Four years ago, astronomers detected ‘Oumuamua—the first interstellar object known to pass through our solar system. The object featured an array of strange and inexplicable characteristics, some of which are consistent with an icy shard ripped from a Pluto-like object, according to new research.
“We suggest ‘Oumuamua was probably thrown out of a young star system about half a billion years ago,” declare the authors of two new research papers published in the Journal of Geophysical Research: Planets. Because the object exhibits features seen on Pluto and Neptune’s moon Triton, the authors, planetary scientist Alan Jackson and astrophysicist Steven Desch, both from Arizona State University, say “‘Oumuamua may be the first piece of an exoplanet brought to us.”
Of course, what they mean is that it’s the first known piece of an exoplanet brought to us; and by exoplanet they mean an exo-dwarf planet, as Pluto is technically not a planet (if these corrections are not what they intended to say, then I’m saying they should be).
In their papers, Jackson and Desch classify ‘Oumuamua as an “ex-Pluto,” which I think is pretty cool. Indeed, astronomers often compare exoplanets and other astronomical phenomena to those in orbit around our Sun, referring to hot Jupiters, super-Earths, and sub-Neptunes, for example. We can now add “ex-Plutos” to the list of known astronomical objects, and by virtue of this, “Plutos” in general—small icy worlds located in the Kuiper belts (another analogous term borrowed from our solar system) of distant star systems.
Until this traveler from afar paid us a visit, “we’ve had no way to know if other solar systems have Pluto-like planets, but now we have seen a chunk of one pass by Earth,” said Desch in an AGU statement. Well, that’s assuming this interpretation is correct, which if it is, means ‘Oumuamua would be the first evidence that Pluto-like objects exist elsewhere in the galaxy.
‘Oumuamua didn’t stay for long when it visited our neighborhood in 2017, as it was traveling at speeds reaching 196,000 miles per hour (315,430 km/h). It’s hard to comprehend that kind of speed, but saying it traversed 54 miles each second (87 km/s) helps somewhat.
The interstellar object was fast, but it was also weird. ‘Oumuamua is fairly small—about half the size of a city block—but exceptionally thin, featuring a depth of around 115 feet (35 meters). So weird and unprecedented is this shape that at least one scientist said ‘Oumuamua might not be natural at all and instead some kind of probe dispatched by extraterrestrials. The object is also very shiny (i.e. it has a high albedo), it has a faint comet-like coma, and it exhibits a slight rate of acceleration seemingly not caused by gravity.
For the first of the two studies, Jackson and Desch considered several different types of ice that could exist on such an object. They did this to determine how the evaporation of ice might contribute to the observed non-gravitational acceleration of the object. The scientists made calculations of how quickly these various ices sublimated (when a solid changes directly to gas) when ‘Oumuamua passed by our Sun. Factors like mass, shape, and reflectivity were also taken into account to explain the propulsion-like effect produced by the sublimating ice.
Solid nitrogen turned out to be the best match. That’s a very interesting result, as Pluto and Triton are known for their solid nitrogen-rich surfaces and for similar albedos to the one described for ‘Oumuamua.
Nitrogen could also explain the object’s unusual shape. ‘Oumuamua had only recently taken on its pancake-like appearance, a consequence of having flown close to the Sun, according to the study. The resulting meltage caused the object to lose upwards of 95% of its total mass, and as the ice evaporated, “the shape of the body would have become progressively more flattened, just like a bar of soap does as the outer layers get rubbed off through use,” said Jackson.
In the second paper, the authors estimated the rate at which Pluto-like objects might have large chunks of ice ripped from their surfaces during their youth. They also estimated the rate at which these pieces would go interstellar and make the long trek to our solar system.
“A similar fragment, generated in another solar system, after travelling for about a half billion years through interstellar space, would match the size, shape, brightness, and dynamics of [‘Oumuamua],” wrote the authors in the second paper. “The odds of detecting such an object, as well as more comet‐like objects like the interstellar object 2I/Borisov, are consistent with the numbers of such objects we expect in interstellar space if most stellar systems ejected comets and [nitrogen] ice fragments with the same efficiency our solar system did.”
Object 2I/Borisov, in case you’re wondering, was detected in 2019, and it’s the second known interstellar object to pass through our solar system.
Matthew Knight, an astrophysicist at the U.S. Naval Academy and an expert on ‘Oumuamua, was impressed with the comprehensiveness of the two studies.
“The authors have done an excellent job of meeting various observational and theoretical constraints with a simple and self-consistent model,” Knight, who was not involved with the new research, said in an email. “Their key idea, that ‘Oumuamua was composed primarily of highly reflective nitrogen ice, is both creative and satisfyingly plausible, since we have ample evidence that nitrogen ice is common on the surface of Pluto and other large objects in the outer solar system.”
Knight said these ideas have “a good chance to ultimately be accepted as the best explanation for ‘Oumuamua.”
As it stands, we only know of two interstellar objects, ‘Oumuamua and 2I/Borisov, but that could soon change thanks to the upcoming Vera C. Rubin Observatory and the 10-year Legacy Survey of Space and Time project.
“It’s anticipated that LSST should find approximately one per year, so when we have 10 or 20 known objects, we’ll be in a much better position to make a statistical assessment,” said Knight. “It will be very exciting to see how these results change our understanding of how our solar system works and reveal how similar—or not—our solar system is to other solar systems.”