The debate continues: ‘Oumuamua could be a remnant of a Pluto-like planet

The mysterious pancake-shaped object called ‘Oumuamua (Hawaiian for “messenger from far arrives first”) caused considerable controversy earlier this year with the publication of Harvard astronomer Avi Loeb’s best-selling new book claiming it could be a piece of alien technology. to be. 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 previously reported, in late 2017, our solar system received its very first known interstellar visitor: a bizarre cigar-shaped object whizzing past at 27 miles per second. Since then, scientists have wondered about the origin and unusual features of ‘Oumuamua. 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 approaching Earth. Other telescopes around the world soon sprang into action, measuring the object’s different characteristics.

Since it had a hyperbolic or escape orbit around the sun, ‘Oumuamua is unlikely to pass our way again. So astronomers only had a short window of time to gather as much data about the object as possible before it continued its merry path. For one thing, ‘Oumuamua accelerated away from our sun much faster than could be explained by gravity alone – that is, via a “rocket effect” common to comets, caused by sunlight evaporating the ice that such bodies are made of. . Although its strange orbit was initially categorized as a comet, imaging did not show any indication of the release of gas and dust, as is typical when a comet approaches the sun. Its elongated, cigar-like shape, combined with its relatively fast rotation, led to an early suggestion that it could also be an asteroid.

Oumuamua caused some media frenzy in October 2018 when Loeb and his then postdoc Shmuel Bialy submitted a preprint (since publication) 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 spacecraft around our solar system or beyond. Loeb and Bialy concluded their paper with a more exotic, highly speculative scenario, suggesting that the object could in fact be a very thin solar sail – specifically, “a fully operational probe deliberately sent to Earth’s vicinity by a 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 look for evidence rather than assume that everything we see in the sky must be rocks.” The book received widespread critical acclaim, with an appropriate degree of mild skepticism.

Others were tougher in their assessment, such as astrophysicist Ethan Siegel, who wrote a strongly articulated counter-argument at Forbes:

Despite the fact that the object’s spectral signatures – its color, reflectivity, size, etc. – are consistent with natural origins, Loeb offers only loud, indecent speculations about aliens and rant about groupthink in the community. Coupled with inadequate data, which is the only data we have, he can’t possibly prove him wrong … There are literally hundreds of astronomers working in this field, and Loeb continues to ignore them all – their work, their data, their conclusions and the full evidence available – instead focusing on his own idea that has no compelling data to back it up … [R]In addition to addressing their scientific objections, he no longer listens to other astronomers at all, opting instead to try his scientific case in the most unscientific place imaginable: the court of public opinion.

“Everyone is interested in aliens and it was inevitable that this first object outside the solar system would make people think of aliens,” said Steven Desch of ASU, 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 come up with a natural explanation – a chunk of nitrogen ice – that matches everything we know about ‘Oumuamua. long in science, and too soon to say we 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 – perhaps a long-period comet or debris disk, or even a super Earth – torn apart by tidal forces if it passed too close to his guest star. Specifically, the astronomers showed that ‘Oumuamua-like interstellar objects can be produced by extensive tidal fragmentation during close encounters of their mother bodies with their host stars, and then ejected into interstellar space.

Desch and his co-author, fellow ASU astrophysicist Alan P. Jackson, also suggest that “ Oumuamua is likely from another solar system, a fragment of an exoplanet that was impacted about half a billion years ago, and it is its older system. They suggest that the parent body likely had features similar to Pluto. In particular, Pluto, like Neptune’s moon Triton, is covered in nitrogen ice.

Desch and Jackson argue that ‘Oumuamua is also composed of solid nitrogen ice, which may explain the object’s strong thrust from the sun. They calculated the value for how reflective the body should be to account for that strong push and how quickly different types of ice would sublimate. Solid nitrogen matched their calculations exactly. ‘Oumuamua is made from frozen nitrogen, which also explains its unusual flattened shape. “As the outer layers of nitrogen evaporated, the shape of the body would have become more and more flattened, much like a bar of soap does when the outer layers are rubbed off with use,” Jackson explained.

Oumuamua’s study has provided useful new insights into planetary systems outside our own solar system, said Desch and Jackson, who hope that more such interstellar objects will be discovered when a new generation of telescopes comes online, most notably the Vera Rubin. observatory. / Large Synoptic Survey Telescope in Chile. “It is hoped that in a decade or so we can get statistics on what kinds of objects pass through the solar system and whether chunks of nitrogen ice are rare or as common as we have calculated,” Jackson said. “Either way, we should be able to learn a lot about other solar systems and whether they have gone through the same kind of collision histories as ours.”

DOI: Alan P. Jackson et al., Journal of Geophysical Research: Planets, 2021.10.1029 / 2020JE006706.

DOI: SJ Desch et al., Journal of Geophysical Research: Planets, 2021.10.1029 / 2020JE006807 (About DOIs).

Table image by William Hartmann