New Theory Suggests ‘Oumuamua Is a Nitrogen Ice Pancake

When the interstellar visitor swept around the sun, it got a speed boost, probably because some of the ice vaporized

A painting of 'Oumuamua shows it as a red, flat, circular chunk of rock with a crater in the middle
'Oumuamua is shaped less like a cigar, as was originally proposed, and more like a pancake, according to recent research. Illustration by William Hartmann

When astronomers first detected ‘Oumuamua in 2017, it was shooting through our solar system so fast, it could only have originated from a faraway star. The strange object is long gone, but astronomers are still working to solve the mysteries it raised.

Two studies published on March 16 in JGR Planets present the case that the interstellar traveler might be a large chunk of nitrogen ice that chipped off of a Pluto-like exoplanet millions of years ago, during the formation of a solar system far, far away. The nitrogen iceberg theory is the second natural explanation for ‘Oumuamua’s behavior, following research last year that suggested it could be a rocky fragment of a planet ripped apart by its star. Both are a far cry from the extraterrestrial theories that emerged when ‘Oumuamua was first spotted.

“When I first started reading [the new research], I was skeptical … but it does tick a lot of the necessary boxes,” says Carnegie Institution for Science astronomer Scott Sheppard, who was not involved in the work, to Maria Temming at Science News. “It’s definitely plausible that this could be a fragment of an icy dwarf planet,” but the case isn’t closed yet.

‘Oumuamua was first observed by the Pan-STARRS1 telescope on Maui in Hawaii, and as it was the first object recorded from beyond our solar system, its name means “a messenger from afar arriving first,” Jason Daley wrote for Smithsonian in 2017.

Telescopes around the globe turned toward the object to gather as much data as possible during its short visit. They found several strange features: no visible dust or comet tail, a unique oblong shape—the object is ten times longer than it is tall—and a reddish color that resembles objects in the outer solar system. Observations suggested that ‘Oumuamua had been tumbling around the Milky Way for hundreds of millions of years before it happened to swing between Mercury and the sun.

When ‘Oumuamua left our solar system, it sped away a little faster than it had entered. That speed boost resembled the boost that comets get when they zoom away from the sun. The heat from the sun vaporizes some of the comet’s ice, which propels the now-lighter object forward. In the new study, the researchers analyzed how several kinds of ice made of hydrogen, nitrogen and oxygen would have behaved in the same situation.

As it turned out, a large chunk of nitrogen ice would act the same way that ‘Oumuamua did. But nitrogen gas would have been difficult for astronomers to measure with the telescopes they had at hand.

"In essence, there was a tail like one would expect for a comet, it is just that because of what it is made of, we didn't detect it," says study co-author Arizona State University astrophysicist and planetary scientist Alan Jackson to Charles Q. Choi at Space.com.

The researchers also identified plausible sources of nitrogen icebergs. Dwarf planets and other planetary bodies at the edges of solar systems, like Pluto and Neptune’s moon Triton, are covered in nitrogen ice. Between the two papers, the Jackson and co-author Steven Desch suggest that early in the formation of a far-away solar system, planetary bodies collided, and a chunk of nitrogen was lopped off.

Eventually, it wandered to our corner of the Milky Way. As it approached the sun, its faces wore down until it became a flat, pancake-like version of itself.

“The idea is pretty compelling,” says Yale University astronomer Garrett Levine, who was not involved in the work, to Science News. “It does a really good job of matching the observations.”

‘Oumuamua is now too far away to take new measurements, and it’s unlikely to come back for another visit. But astronomers are optimistic that more interstellar objects are out there to observe. A second interstellar object, a rogue comet called 2I/Borosov, was just detected in 2019. The Vera Rubin Observatory in Chile is expected to spot about one interstellar object per year once it begins operations, currently scheduled for 2023.

Until then, ‘Oumuamua offers ample excitement for astronomers. Desch tells Space.com, "The thought that what we saw could be a chunk of an actual exoplanet is thrilling."