ASU astrophysicists Steven Desch and Alan P. Jackson set out to explain the strange features of & # 39; Oumuamua and determined that it was likely a piece of a Pluto-like planet from another solar system.
The mysterious pancake-shaped object called "Oumuamua" (Hawaiian for "messenger from afar arrives first") sparked considerable controversy earlier this year when the new bestselling book by Harvard astronomer Avi Loeb argued that it could be one trade extraterrestrial technology. Now, two astrophysicists at Arizona State University (ASU) argue that the secret of at least one aspect of the object's unusual properties lies in solid nitrogen ice. They described their findings in two new articles published in the Journal of Geophysical Research: Planets.
As we have already reported, our solar system received its first known interstellar visitor in late 2017: a bizarre cigar-shaped object that rushes by at 44 kilometers per second. Since then, scientists have puzzled over the origin and unusual properties of & # 39; Oumuamua. It was first discovered by the University of Hawaii Pan-STARRS1 telescope, which is part of NASA's near-earth object observation program to track asteroids and comets as they approach Earth. Other telescopes around the world soon came into action, measuring the various properties of the object.
Since there was a hyperbolic or escape path around the sun, it is unlikely that Oumuamua will pass our way again. So the astronomers only had a short window of time to collect as much data as possible on the object before it went on its merry way. For starters, Oumuamua accelerated away from our sun much faster than could be explained by gravity alone – i.e. H. By a "rocket effect" common in comets, caused by sunlight vaporizing the ice that makes up such bodies. While its strange orbit originally categorized it as a comet, the imaging showed no evidence of the release of gas and dust that is typical when a comet approaches the Sun. Its elongated, cigar-like shape, combined with its relatively fast rotation, led to an early indication that it could also be an asteroid.
& # 39; Oumuamua caused another stir in the media in October 2018 when Loeb and his then postdoc Shmuel Bialy presented a preprint (which has since been published) to the Astrophysical Journal. As we reported at the time, many of their analyzes discussed the possibility of solar radiation pressure or the momentum transfer of photons hitting an object. That just happens to be the driving idea behind "solar sails" that could one day propel spaceships around our solar system or beyond. Loeb and Bialy completed their work on a more exotic, highly speculative scenario, suggesting that the object might actually be a very thin solar sail – specifically, "a fully functional probe that was purposely sent near Earth by an alien civilization ".
Enlarge /. This painting by William K. Hartmann shows a concept of the Oumuamua object as a pancake-shaped disc.
Loeb expanded this hypothesis in his new book (published January), Extraterrestrials: The First Signs of Intelligent Life Beyond Earth, arguing, "We should be open-minded and look for evidence rather than assume that everything is in heaven see, must be rock. ”The book received its share of glowing reviews with a fair amount of mild skepticism.
Others were tougher in their assessment, such as the astrophysicist Ethan Siegel, who wrote a strongly formulated counter-argument at Forbes:
Despite the fact that the object's spectral signatures – color, reflectivity, size, etc. – are consistent with a natural origin, Loeb only offers loud, immodest speculations about aliens and diatribes about community groupthink. Coupled with insufficient data, which is the only data we have, it cannot turn out to be wrong. There are literally hundreds of astronomers working in this field and Loeb continues to ignore all of them – their work, their data, their conclusions, and the full range of evidence available – instead focusing on his own idea, which does not contain convincing data, In order to support them … (R) besides addressing their scientific objections, he stops listening to other astronomers fully instead of choosing to try his scientific case in the most unscientific place you can imagine: the court of the public Opinion.
"Everyone is interested in aliens, and it was inevitable that this first non-solar system object would make people think of aliens," said Steven Desch of ASU, co-author of the two new articles, The Loeb Hypothesis. "But in science it's important not to reach conclusions. It took two or three years to come up with a natural explanation – a piece of nitrogen ice – that goes with everything we know about 'Oumuamua. That hasn't been that long in science, and it's not that long either. " soon to say we have exhausted all natural explanations. "
Last year, a team of astronomers published a paper suggesting that Oumuamua could be the fragment of another, larger parent body – perhaps a long-period comet, or a debris disk, or even a super-Earth planet – that has been torn apart by tidal forces like it passed too close to its host star. In particular, the astronomers showed that 'Oumuamua-like interstellar objects can be created through extensive tidal fragmentation during close encounters between their parent bodies and their host stars and then ejected into interstellar space.
Desch and his co-author, ASU astrophysicist Alan P. Jackson, also suggest that & # 39; Oumuamua likely came from another solar system, a fragment of an exoplanet that was destroyed by impact about half a billion years ago , and has thrown it out of its parent system. They suggest that the parent body likely had properties similar to Pluto. In particular, like Neptune's moon Triton, Pluto is covered in nitrogen ice.
Enlarge /. Illustration of a plausible story for & # 39; Oumuamua: origin in its parent system about 0.4 billion years ago; Cosmic ray erosion during their journey to the solar system; and passage through the solar system, including its closest approach to the sun on September 9, 2017, and its discovery in October 2017.
S. Selkirk / ASU
Desch and Jackson claim that & # 39; Oumuamua is also made of solid nitrogen ice, which may be responsible for the strong pressure on the object from the sun. They calculated the value of how reflective the body would have to be to account for this heavy pressure and how quickly different types of ice would sublimate. Solid nitrogen exactly matched their calculations. 'Oumuamua made from frozen nitrogen also explains its unusual shape, which could be a pancake-like disc. "As the outer layers of nitrogen evaporate, the shape of the body flattens progressively, just like a bar of soap when the outer layers are rubbed off with use," Jackson said.
The & # 39; Oumuamua study has provided useful new insights into planetary systems outside of our own solar system, according to Desch and Jackson, with the hope that even more such interstellar objects will be discovered when a new generation of telescopes goes online, particularly the Vera Rubin Observatory / Large synoptic survey telescope in Chile. "We hope that in about a decade or so we can get statistics on what types of objects are passing through the solar system and whether chunks of nitrogen ice are rare or as common as we calculated," Jackson said. "In any case, we should be able to learn a lot about other solar systems and whether they have gone through the same collision histories as ours."
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 (Via DOIs).
Listing picture by William Hartmann