A simpler, more mundane explanation for ‘Oumuamua’s strange behaviour

In 2017, a mysterious body dubbed “Oumuamua entered the solar system from interstellar space and then appeared to slowly accelerate away from the Sun as it headed back out.

Without the coma or dust tail of a typical comet, and with an unusual shape somewhere between a cigar and a pancake, the unresolved body resembled an asteroid of some sort more than a comet. The observed acceleration was even more mysterious, leading to widespread speculation and even suggestions the object might be some sort of alien spacecraft.

It turns out there’s a much more mundane explanation that answers most, if not all, the questions ‘Oumuamua raised: outgassing hydrogen, a mechanism likely common among icy comets moving into the warmth of the Sun. No alien technology necessary.


An artist’s impression of ‘Oumuamua as it warmed up approaching the sun from interstellar space and began outgassing hydrogen, giving it a small but intriguing acceleration. Image: NASA, ESA and Joseph Olmsted and Frank Summers of STScIA simple, if somewhat disappointing, explanation for ‘Oumuamua

“A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result,” said Jennifer Bergner, a University of California assistant professor of chemistry who studies the reactions that occur on icy bodies in the deep-freeze vacuum of space.

“Our thought was: If this was happening, could you actually trap it in the body, so that when it entered the solar system and it was warmed up, it would outgas that hydrogen? Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?”

Bergner and Darryl Seligman, now a National Science Foundation postdoc at Cornell University, found that research published decades ago showed that when ice is bombarded by high-energy particles like cosmic rays, molecular hydrogen is produced in abundance and trapped in the ice.

As it turns out, cosmic rays can penetrate ice to a depth of tens of metres, converting a quarter or more of the water therein to hydrogen gas.

“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,” Bergner said. “But because ‘Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration.”

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