On October 19, 2017 our Solar System received an unexpected visitor.
The rocky, elongated object came to us from beyond the realms of the Sun — the first ever recorded interstellar object to cruise through our neck of the galaxy. We dubbed it 'Oumuamua, which means "A messenger from afar arriving first" in Hawaiian.
Since then, scientists have puzzled over this cigar-shaped alien body. Where did it come from? Is it a misshapen comet? An alien probe from a distant world?
It turns out 'Oumuamua's origins are even more unusual than we could have supposed.
A new study, published Monday in the journal Nature Astronomy is the first to propose a comprehensive origin story for the mysterious interstellar object.
It suggests 'Oumuamua was once part of a planet that got too close to its host star. Like Icarus flying too near to the Sun, the planet was destroyed in its close encounter, violently shredded by the star's gravity into long, thin, rocky pieces.
Violent origins — Yun Zhang, a researcher at the Côte d’Azur Observatory in France and lead author of the new study, began the quest to uncover 'Oumuamua's origins because there were just no good theories out there to explain the object's weird features.
'Oumuamua resembles a cosmic cigar. And while many assumed 'Oumuamua is a comet, it lacks the telltale coma, or envelope surrounding a comet’s core, and tail of gas and dust. It also has a dry surface more akin to that of rocky asteroids.
Another strange characteristic of 'Oumuamua is how fast it travels through the Solar System. It slingshotted past the Sun at a speed of 196,000 miles per hour — too fast to be explained by the force of gravity alone.
"It really is a mysterious object," Zhang tells Inverse. "We did not know any scenario that can produce that shape before."
To unravel 'Oumuamua's mysteries, Zhang and her team created a series of computer simulations to model what would happen when three types of objects — half-mile-wide planetesimals, frozen objects similar to comets, and larger planets such as super-Earths — came too close to their host star.
The study found that if any of these objects came as close as 220,000 miles to their host star, they would be torn apart by the star's tidal forces. They would essentially shredded into elongated fragments, and then ejected out into interstellar space. The smaller the object, the closer it would have to get for this process to take place.
In 'Oumuamua's case, that means it could be the shrapnel left after one of these destructive events — the remains of a planet that came too close for comfort to its star, somewhere out in our galaxy.
"Our scenario is still hypothetical, but it’s the only scenario that can explain all the puzzling characteristics of 'Oumuamua," Zhang says. The theory is just that, however — the researchers can't prove this is the true origin tale of 'Oumuamua.
"Although we cannot verify the actual thing that happened, but we have high confidence about this scenario."
'Oumuamua population — If the findings hold up, they suggest there may be many more 'Oumuamua-type objects out there.
The process described in the study would take place in planetary systems around low-mass stars or white dwarfs which would have strong enough tidal forces to rip the planets that come too close to shreds. Reassuringly, our Sun isn't dense enough to exert such strong forces. It does exert some tidal forces on the Earth, but the effect is smaller than that of the Moon.
On average, each of the qualifying planetary systems could eject about a hundred trillion objects like 'Oumuamua over the course of their lifespan, according to the researchers. To know whether that is the case, they need a bigger sample size than a single strange space rock, Zhang explains.
"If we can observe many more interstellar objects and we can compare their properties with 'Oumuamua, we can compare the formation and ejection we propose, and compare the occurrence rate, and then we will see how many interstellar objects can be actually produced from our scenario," Zhang says.
There's hope we will find more objects like 'Oumuamua to help unravel these mysteries. Just two years after the discovery of 'Oumuamua, a second interstellar object was spotted by an amateur astronomer. The object, dubbed 2I/Borisov after its spotter, Gennady Borisov, is more similar to comets that formed inside the Solar System.
“We have two interstellar visitors, and each of them is completely different,” Michal Drahus, a postdoctoral scholar at Jagiellonian University in Poland, who co-authored a study on 21/Borisov in 2019, told Inverse at the time.
“One is unlike anything we’ve ever seen in the Solar System, and the other one is completely similar to the Solar System’s comets.”
If scientists are able to observe more of these interstellar objects, they may offer a unique opportunity to study the composition of planetary bodies that form outside of our Solar System. They could also offer clues as to whether or not these interstellar objects are capable of carrying life with them. This is the foundation of a theory known as "panspermia."
The theory of panspermia suggests that life didn’t originate on Earth at all. Instead, so the theory goes, it originated elsewhere in the Universe, and was transported to our planet via interstellar objects like 'Oumuamua, which travel between planetary systems, seeding them with the ingredients for life as they go.
Abstract: The first discovered interstellar object (ISO), ‘Oumuamua (1I/2017 U1) shows a dry and rocky surface, an unusually elongated shape, with short-to-long axis ratio c∕a≲ 1∕6, a low velocity relative to the local standard of rest (~10 km s−1), non-gravitational accelerations and tumbles on a timescale of a few hours1,2,3,4,5,6,7,8,9. The inferred number density (~3.5 × 1013−2 × 1015 pc−3) for a population of asteroidal ISOs10,11 outnumbers cometary ISOs12 by ≥103, in contrast to the much lower ratio (≲10−2) of rocky/icy Kuiper belt objects13. Although some scenarios can cause the ejection of asteroidal ISOs14,15, a unified formation theory has yet to comprehensively link all ‘Oumuamua’s puzzling characteristics and to account for the population. Here we show by numerical simulations that ‘Oumuamua-like ISOs can be prolifically produced through extensive tidal fragmentation and ejected during close encounters of their volatile-rich parent bodies with their host stars. Material strength enhanced by the intensive heating during periastron passages enables the emergence of extremely elongated triaxial ISOs with shape c∕a≲ 1∕10, sizes a ≈ 100 m and rocky surfaces. Although volatiles with low sublimation temperature (such as CO) are concurrently depleted, H2O buried under surfaces is preserved in these ISOs, providing an outgassing source without measurable cometary activities for ‘Oumuamua’s non-gravitational accelerations during its passage through the inner Solar System. We infer that the progenitors of ‘Oumuamua-like ISOs may be kilometre-sized long-period comets from Oort clouds, kilometre-sized residual planetesimals from debris disks or planet-sized bodies at a few astronomical units, orbiting around low-mass main-sequence stars or white dwarfs. These provide abundant reservoirs to account for ‘Oumuamua’s occurrence rate.