Â鶹ÒùÔº

A spiral structure in the inner Oort cloud

We typically think of the Oort cloud as scattered ice balls floating far from the sun, yet still tied to it gravitationally. Occasionally, some wayward gravitational perturbation will knock one of them a weird way and create a long-period comet, which might briefly delight us lowly humans by providing something interesting in the sky to look at.

But what the Oort cloud actually looks like and how it is affected by forces greater than just our solar system has remained somewhat of a mystery. A posted to the arXiv preprint server from researchers at the Southwest Research Institute and the American Museum of Natural History tries to shine a light on what this invisible part of the solar system looks like—at least the part that is only 1,000 to 10,000 times farther away from the sun than Earth is.

That part called the "inner" Oort cloud is considered slightly more populated than the "outer" Oort cloud, which ranges from 10,000 AU to 100,000 AU. Overall, potentially trillions of icy bodies are thought to be floating deep in space, though we only ever see the ones that show up in the inner solar system as long-period comets.

Estimating the cloud's structure requires more than understanding the planet's gravitational forces. While they still have an impact, there is a larger player in the orbital mechanics of these icy rocks—the galaxy itself.

There's a concept known as the "galactic tide." As our solar system moves through the galaxy, it is subjected to gravitational forces of other objects, like stars and black holes, that are closer or farther away from it. Like Earth's moon forces the water on the surface towards it due to its gravity, the galactic center, where most of the galaxy's mass is, affects large objects in our solar system.

For the planets, this influence is drowned out by their gravitational bond to the sun. But for Oort cloud objects, it plays a major role in determining their positioning. New long-period comets are formed when a nuance in the galactic tide either forces them into the inner solar system itself or causes them to collide with one another, sending one off on a trajectory toward the sun.

Modeling this complex dynamic is hard, and the researchers, including lead author David Nesvorný, had to rely on a supercomputer at NASA to run their analytical model and compare it to previous simulations of the structure of the Oort cloud. They found something intriguing hiding in the data.

According to their model, the Oort cloud looks like a spiral disk about 15,000 au across, offset by the ecliptic by about 30 degrees. But more interestingly, it has two spiral arms that almost make it look like a galaxy.

These spiral arms, which are located nearly perpendicular to the galaxy's center, resulting from the influence of the Galactic tide, are represented in the mathematical model by a phenomenon known as the Kozai-Lidov effect. In this quirk of celestial mechanics, large bodies are affected by "Kozai oscillations" that result from the gravitational influence of objects that are much farther away but, in the aggregate, still have an impact on the mechanics of a body.

The changes those oscillations make take a long time, but according to the researcher's analysis, they almost solely determine the shape of the inner Oort cloud. The gravitational pull of the solar system's planets or nearby passing stars doesn't seem to have much effect.

According to the paper, taking a picture of this two-armed spiral will be exceedingly difficult. The authors suggest doing so would either require direct observation of a large number of objects in that space (which is unlikely in the near term) or separation of radiation from those objects that eliminates background and foreground sources so it could track the specific structure.

As of now, neither observational method has any resources dedicated to it. But, if we want to learn more about the home of any potential new comets and their impact on us, it wouldn't be a bad idea to start planning how to look.