National Science Foundation postdoctoral fellow at Cornell University, adding that their conclusion about the source of ‘Oumuamua’s acceleration should close the book on the comet.

Bergner and Seligman were postdoctoral fellows at the University of Chicago when they began collaborating on the paper published in the journal Nature.