1st known interstellar visitor 'Oumuamua is an 'exo-Pluto' — a completely new class of object, scientists say

The first interstellar object to visit the solar system may have been a fragment of an icy exoplanet, research suggests.

When 1I/'Oumuamua was first spotted in 2017, astronomers quickly determined that it came from outside the solar system. But although it was initially classified as a comet from another star system, it may actually be the skin of an "exo-Pluto," a completely unexpected class of Pluto-like objects anticipated to visit the sun.

"Everything about this object is consistent with it being a slab of nitrogen ice like you see on the surface of Pluto," said Steve Desch, an exoplanet researcher at Arizona State University. Desch presented his findings in July at the Progress in Understanding the Pluto Mission: 10 Years after Flyby conference in Laurel, Maryland.

Instead of being a mix of water ice, rock and carbon-rich material left over from the formation of the solar system, 'Oumuamua appears to be almost pure nitrogen ice. And rather than being a compact ball, the visitor is more elongated than any known body in the solar system and starkly different from the interstellar Comets 2I/Borisov and 3I/ATLAS, the only other known interstellar visitors.

"'Oumuamua is in a different category of object," Desch told Space.com by email. "It's much harder to find, but there are a lot more of them."

"We weren't expecting objects like this"

Planets arise from the cloud of gas and dust left over after a star is born. The first few million years are chaotic as the growing worlds jostle for their place around the young star.

In the solar system, the dance of the giant planets cast out a wealth of material. Most of the icy stuff was ejected; scientists think the icy bodies in the Kuiper Belt beyond Neptune today make up only a small portion of the original ejecta. Early on, there may have been enough material to create as many as 2,000 Pluto-like objects, along with 6,000 other, larger dwarf planets, according to Desch.

"Each Pluto would have been pummeled with a Vesta-mass of material," Desch said at the conference, referring to the second-largest object in the asteroid belt. (The largest, Ceres, is also classified as a dwarf planet.)

These collisions would have carved out some of the outermost layer of the wannabe planets. Observations made by NASA's New Horizons spacecraft during its 2015 flyby suggest that most of Pluto's surface is made of nitrogen ice, with some water ice acting as "bedrock." Although some of this base layer was likely ejected as well, Desch and his colleague Alan Jackson, also of Arizona State University, used simulations to determine that most of the material scraped from the baby Plutos was nitrogen.

During the shake-up of the solar system, these objects would have been redistributed. Passing by the sun too often would have caused many of them to evaporate quickly. Some would have been hurled inward, toward the sun. Others would have been tossed outward by Jupiter’s gravity. A handful of that group may have been captured in the Oort cloud at the very edge of the solar system, but most would have ended up adrift in interstellar space.

If planetary dances are common around other stars — and a growing number of observations suggest that they may be — then fragments of exo-Plutos may be ejected alongside comets and full-size planets.

There are hints that some objects classified as comets may actually be chunks of Pluto. In 2018, a separate research team reported that the unusual chemistry of Comet C/2016 R2 hints that it might be a collisional fragment from a Kuiper Belt object. Two other comets, C/1908 R1 Morehouse and C/1961 R1 Humason, have similar nitrogen-rich compositions that could classify them as scrapings from a proto-Pluto.

In a pair of papers published in 2018 and 2021 in the Journal of Geophysical Research: Planets, Desch and Jackson more fully explored how the unusual properties of 'Oumuamua would be better explained by a fragment of a Pluto-like object than by a comet.

"Because we had hardly seen such objects in the solar system, we weren't expecting objects like this," Desch told Space.com. "But we should have. Fragments of icy surfaces from Pluto-like dwarf planets were almost certainly ejected from our solar system, and 'Oumuamua made us come to grips with how much material must have been ejected."

An unlikely comet

When astronomers first spotted 'Oumuamua, it didn't quite meet their expectations of an exocomet. Although its rapid speed was one of the first signs of its extrasolar origin, it was moving much more slowly than anticipated. Solar system comets are made of water ice, silicates and carbon-rich material, while 'Oumuamua was nitrogen-rich. At about 330 feet (100 meters) in diameter before the sun began to melt its ice, 'Oumuamua was also far smaller than most comets, which typically range from about a few kilometers to tens of kilometers in diameter.

Finally, the object had an unusual shape that puzzled astronomers. Eventually, they determined that 'Oumuamua didn't have the roughly spherical core typically seen in comets; instead, it was elongated, or "pancake-shaped," Desch said.

'Oumuamua's low speed could be explained by its ejection from a young star. As stars age, gravitational interactions with their neighbors provide an occasional speed boost. If a fragment from an icy world was ejected early, the star would be traveling relatively slowly, imparting that reduced speed to its expelled material.

The nitrogen-rich material also suggested a youthful lifetime. Exposure to cosmic rays erodes the nitrogen ice, leaving behind water-ice objects that are likely more plentiful. Desch and Jackson estimate that 'Oumuamua is less than 2 billion years old, and perhaps as young as 500 million years old. They suspect it came from a young system, perhaps in the Perseus arm, the closest spiral in the Milky Way to the sun's location in the Orion arm.

The short-lived nitrogen is what made 'Oumuamua so easy to detect. While the water-ice leftovers may be more plentiful, nitrogen ice shines more brightly. But it also evaporates easily; Desch and Jackson estimate that by the time 'Oumuamua was observed, it had lost more than 90% of the mass it had brought into the solar system.

Altogether, it looks as though chunks of exoplanets may be quite plentiful.

"I think these objects are strong support for the idea that fragments of Pluto surfaces are part of the population of things ejected from the solar system," Desch said.

Indeed, the prompt discovery of 'Oumuamua suggests that interstellar objects may be an order of magnitude more abundant than formerly thought. Desch said he expects astronomers will find many more interstellar visitors using the Pan-STARRS and ATLAS surveys that found 'Oumuamua and ATLAS, as well as the newly operational Vera Rubin Observatory. By studying objects from beyond the solar system, researchers may be able to understand more about the outermost dwarf planets.

"More observations of 'Oumuamua-like objects … would tell us a lot about the composition of Plutos," Desch said.

He pointed to observations from New Horizons that suggest our own Pluto may have had a thick coat of nitrogen ice that was lost by impacts and other processes over the 4.5 billion-year life of the solar system.