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 its origin and unusual characteristics ever since. A new paper in Nature Astronomy offers a new comprehensive model to explain some of the object's oddities. 'Oumuamua, as it is called, 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.

"Our objective is to come up with a comprehensive scenario, based on well understood physical principles, to piece together all the tantalizing clues," said co-author Douglas Lin of the University of California, Santa Cruz. "We showed that 'Oumuamua-like interstellar objects can be produced through extensive tidal fragmentation during close encounters of their parent bodies with their host stars, and then ejected into interstellar space."

The interstellar object 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. The team dubbed it 'Oumuamua (Hawaiian for "messenger from afar arriving first"). Other telescopes around the world soon kicked into action, measuring the object's various characteristics, which turned out to be very odd, indeed. For starters, it was accelerating away from our Sun much faster than could be explained by gravity alone. As Ars' John Timmer wrote in 2018,

Its odd orbit had initially had it categorized as a comet, as these tend to have more extreme orbits. But imaging didn't show any indication of gas and dust being released, as is typical when a comet approaches the Sun. That imaging also revealed that it had an elongated, cigar-like shape. Combined with its relatively rapid rotation, this would indicate that 'Oumuamua had to be fairly robust, leading to the conclusion that it was probably an asteroid.

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 for observation to gather as much data as they could about the object before it went on its merry way.

'Oumuamua stirred up a bit of media excitement again in October 2018, when Harvard astronomer Avi Loeb and his then-post-doc, 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."

That short phrase may have launched a thousand hyperbolic headlines, but astronomers are pretty much in consensus that 'Oumuamua is a naturally occurring object and not the result of alien intelligence, despite its strange characteristics. As astrophysicist Katie Mack noted on Twitter at the time, "The thing you have to understand is: scientists are perfectly happy to publish an outlandish idea if it has even the tiniest *sliver* of a chance of not being wrong. But until every other possibility has been exhausted a dozen times over, even the authors probably don’t believe it."

And that brings us to this latest paper, detailing a comprehensive theory for 'Oumuamua's formation that accounts for all of its strange characteristics. Lin and his co-author, Yun Zhang of the Chinese Academy of Science's National Astronomical Observatories, ran several numerical simulations for the kinds of destructive events most likely to lead to unusually elongated fragments like 'Oumuamua. Tidal disruption—the large forces created when a small body passes very close to a much larger one, like a star—proved to be the best match.

Under this scenario, 'Oumuamua's parent body would have been torn apart by those tidal forces, producing numerous elongated fragments that were then ejected into interstellar space. Lin and Zhang also produced thermal modeling demonstrating how the surfaces of those fragments would melt when close to the star, recondensing when they were farther way to form a stabilizing crust.

"Heat diffusion during the stellar tidal disruption process also consumes large amounts of volatiles, which not only explains 'Oumuamua's surface colors and the absence of visible coma, but also elucidates the inferred dryness of the interstellar population," Zhang said. "Nevertheless, some high-sublimation-temperature volatiles buried under the surface, like water ice, can remain in a condensed form."

'Oumuamua is probably not the only object of its kind. Last summer, astronomers observed an interstellar comet, dubbed 2I/Borisov, hurtling into our Solar System from the direction of the constellation Cassiopeia. It's very likely that large swaths of interstellar space may be sprinkled with these kinds of objects—'Oumuamua and 2I/Borisov are just the first ones we've observed.

When the Vera C. Rubin Observatory comes online later this year (coronavirus willing) in Chile, astronomers are optimistic that we'll discover even more interstellar objects visiting our humble Solar System. If those new objects also exhibit the same unusual properties as 'Oumuamua, that would indicate that Zhang and Lin's process might be quite widespread.

"The expected discovery of a population of ‘Oumuamua-like objects within the Solar System over the next several years... has the potential to transform our thinking about and investigations of minor planets within extrasolar systems," Dmitri Veras of the University of Warwick, Coventry, in the UK, wrote in an accompanying Nature commentary. "The timing would be fortuitous, given the mounting detections of individual planetesimals outside of the Solar System. The detailed disruption and thermal models of ‘Oumuamua employed by Zhang and Lin provide a comprehensive, self-consistent history of this object, and a foundation for exploring future detections."

DOI: Nature Astronomy, 2020. 10.1038/s41550-020-1065-8 (About DOIs).