Insights into an alien comet that entered the Milky Way

By Mark Waghorn via SWNS

An alien comet that entered the Milky Way from another galaxy is being powered by hydrogen gas from ice reserves, according to new research.

Shaped like a giant cigar, it is the first known 'visitor' from outside our solar system.

Some even suspected it of being a spaceship. But there is a surprisingly simple explanation for its weird orbit, say astronomers.

Named Oumuamua - Hawaiian for 'messenger from afar' - it is 1,300 feet long by and 131 feet wide. NASA has never seen anything like it.

It was spotted in 2017 - firing the imaginations of scientists and the public alike.

Unusually, there was no bright coma or dust tail and its small size more befitted an asteroid.

The fact it was accelerating away from the sun instead of around it also perplexed experts.

Now a US team has shown the mysterious deviations from a hyperbolic path were caused by the outgassing of hydrogen as the comet warmed up in the sunlight.

It was so small its gravitational deflection around the sun was slightly altered by the tiny push created when gas spurted out.

Most comets are essentially dirty snowballs that periodically approach the sun from the outer reaches of the solar system.

When warmed by sunlight, a comet ejects water and other molecules, producing a bright halo or coma around it and often tails of gas and dust.

The ejected gases act like the thrusters on a spacecraft to give the comet a tiny kick that alters its trajectory slightly from the elliptical orbits typical of other solar system objects, such as asteroids and planets.

Lead author Dr. Jennifer Bergner, of California University in Berkeley, studies the chemical reactions that occur on icy rocks in the cold vacuum of space.

She said: "A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result.

"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?"

When ice is hit by high-energy particles akin to cosmic rays, molecular hydrogen (H2) is abundantly produced and trapped within the ice.

In fact, cosmic rays can penetrate tens of meters into ice, converting a quarter or more of the water to hydrogen gas.

Dr. Bergner said: "For a comet several kilometers 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."

To date, all the comets observed in our solar system — the short-period comets originating in the Kuiper belt and the long-period comets from the more distant Oort cloud have ranged from around one kilometer (0.6 miles) to hundreds of kilometers across.

Co-author Dr. Darryl Seligman, of Cornell University in New York, said: "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."

Comets are icy rocks left over from the formation of the solar system 4.5 billion years ago, so they can tell astronomers about the conditions that existed when our solar system formed.

Interstellar comets can also give hints about the conditions around other stars surrounded by planet-forming disks.

Dr. Bergner said: "Comets preserve a snapshot of what the solar system looked like when it was in the stage of evolution that protoplanetary disks are now.

"Studying them is a way to look back at what our solar system used to look like in the early formation stage."

Faraway planetary systems also seem to have comets, and many are likely to be ejected because of gravitational interactions with other objects in the system.

Some of these rogue comets should occasionally enter our solar system, providing an opportunity to learn about planet formation in other systems.

Dr. Seligman said: "The comets and asteroids in the solar system have arguably taught us more about planet formation than what we've learned from the actual planets in the solar system.

"I think that the interstellar comets could arguably tell us more about extrasolar planets than the extrasolar planets we are trying to get measurements of today."

Their conclusion about the source of 'Oumuamua's acceleration should close the book on the comet. Since 2017 the researchers have identified six other small comets with no observable coma.

Dr. Bergner said: "The main takeaway is that 'Oumuamua is consistent with being a standard interstellar comet that just experienced heavy processing.

"The models we ran are consistent with what we see in the solar system from comets and asteroids. So, you could essentially start with something that looks like a comet and have this scenario work."

The idea also explains the lack of a dust coma.

Dr. Seligman said: "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."

The study is published in Nature.