'Oumuamua was an iceberg of molecular hydrogen, scientists claim

June 9 (UPI) -- Ever since scientists spotted 'Oumuamua, the solar system's first documented interstellar visitor, there has been some debate over whether the object was a comet or an asteroid.

Now, astronomers argue 'Oumuamua was a neither a comet nor an asteroid, but a new type of object.

"We hypothesize in the paper that 'Oumuamua was a hydrogen iceberg," Darryl Seligman, postdoctoral research fellow at the University of Chicago, told UPI in an email.

Seligman and his research partner, Gregory Laughlin of Yale University, published their hypothesis on Tuesday in the Astrophysical Journal Letters.

By the time astronomers first saw 'Oumuamua, it was already headed on its way back toward interstellar space, but scientists were able to make out its shape, clock its spin rate and calculate its mass.

While initial analysis of the object's trajectory suggested it moved like a comet, 'Oumuamua didn't look like one. It didn't have a tail and it was dark.

To better understand the object's composition, Seligman and Laughlin took a closer look at 'Oumuamua's movement through our solar system. Observations revealed a slight acceleration on the object's path away from the sun, a surprise.

Fighting against the sun's gravitational pull should have caused a slight slowdown, not an acceleration. Seligman and Laughlin determined that the acceleration was most likely caused by outgassing.

"What happens is that a comet has ice, or volatiles, on the surface," Seligman said. "When the comet gets close enough to the sun, the photons from the sun can provide enough energy to sublimate the ice."

Once the sun's heat severs the bonds between molecules in the ice, the solar energy begins to quickly excite the now gaseous molecules.

"Now, this gas produces an outflow, in which the molecules explode off of the surface of the sun away from the comet, generally in the direction of the sun," Seligman said. "By the conservation of momentum, this outflow will also back react on the comet, and push it away from the sun. We see this type of behavior in solar system comets all of the time."

The outgassing of 'Oumuamua was different than most comets, however. The interstellar visitor was without a tail, or coma, and the object's acceleration was too great to be explained by water ice.

When Seligman and Laughlin looked for better candidates, they settled on molecular hydrogen. Their calculations showed the outgassing of modest amounts of molecular hydrogen ice would produce sharp acceleration without yielding a visible coma.

The discovery has implications for tracking down 'Oumuamua's cosmic origins.

"Molecular hydrogen ice is a weird thing to think about, simply because H2 will sublimate at approximately 6 degrees Kelvin, which is only a few degrees above the cosmic microwave background at 2.7 Kelvin," Seligman said.

"So there are not many places in the universe that are thought to be able to get cold enough to freeze out hydrogen," Seligman said. "Certainly, you won't be finding frozen hydrogen most places in a protoplanetary disk where comets and asteroids form."

The best place to find molecular hydrogen ice is someplace really cold, like inside a failed stellar core in a giant molecular cloud, some of the coldest, densest places in the universe.

Seligman and Laughlin hypothesize that what's called a prestellar core, a dense orb of star-making material, failed to generate fusion. Without fusion, the core remained especially cold and accumulated chunks of hydrogen ice that were eventually ejected into interstellar space.

Such an origin story not only explains 'Oumuamua's acceleration through our solar system, but also its unusual baguette-like shape.

The weathering of a hydrogen iceberg would naturally produce an especially oblong object. When it passed through our solar system, 'Oumuamua was six times longer than it was wide. Seligman likened the process to using a bar of soap.

"You start with a bar of soap which is somewhat elongated, maybe 2:1, and as you continuously use it, you take off roughly equal amounts of soap everywhere," Seligman said. "So you always take a larger percentage of soap from the small axis as the long axis."

"This continuous removal of material evenly off of the surface naturally produces a smaller but more elongated object," he said. "That is what we envisioned happened to Oumuamua, but instead of soap, it was hydrogen coming off the surface, both due to the solar photons and the galactic cosmic rays that were hitting it after it was ejected from the Giant Molecular Cloud."

The hypothesis proferred by Seligman and Laughlin, however, can't be directly proven. To better understand 'Oumuamua, scientists need to start by detecting and studying a lot more instrestellar objects -- specifically, hydrogen icebergs -- the researchers said. And they need to find them before they make their closest apporach to the sun.

But, according to Seligman, the ultimate goal is to acquire material samples.

"I think advocating for an interception mission to an interstellar object is an important next step," he said.