The first-known interstellar object in our solar system, known as 'Oumuamua, continues to defy scientific explanation. Now, one of the latest explanations for what the cigar-shaped interloper is made of — a "nitrogen iceberg" — has also been shot down.
In a recent attempt to explain 'Oumuamua, researchers described it as a nitrogen iceberg. But astrophysicists at Harvard say that's impossible, and explain why in a new paper published Nov. 5 in the journal New Astronomy.
In October 2017, when astronomers first caught sight of 'Oumuamua zipping through our solar system, it was making its exit at nearly 57,000 mph (92,000 km/h) — way too fast to have originated in our solar system.
As the flat, wonky-shaped object passed the sun, tumbling end-over-end, it accelerated at a pace that couldn't be explained by the gravitational pull of the sun. And astronomers couldn't find any visible evidence of a propellant, such as water vapor or gases escaping the object and thrusting it forward.
Not only are scientists unsure what propelled 'Oumuamua on its slingshot visit into and out of our solar system, they also don't know what it is made of.
But in March, Arizona State University astrophysicists Alan Jackson and Steven Desch said they had figured it out. The team published two papers announcing that 'Oumuamua was most likely a chunk of nitrogen ice that popped off a Pluto-like planet somewhere outside our solar system, Live Science previously reported.
The theory would solve the invisible propellant mystery, because as 'Oumuamua approached the sun, evaporating nitrogen gas would have pushed the object and been invisible to telescopes. And, astronomers know that nitrogen ice exists in our solar system because they've found it on Pluto, so it's not unreasonable to suspect that chunks of nitrogen ice occasionally split away from exo-Plutos.
Why it might not be nitrogen
But not everyone agrees with this conclusion.
"The moment I saw those papers, I knew that there was no physical mechanism for it to work. And not even the error budget for it to work," said Amir Siraj, an astrophysicist at Harvard University, referring to the amount of error for the prediction to still be realistic.
According to Siraj and his co-author, Harvard astrophysicist Avi Loeb, Jackson and Desch's conclusion that 'Oumuamua is a nitrogen iceberg is flawed because there isn't enough nitrogen in the universe to make an object like 'Oumuamua, which is somewhere between 1,300 and 2,600 feet (400 and 800 meters) long and between 115 and 548 feet (35 and 167 m) wide.
Pure nitrogen is rare, Siraj said, and has been found only on Pluto, where it makes up about 0.5% of the total mass. Even if all of the nitrogen ice in the universe was scraped off every Pluto-like planet that's predicted to exist, there still wouldn't be enough nitrogen to make 'Oumuamua.
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Siraj and Loeb calculated that the mass of exo-Plutos needed to make a nitrogen iceberg the size of 'Oumuamua would exceed the mass of stars, requiring — at a minimum — more than 60 times the mass per star needed to make all the planets in our solar system. "But that's crazy," Siraj said. "It's preposterous."
Siraj and Loeb made many conservative assumptions in their calculations, Siraj said, such as ignoring the effects from cosmic rays, subatomic particles that are constantly flying through space at the speed of light and degrading everything they slam into, including objects like 'Oumaumua. When cosmic rays are taken into consideration, Siraj calculated that about 1,000 times the entire mass of stars in the galaxy would be necessary to generate all the exo-Plutos to build 'Oumuamua.
However, Jackson and Desch said their careful calculation of the number of nitrogen fragments flying around in space is not an overestimate and is consistent with previous research predicting how many 'Oumuamua-like objects exist in space.
"Siraj and Loeb did not find that we made a mistake, and so they should have accepted the numbers we got," Desch told Live Science in an email. "Instead, they attempted their own back-of-the-envelope calculation and made a great number of approximations and estimates, and came up with different numbers that they say aren't favorable."
A very large window of error is necessary when estimating the number of objects based on a single observation, Jackson said, as is the case with 'Oumuamua; astronomers have never seen anything else like it. Siraj and Loeb calculated that the mass needed to make 'Oumumua was very high, he said, because they used a very high estimate for the number of 'Oumuamua-like objects in space.
"They are attempting to manufacture controversy when none exists," Desch said.
According to Siraj, however, the mystery of 'Oumuamua is still not solved. Some experts may be eager to jump to conclusions about 'Oumuamua, he said, because as long as it's a mystery, the possibility of artificial origin is still on the table. "If it's still unexplained, you have to consider all possibilities."
But that's what makes 'Oumuamua so fascinating, he added. "I don't really care what it is, because every single possibility is an astrophysical object we've never seen before, so that's why it's exciting."
Correction: This article was updated at 11:25 am ET to correctly state that Siraj and Loeb calculated the amount of nitrogen required would be more than 60 times the mass per star needed to make all the planets in our solar system, not two times the mass. And, they calculated that about 1,000 times the entire mass of stars in the galaxy, not the sun, is required when cosmic rays are taken into account.
Originally published on Live Science.