Scientists finally solve the mystery of ‘interstellar object’ Oumuamua

When Comet Oumuamua whizzed past Earth in 2017, scientists were baffled by its unusual features.

Comets speed up when they get close to the sun because, as they heat up, the ice stored inside turns into water vapor that is ejected outward and acts as a propulsion.

This expulsion of gas manifests as a dust tail or bright halo called a “coma” – but Oumuamua had none of these things and was still accelerating.

This led many to suggest that it was an alien spacecraft powered by an alien engine.

But now researchers from the University of California, Berkeley and Cornell University in the US have come up with a new, simpler explanation.

The lack of a trail may be because the tiny comet was expelling a thin shell of hydrogen gas that was invisible to telescopes.

Dr. Jenny Bergner, the first author of the new study, said: “For a comet several kilometers across, the outgassing would come from a very thin shell relative to the bulk of the object, so both in terms of composition and in terms of acceleration, you wouldn’t necessarily expect this to be an observable effect.

“But because Oumuamua was so small, we think it produced enough force to drive this acceleration.”

Nicknamed “dirty snowballs” by astronomers, comets are balls of ice, dust and rock that typically emanate from the ring of icy material called the Oort Cloud at the outer edge of our solar system.

They move into the inner solar system as different gravitational forces detach them from the Oort cloud, becoming more visible as they get closer to the heat given off by the sun.

As they approach, the comets melt, releasing a stream of water vapor, dust, and other molecules that are blown off their surfaces by solar radiation and plasma.

This manifests a cloudy and outward tail, giving them an outward kick that slightly changes the shape of their orbit around the sun.

Also around a comet is a thin and gaseous atmosphere filled with more ice and dust called a coma.

However, on October 19, 2017, scientists in Hawaii spotted an object sailing past Earth that looked and behaved slightly differently.

First, it was moving very fast, traveling at about 97,200 mph (156,428 km/h) — a speed that scientists concluded could not have been caused by the sun’s gravity.

Further analysis revealed that Oumuamua had an unusually elongated shape, like a cigar, and tumbled through space.

These observations suggested that the object was not bound to the sun and was therefore the first observation to come from outside the solar system.

While it accelerated similarly to other comets, it was also much smaller than normal, with a length of only about 115 meters.

This, plus the fact that it was quite far from the sun, meant it wouldn’t be able to produce enough water vapor to give it the non-gravity it exhibited.

Plus, it didn’t have the trademark tail or coma, which led the SETI Institute — which stands for Search for Extra-terrestrial Intelligence — to say there was a possibility it was “an alien artifact.”

For the new study, published in Naturethe scientists wanted to test a new theory that the comet was actually propelled by undetectable hydrogen gas.

Some have suggested that the comet was actually an iceberg made of solid hydrogen or nitrogen, as these could have evaporated at the distance Oumuamua was from the sun.

However, such materials have never been observed before and the conditions that would lead to their formation are unclear.

So the new team looked at previous experiments on how high-energy particles, such as cosmic rays from interstellar space, would affect ice trapped in a comet.

They found that they could penetrate tens of meters deep into the rock, reach deeply embedded ice and convert it into hydrogen gas.

This would remain locked in the rock until it got close to the sun, where the heat would change the structure of the solid ice and cause the gas to be expelled.

Models showed that the force of this expulsion of gas would be enough to accelerate the tiny object from its hyperbolic orbit around the sun.

Until now, our understanding of comets smaller than those only a few kilometers across has been limited by a lack of observations.

But since the arrival of Oumuamua, more and more comas and tailless comets have been spotted behaving in a similar way.

This research proves that, unfortunately, they are not necessarily signs of extraterrestrial life, and actually behave as might be expected.

“The great thing about Jenny’s idea is that it’s exactly what should happen with interstellar comets,” says lead author Dr. Darryl Seligman.

“We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it’s just the most common explanation.”