Detected in 2017 and observable for only a few weeks, the first known interstellar object to pass through the solar system confounded astronomers, sparking speculation it could be a giant snowflake, hydrogen iceberg or piece of a Pluto-like planet – even an alien probe, an idea promoted in a best-selling book.

Each theory attempted to resolve an apparent contradiction: How could the interstellar interloper named Oumuamua (Hawaiian for “first messenger from afar,” and pronounced oh-MOO-uh-MOO-uh) be accelerating like a comet, but lack a comet’s bright, dusty tail?

“In every way we expected these interstellar comets to behave, Oumuamua just acted exactly the opposite,” said Darryl Seligman, a postdoctoral researcher in the Department of Astronomy in the College of Arts and Sciences, and a National Science Foundation postdoctoral fellow. “It was a complete mystery from the very beginning.”

Now Seligman and colleagues believe they’ve unlocked another piece of the puzzle. In recently published research, Seligman helps put forward the most plausible explanation yet for what Oumuamua likely is – an icy planetary building block, broadly similar to comets – and argues that similar small bodies may be found in our midst. He calls them “dark comets.”

“It turns out that we needed something that was as intensely scrutinized as Oumuamua, because it was interstellar, to realize that there’s this new population of dark comets within the solar system itself,” Seligman said.

Insights from Oumuamua and dark comets – one of which a spacecraft will visit in less than a decade – could advance our understanding of planet formation in this solar system and others.

Natural explanation

In a paper published March 22 in Nature, Seligman and first author Jennifer Bergner, an assistant professor of chemistry at the University of California, Berkeley, argue that hydrogen gas released by warming of amorphous water ice likely drove Oumuamua’s nongravitational acceleration, or acceleration beyond the sun’s influence.

Outgassing had been Seligman’s focus since Oumuamua was discovered while he was a doctoral student at Yale University, but every explanation ran into a theoretical or observational problem. A breakthrough came when Seligman’s calculations showed that Oumuamua didn’t receive enough sunlight to power typical cometary outgassing by water or carbon dioxide, suggesting it might instead be composed of what are called hyper-volatiles – perhaps carbon monoxide or an exotic ice such as solid hydrogen or solid nitrogen. That led to Seligman’s own outside-the-box idea about hydrogen icebergs, one he now considers “out there,” but a step in the right direction.

Then Bergner, a former colleague at the University of Chicago, pointed to experiments involving amorphous ice, a disordered type of ice (as opposed to crystalline) that forms at very cold temperatures. It was shown in labs to trap pockets of molecular hydrogen when energetic radiation (akin to cosmic rays) hit the ice. This radiolytically produced hydrogen is released when the ice is warmed and the amorphous ice becomes crystalline, rearranging its structure.

Given Oumuamua’s size – at roughly 400 feet wide and shaped like a pancake, an order of magnitude smaller than most comets – the researchers concluded such hydrogen outgassing was sufficient to power nongravitational acceleration. It was also consistent with missing dust: Oumuamua would not be sublimating dusty surface ice like comets with reflective tails, just releasing trapped interior gas.

“The main takeaway is that Oumuamua is consistent with being a standard interstellar comet that just experienced heavy processing,” Bergner said. “So you could essentially start with something that looks like a water-rich comet and have this scenario make sense.”

Seligman said it’s the most natural explanation: A water-rich comet is exactly what you would expect something ejected from an extrasolar planetary system to be.

“We’ll see how the scientific community responds, but it definitely feels to me like this has to be the right answer,” he said. “It explains all the mysterious properties, and I can’t think of a single theoretical barrier – unlike every other theory that’s out there about Oumuamua.”

Dark comets?

Seligman’s years of work on Oumuamua made him wonder: What if there are objects like it in the solar system, accelerating but not producing a trail of dust?

Davide Farnocchia, a navigation engineer at NASA’s Jet Propulsion Laboratory, identified a sample of seven asteroids with well-known trajectories that appeared to fit the description. Each exhibited nongravitational acceleration too strong to be caused by solar radiation pressure or the Yarkovsky effect, when sunlight is absorbed and re-emitted as heat.

The asteroid with the strongest acceleration became the focus of “2003 RM: The Asteroid that Wanted to be a Comet,” published Feb. 9 in The Planetary Science Journal with Farnocchia as lead author. Seligman broadened the analysis to six more near-Earth objects in a companion paper published Feb. 15, “Dark Comets? Unexpectedly Large Nongravitational Accelerations on a Sample of Small Asteroids.”

The researchers don’t know conclusively what is causing the objects’ acceleration, but as with Oumuamua, believe the most likely source is some form of outgassing that is gentler than a typical comet’s.

“They are for sure there, and they are for sure accelerating,” Seligman said of the dark comets. “They are a new type of object that’s separate from Oumuamua, but Oumuamua is what led us to the discovery.”

Cosmic context

More observations will be needed to confirm dark comets as a new class of small bodies, and to find new ones. Farnocchia pointed to Pluto’s planetary demotion as an example of how objects’ definitions may change based on new scientific knowledge.

“We always thought that asteroids and comets were completely separate classes of objects,” he said. “But the reality is that you might have more of a continuum, and these objects that we saw could be kind of filling the gaps between the two classes.”

Seligman has submitted a proposal for observing time on NASA’s James Webb Space Telescope, and Japan’s Hayabusa2 spacecraft is slated to rendezvous with one of the dark comet candidates in 2031. After showing that a spacecraft could have intercepted Oumuamua with enough notice, Seligman continues to work on concept studies for missions that could get up-close looks at a future interstellar visitor, more of which are expected to be found by new telescopes including the Vera C. Rubin Observatory in Chile.

“Studying interstellar comets and small bodies in our solar system, you can put the Earth and solar system within their cosmic context,” he said. “It’s taught us about the formation and evolution of the planets, which is essentially where we come from.”

The researchers were supported by NASA and the National Science Foundation.