'Oumuamua could be a relic from giant clouds where stars are born

When the mysterious object 'Oumuamua zoomed through our solar system back in 2017, it was clear it was unlike any interstellar object astronomers had ever seen.

It was strangely shaped. It wasn't an asteroid. It seemed to behave like a comet — it changed speed as it flew around the sun like comets do. But it had no signature cometary tail. It was a cosmic mystery with no good answer, until now. 

In a new study, scientists suggest what they describe as the most likely explanation for all of its oddities. They think 'Oumuamua is a dense chunk of exotic hydrogen ice mixed with a little dust. 

"If we're right and 'Oumuamua is a cosmic hydrogen iceberg, that's fundamentally a new type of astrophysical object," said Darryl Seligman, a postdoctoral fellow at the University of Chicago.

Hydrogen ice forms at spectacularly low temperatures, only six degrees above absolute zero. 

"The only places you could even feasibly form solid macroscopic hydrogen icebergs are in the coldest, densest, and darkest parts of the galaxy. And those are the giant molecular cloud cores."

In our Milky Way galaxy, there are clouds of gas and dust that form molecular clouds. Among the most famous are the so -called "Pillars of Creation" seen in images captured by the Hubble Space Telescope. Light doesn't penetrate inside these clouds, so hydrogen — which normally exists in a gaseous state — freezes into dense pre-stellar cores. In many of these molecular clouds, these become the building blocks of new stars. But this isn't the fate of all of them.

"What you do is you take a starless core that fails to form a star, you build up these large macroscopic hydrogen icebergs and eventually the cloud disperses and these things are just gently released into the galaxy," Seligman told Quirks & Quarks' Bob McDonald. 

Parsing out clues from 'Oumuamua's behaviour

The starting point for Seligman in solving this mystery was a study describing how the sun's gravitational acceleration couldn't explain 'Oumuamua's trajectory. 

"There was an extra force that was acting on 'Oumuamua and always pushing it away from the sun," he explained.

The sun's energy provides that extra force by heating up the ice, turning it into gas that explodes toward the sun pushing 'Oumuamua in the opposite direction. 

Seligman and his supervisor asked themselves: "What species of ice could have powered the non-gravitational acceleration of 'Oumuamua just from the amount of energy 'Oumuamua received from the sun?"

There's probably about one passing through the inner solar system at any given time.- Darryl Seligman, University of Chicago

"The constraint is shockingly powerful," said Seligman. "Almost nothing works."

Ordinary comets are mostly made of water ice, but their calculations showed that wouldn't provide enough thrust to explain the observations. Their next possibility was hydrogen ice. 

"Based on the fact that hydrogen ice is not very well held together, that seems like one of the only possible — and certainly the most likely accelerant," he said.

Hydrogen icebergs in our solar system likely extremely common

It's entirely possible objects like 'Oumuamua have been visiting our solar system in some numbers, but we've never seen them.

Seligman said 'Oumuamua was very small and dim when scientists first spotted it quite close to Earth — about 40 lunar distances away. And the telescope that discovered it has only been in operation since 2010. 

"The fact that we saw one of those in the last couple of years when Pan-STARRS, which is the survey in Hawaii that detected 'Oumuamua was operating, means that there's probably about one passing through the inner solar system at any given time," he said.

In a previous study, he showed that when the Vera Rubin Observatory comes online, we should be able to detect a lot more of these hydrogen icebergs sooner than we observed 'Oumuamua.

Given the effort we put into studying comets, asteroids and meteorites to learn about the early formation of our solar system, Seligman said hydrogen icebergs will give us an even earlier glimpse into the conditions that give rise to stars. 

"If the giant molecular cloud cores — the sites of star formation — are locking up their material into solid hydrogen icebergs, and they're making so many of them that they're passing through our solar system all the time, that means that we could up close study the most pristine and primordial material imaginable in the galaxy," he said.

Produced and written by Sonya Buyting