Scientists Think They’ve Uncovered Mysterious Arms Near the Boundary of the Solar System

• The Oort cloud is a shell of icy objects that forms the very outskirts of our Solar System.

• Recently, a group of researchers discovered that the inner portion of the Oort cloud likely has spiral arms that make it look like a galaxy.

• The arms were brought into existence by “galactic tides”—gravitational forces that galaxies are able to exert on the objects inside of them.

There’s a weird quirk built into humanity’s study of the universe at large: we can see most other stellar systems and galaxies better than we can see our own, which sounds nicely profound, but it’s really just a result of the fact that it is harder to see the entirety of something from the inside than it is from the outside.

As a result, the borders of our Solar System remain a bit of a mystery to scientists, even as we develop telescopes that can see light years away and deep into the cosmic past. The epitome of that might be the Oort cloud—a truly massive shell made up of rocks and debris that forms a border of sorts around our little celestial neighborhood.

To put its location and size into perspective, the Voyager space probes (the objects that we have managed to send farthest from Earth) would need 300 years from their launch to reach the projected inner edge of the Oort cloud. To come out the other side, they would need about 30,000 years.

So yeah—big, mysterious, and simultaneously too far to get to and too close to properly see. As a result, studying the Oort cloud is more about simulation and extrapolation (largely from the paths of comets) than it is about direct observation. And through one of these simulations, a group of researchers believes they’ve managed to uncover something huge hidden in the makeup of the inner cloud: spiral arms that make the overall structure appear almost like a galaxy.

“As the galactic tide acts to decouple bodies from the scattered disk [of the Oort cloud],” the researchers wrote in their study, which is currently hosted on the preprint server arXiv and is scheduled for publication in The Astrophysical Journal, “it creates a spiral structure in physical space that is roughly 15,000 [astronomical units] in length. The spiral is long-lived and persists in the inner Oort cloud to the present time.”

Critically, we are talking about the inner Oort cloud here. The Oort cloud can (in general) be split into an inner cloud and an outer cloud, with the inner cloud forming a rough disk shape and the outer cloud being relatively spherical. The inner and outer clouds formed a bit differently and—as a result of their differing distances from the largest gravity well in the vicinity (our Sun)—they have behaved differently over time.

One of the major differences between the two regions is how they are impacted by the gravitational effects of the Milky Way galaxy in which they (and we all) reside. Because it is less tightly bound to the Sun, the outer Oort cloud is much more susceptible to what are called “galactic tides”—the forces exerted by a galaxy on the objects inside it that can shape, twist, and deform large-scale objects and orbital paths.

That said, the inner Oort cloud is affected by these galactic tides as well, albeit over a much longer time frame—like, age-of-the-Solar-System long.

“In the inner Oort cloud […],” the authors wrote, “structures in the spatial distribution of bodies can form and ‘freeze’ over timescales comparable to the age of the Solar System. This raises the question of how the inner Oort cloud would look to a distant observer and/or whether there are any diagnostic features that would facilitate its detection.”

To figure this out, the team ran several simulations that traced the formation of the Oort cloud and the paths of the individual objects inside it. They also included the potential disruptive effects of passing stars in these simulations to make sure that the large-scale shaping effects they were seeing could not be attributed to an interloping stellar body.

And in the end, they had their answer: the inner Oort cloud has arms, and those arms were brought into existence by the galactic tides experienced by the structure over the course of its formation and evolution. As for proving this observationally, however, the team reiterates that “direct observational detection of the Oort spiral is difficult,” and maintains that the structure will have to be determined from “detection of a large number of objects” in the radius of the inner Oort cloud or from “thermal emission from small particles in the Oort spiral.”

For now, we’re just going to have to trust the simulations, and rework our mental pictures of the Solar System once again.

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