The mysterious pancake-shaped object dubbed ‘Oumuamua (Hawaiian for “messenger from afar arriving first”) generated considerable controversy earlier this year with the publication of Harvard astronomer Avi Loeb’s bestselling new book arguing that it could be a piece of alien technology. Now two astrophysicists at Arizona State University (ASU) are counter-arguing that the secret to at least one aspect of the object’s unusual properties lies in solid nitrogen ice. They described their findings in two new papers published in the Journal of Geophysical Research: Planets.

As we reported previously, in late 2017, our Solar System received its very first known interstellar visitor: a bizarre cigar-shaped object hurtling past at 44 kilometers per second. Scientists have been puzzling over the origin and unusual characteristics of ‘Oumuamua ever since. It was first discovered by the University of Hawaii’s Pan-STARRS1 telescope, part of NASA’s Near-Earth Object Observations program to track asteroids and comets that come into Earth’s vicinity. Other telescopes around the world soon kicked into action, measuring the object’s various characteristics.

Because it had a hyperbolic, or escape, orbit around the Sun, ‘Oumuamua is unlikely to pass our way again. So astronomers only had a brief window of time to gather as much data as they could about the object before it went on its merry way. For starters, ‘Oumuamua was accelerating away from our Sun much faster than could be explained by gravity alone—i.e., via a “rocket effect” that is common in comets, caused by sunlight vaporizing the ice such bodies are made of. While its odd orbit initially had it categorized as a comet, imaging didn’t show any indication of gas and dust being released, as is typical when a comet approaches the Sun. Its elongated, cigar-like shape, combined with its relatively rapid rotation, led to an early suggestion that it could also be an asteroid.

‘Oumuamua stirred up a bit of media excitement again in October 2018, when Loeb and his then-postdoc Shmuel Bialy submitted a preprint (since published) to the Astrophysical Journal. As we reported at the time, much of their analysis discussed the possibility of solar radiation pressure, or the momentum transfer of photons striking an object. That just happens to be the driving idea behind “solar sails” that may one day power spacecraft around our Solar System or beyond. Loeb and Bialy closed their paper with a more exotic, highly speculative scenario, suggesting that the object might actually be a very thin solar sail—specifically, “a fully operational probe sent intentionally to Earth vicinity by an alien civilization.”

Loeb expanded on that hypothesis in his new book (released in January), Extraterrestrial: The First Signs of Intelligent Life Beyond Earth, arguing that “we should be open-minded and search for evidence rather than assume that everything we see in the sky must be rocks.” The book received its share of glowing reviews, with a suitable degree of gentle skepticism.

Others were more harsh in their assessment, such as astrophysicist Ethan Siegel, who wrote a strongly worded counter-argument over at Forbes:

Despite the fact that the spectral signatures of the object—its color, reflectivity, size, etc.—are consistent with a natural origin, Loeb offers only loud, immodest speculation about aliens and diatribes about community groupthink. Coupled with inadequate data, which is the only data we have, he’s impossible to prove wrong…. There are literally hundreds of astronomers who work in this field, and Loeb continues to ignore all of them—their work, their data, their conclusions, and the full suite of evidence at hand—instead focusing on his own idea which has no convincing data to back it up… [R]ather than address their scientific objections, he’s stopped listening to other astronomers entirely, instead choosing to try his scientific case in the most unscientific place imaginable: the court of public opinion.

“Everybody is interested in aliens, and it was inevitable that this first object outside the solar system would make people think of aliens,” said ASU’s Steven Desch, co-author of the two new papers, of the Loeb hypothesis. “But it’s important in science not to jump to conclusions. It took two or three years to figure out a natural explanation—a chunk of nitrogen ice—that matches everything we know about ‘Oumuamua. That’s not that long in science, and far too soon to say we had exhausted all natural explanations.”

Last year, a team of astronomers published a paper suggesting that ‘Oumuamua may be the fragment of another, larger parent body—a long-period comet or debris disk, perhaps, or even a super-Earth planet—torn apart by tidal forces as it passed too close to its host star. Specifically, the astronomers showed that ‘Oumuamua-like interstellar objects can be produced through extensive tidal fragmentation during close encounters of their parent bodies with their host stars, then ejected into interstellar space.

Desch and his co-author, fellow ASU astrophysicist Alan P. Jackson, also suggest that ‘Oumuamua likely originated from another solar system, a fragment of an exoplanet knocked off by an impact roughly a half-billion years ago, throwing it out of its parent system. They suggest that parent body likely had characteristics similar to Pluto. Specifically, Pluto, like Neptune’s moon Triton, is covered in nitrogen ice.

Desch and Jackson contend that ‘Oumuamua is also composed of solid nitrogen ice, which can account for the object’s strong push away from the Sun. They calculated the value for just how reflective the body would have to be to account for that strong push and how quickly various types of ice would sublimate. Solid nitrogen exactly matched their calculations. ‘Oumuamua being made of frozen nitrogen also explains its unusual flattened shape. “As the outer layers of nitrogen 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,” Jackson explained.

The study of ‘Oumuamua has yielded useful new insights into planetary systems outside our own Solar System, according to Desch and Jackson, who are hopeful even more such interstellar objects will be discovered as a new generation of telescopes comes online, notably the Vera Rubin Observatory/Large Synoptic Survey Telescope in Chile. “It’s hoped that in a decade or so we can acquire statistics on what sorts of objects pass through the Solar System, and if nitrogen ice chunks are rare or as common as we’ve calculated,” said Jackson. “Either way, we should be able to learn a lot about other solar systems, and whether they underwent the same sorts of collisional histories that ours did.”

DOI: Alan P. Jackson et al., Journal of Geophysical Research: Planets, 2021. 10.1029/2020JE006706.

DOI: S.J. Desch et al., Journal of Geophysical Research: Planets, 2021. 10.1029/2020JE006807  (About DOIs).

Listing image by William Hartmann

https://arstechnica.com/?p=1750596