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SYFY WIRE Bad Astronomy

Is the alien chunk 'Oumuamua actually a hydrogen iceberg?

By Phil Plait
Artwork depicting what ‘Oumuamua might look like; observations indicate it’s highly elongated. But where did it come from? Credit: ESO / M. Kornmesser

Every time I wonder just how weird 'Oumuamua is, some scientists step in and come up with ideas that are even weirder.

Is it a comet? (maybe) Is it a spaceship? (No) Is it the remains of a shattered alien planet? (cooool, but doesn't explain everything) Is it a giant fractal snowflake? (What? Well, yeah, possibly)

The newest: It may be a hydrogen iceberg slowly grown in the core of a gigantic star-making factory and nudged into a galactic orbit.

OK, wow. Yeah, let's back up. 'Oumuamua* is the first interstellar object ever found to pass through our solar system. Unlike the planets and asteroids and comets that orbit the Sun, this thing came barreling in from deep space, literally from somewhere else in the galaxy. We know this from its orbit: Due to the Sun's gravity, objects orbiting it move at speeds that depends on the shape of their orbit and distance from the Sun. 'Oumuamua is moving way too fast to be bound to the Sun, so it came from Out There, and to Out There will it return.

That's not the only weird thing about it. It was expected to be a comet — a mix of ice and rock — because those are easiest to dislodge from a star and send into interstellar space. But it didn't act like one. It got to within about 40 million kilometers from the Sun, but despite that didn't show any sort of cometary activity, like having its ice turn to gas, surrounding the solid bit with a huge cloud that reflects sunlight. Nothing like that was seen at all.

Then it got weirder; it was seen to be accelerating away from the Sun on its way out! Again, not a spaceship, but that is likely due to the comet getting warmed by the Sun, having its surface ice turn to gas, and giving the solid body a little push away as it expands.

But again, nothing like this was actually seen by telescopes. So what gives?

That's where the new research comes in. They looked at what kinds of materials could provide that extra kick for 'Oumuamua. Assuming it was covered in various ices that could turn into gas and give it a shove, they went through the physics to find the one what fits the data best. What they came up with was molecular hydrogen: Two hydrogen atoms bound together to make H2. A relatively small amount of hydrogen ice would do the trick pretty efficiently, acting like a rocket motor as it expanded off the surface when warmed by the Sun. Also, H2 gas doesn't emit much light, so it would've been extremely difficult if not impossible to detect. Cool.

But they also found it answered some other questions. 'Oumuamua changes brightness on a regular cycle, getting brighter and dimmer over the course of a few hours. That's a key characteristic of a non-spherical object tumbling as it flies through space. It might be pancake shaped or elongated like a cigar. When we see it face-on (for a pancake) or side-on (for the cigar) we see more sunlight-reflecting surface, so it appears brighter, but when it turns we see less surface, so it looks dimmer. Previous studies have found it more likely that 'Oumuamua is shaped like a pancake, a flattened spheroid something like 115x111x19 meters in size. That's far more non-spherical than bodies we see in the solar system.

This part is neat: If it's a hydrogen iceberg, that shape arises naturally! If it starts off slightly irregular (that is not a perfect sphere), as it approaches the Sun hydrogen will start to sublimate off, turning from ice to gas. Any flattened area will absorb more sunlight, warm more, and erode away quicker. Hydrogen ice sublimates at a very low temperature, so this rapidly started sculpting the shape of 'Oumuamua, flattening it into a pancake.

Since it passed so close to the Sun (which happened before it was discovered, unfortunately) this happened pretty fast. They find that it must have been much larger on approach to the Sun — probably 200 x 200 x 100 meters — but lost up to 80% of its mass by the time it was discovered!

We've all been thinking of 'Oumuamua as a comet that orbited another star and got ejected somehow. But hydrogen ice is extremely volatile, and even if it were on a very distant orbit around a star, it's unlikely to survive for long. So where did it come from?

The authors speculate it formed in the middle of a huge, dense, and extremely chilly giant molecular cloud. These enormous nebulae have extremely cold spots where hydrogen ice can nucleate — attach themselves to grains of dust and then grow in size as more ice sticks to them. They expect a lot of objects like this can be made, most of which are destroyed when stars form, but some of which drift out of the clouds and into space.

Clever! 'Oumuamua being made of lots of hydrogen ice does seem to explain a lot of its characteristics, even the really weird stuff. It occurred to me that once it becomes a gas, sunlight will break apart the H2 molecule to make hydrogen atoms, and those do emit ultraviolet light we can detect. So why didn't we see it? I contacted Darryl Seligman, the lead author of this paper, who told me this was correct! It wasn't seen because we didn't see 'Oumuamua until it was too far away, and no UV observations were made. He hopes that if we find another object like it we'll be ready for it. Perhaps the European Space Agency's Comet Interceptor, designed to catch up to fast-moving objects like 'Oumuamua, will be able to help.

Until then, we'll have to keep scanning the skies, looking for more interstellar interlopers. If this idea is right, then molecular clouds make lots of them, so maybe we won't have to wait too long.

*The pronunciation is a little difficult if you're not used to it; the tick mark is an 'okina, sortof a Hawaiian soft glottal stop; like when you say, "uh-oh." After that, it's OH-oo-moo-ah-moo-ah.