On October 19th, 2017, astronomers made the first-ever detection of an interstellar object (ISO) passing through our Solar System. Designated 1I/2017 U1′ Oumuamua, this object confounded astronomers who could not determine if it was an interstellar comet or an asteroid. After four years and many theories (including the controversial “ET solar sail” hypothesis), the astronomical community appeared to land on an explanation that satisfied all the observations.

The “nitrogen iceberg” theory stated that ‘Oumuamua was likely debris from a Pluto-like planet in another solar system. In their latest study, titled “The Mass Budget Necessary to Explain ‘Oumuamua as a Nitrogen Iceberg,” Amir Siraj and Prof. Avi Loeb (who proposed the ET solar sail hypothesis) offered an official counter-argument to this theory. According to their new paper, there is an extreme shortage of exo-Plutos in the galaxy to explain the detection of a nitrogen iceberg.

In the paper where he broached the possibility, Loeb indicated that ‘Oumuamua’s unusual character and behavior were consistent with a solar sail. This included the highly-reflective nature of the object and its profile, which appeared to be either cigar-shaped or pancake-like. More importantly, its sudden acceleration and deviation from its expected orbit appeared to be the result of radiation pressure, which is precisely how solar sails achieve propulsion.

There was also the way it entered our Solar System, which allowed it to make a flyby of Earth after passing closest to our Sun (perihelion). In other words, its orbital dynamics allowed it to get a close look at the only habitable planet in our Solar System, which is precisely what one might expect of an interstellar probe. These arguments were detailed further in Loeb’s book, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, which we reviewed in a previous article.

At the time of the book’s writing, all attempts to explain ‘Oumuamua in terms of natural phenomena fell short. Basically, there was no single explanation that could account for its brightness, profile, and acceleration while acknowledging that there was no evidence of outgassing. In addition, the sudden acceleration could not be attributed to gravitational forces since these should have been slowing ‘Oumuamua down at the time.

In March of 2021, two researchers from the School of Earth and Space Exploration (SESE) at Arizona State University (ASU) offered a new hypothesis. In two published studies, SESE Exploration Fellow Alan Jackson and Professor Steven Desch argued that ‘Oumuamua may have been a nitrogen ice fragment ejected from a young star system (possibly in the Perseus Arm of our galaxy) ca. 400 to 500 million years ago.

In their first paper, Jackson and Desch addressed the size and compositional constraints of ‘Oumuamua and showed how ‘Oumuamua’s albedo was similar to the nitrogen ices on the surface of Triton and Pluto. In their second paper, they showed how these types of nitrogen ice fragments could be generated by the collision of extrasolar objects similar in composition to Pluto and Kuiper Belt Object (KBOs).

Artist’s Concept of Oumuamua. Credit: William Hartmann

By their estimates, these collisions would generate and eject around 100 trillion (~1014) objects into interstellar space, half of which would be composed of water ice and the other half of nitrogen (N2). This population would be enough to satisfy the statistical significance of ISOs necessary to explain the detection of ‘Oumaumua. Equally important was the fact that an object composed of N2 would not form a tail as it neared our Sun, as there would be no water vapor or CO/CO2 to sublimate. As Siraj explained to Universe Today via email:

“The attraction of the nitrogen iceberg hypothesis is primarily in explaining ‘Oumuamua’s non-gravitational acceleration. Just like for a hypothetical hydrogen iceberg, a nitrogen iceberg’s outgassing activity would not have been detectable in the measurements that were taken of ‘Oumuamua by the Spitzer Space Telescope, which simply limited the abundance of carbon-based
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