Sorry everyone, it’s not an alien spaceship.
Plenty of hypotheses have been put forth to explain the origins of the enigmatic, cigar-shaped, interstellar object named 'Oumuamua. Most recently, it was aliens. More pedestrian explanations have included that it is a kind of comet or asteroid. But a new study published in Journal of Geophysical Research: Planets suggests a new hypothesis: it’s a broken-off hunk of a Pluto-like dwarf planet.
What’s new — Study authors Alan P. Jackson and Steven J. Desch, both scientists at Arizona State University, looked at a plethora of data about the interstellar object to draw one conclusion: 'Oumuamua is likely a fragment of what they describe as an “exo-Pluto” — essentially, a chunk of a dwarf exoplanet.
'Oumuamua was discovered in our solar system in 2017 and quickly fell out of view, but not out of the public’s consciousness. The mysterious interloper was determined to be an interstellar object, but its true nature has been hotly debated.
Here are the facts scientists tend to agree on:
- 'Oumuamua is an interstellar object — that means it came from beyond the boundaries of our solar system.
- 'Oumuamua is cigar-shaped, and tumbling end-over-end through space.
In the new study, Jackson and Desch looked at 'Oumuamua’s albedo — in other words, how much light it reflects — to determine the object’s chemical composition. What they found suggests 'Oumuamua contains nitrogen. Nitrogen ices are most commonly found in the composition of dwarf planets and planetesimals, like Pluto, for example, or its smaller cousin Eris, which reside in the outer solar system in a region called the Kuiper Belt. But 'Oumuamua’s trajectory means that it couldn’t have come from our Kuiper Belt, but rather, from a similar realm in another star system.
“This research is exciting in that we've probably resolved the mystery of what 'Oumuamua is and we can reasonably identify it as a chunk of an ‘exo-Pluto,’ a Pluto-like planet in another solar system,” Desch said in a press release.
Here’s the background — 'Oumuamua was discovered by the Panoramic Survey Telescope and Rapid Response System (PAN-STARRS) observatory in 2017. Pan-STARRS, among other tasks, looks for small bodies in the solar system, including comets and asteroids, by surveying the sky every night and looking for moving objects.
'Oumuamua immediately presented a conundrum — by studying the angle of its trajectory, astronomers were able to determine that it wasn’t originally from our solar system, unlike other known asteroids. Rather, the angle of 'Oumuamua suggested it was swooping into our solar system from elsewhere — and also swooping out.
'Oumuamua was discovered in mid-October 2017, but it was already too distant and faint to study by December. Astronomers had to scramble to learn as much about it as they could, while they could. Subsequent studies have floated various hypotheses for 'Oumuamua. Asteroids. Comets. Hydrogen ice. Avi Loeb, an astronomer at Harvard University, has even suggested it could be an alien craft. Indeed, the idea that it’s a chunk of a planetary object isn’t new, but Jackson and Desch’s analysis goes further than previous research in determining the nature of 'Oumuamua’s progenitor planet.
How they did it — By looking at how 'Oumuamua reflected the Sun’s light, the researchers found signals consistent with the reflection of the Sun on nitrogen ice. In this case, specifically N2, or dinitrogen. Nitrogen often couples with other nitrogen atoms to remain stable. With a low boiling point of -320 degrees Fahrenheit (-196 Celsius), it’s the kind of compound that can only exist as a solid in very cold places — like the Kuiper Belt.
Once they determined 'Oumuamua was likely made of nitrogen ices, they were able to further refine its dimensions. Interestingly, they find evidence to suggest 'Oumuamua is far smaller in length than previously thought — according to this study, it is about 147 feet long, rather than the previous estimates of 3,280 feet long. They believe this could also explain why 'Oumuamua appeared to accelerate, which would happen if nitrogen ices sublimed (went from solid to gaseous states), acting a bit like a natural rocket fuel.
“Being made of frozen nitrogen also explains the unusual shape of 'Oumuamua,” Jackson said in the press release.
“As the outer layers of nitrogen 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.”
What’s next — 'Oumuamua was a “blink and you miss it” cosmic visitor. It’s never expected to return to our solar system. Rather, its current path will put it beyond the orbit of Neptune by 2022.
But in 2019, another interstellar object — 2I/Borisov — was discovered, meaning that we may get other, frequent visitors from beyond our solar system’s edges. There are also proposals to try and send a probe to visit 'Oumuamua while we still can — although as it continues to tumble through space, this gets harder and harder to accomplish. We may be better prepared next time, however: A European Space Agency proposal called the Comet Interceptor could serve as a rapid-response way to race to the next interstellar object that crosses our path.
Maybe that time it really will be aliens. But probably not.
Abstract: The origin of the interstellar object 1I/‘Oumuamua has defied explanation. We perform calculations of the non‐gravitational acceleration that would be experienced by bodies composed of a range of different ices and demonstrate that a body composed of N2 ice would satisfy the available constraints on the non‐gravitational acceleration, size and albedo, and lack of detectable emission of CO or CO2 or dust. We find that ‘Oumuamua was small, with dimensions 45 m × 44 m × 7.5 m at the time of observation at 1.42 au from the Sun, with a high albedo of 0.64. This albedo is consistent with the N2 surfaces of bodies like Pluto and Triton. We estimate ‘Oumuamua was ejected about 0.4‐0.5 Gyr ago from a young stellar system, possibly in the Perseus arm. Objects like ‘Oumuamua may directly probe the surface compositions of a hitherto‐unobserved type of exoplanet: “exo‐plutos”. In a companion paper (Desch & Jackson, 2021) we demonstrate that dynamical instabilities like the one experienced by the Kuiper belt, in other stellar systems, plausibly could generate and eject large numbers of N2 ice fragments. ‘Oumuamua may be the first sample of an exoplanet brought to us.