The inner Oort Cloud has been found to be spiral-shaped.

Back in the 1950s, Dutch astronomer Jan Oort proposed an explanation for the emerging paradox: why, despite their apparent short lifespan, comets continue to arrive from the distant outskirts of the Solar System, even after five billion years of its existence.

This led to the "hypothetical" discovery of the Oort Cloud — a collection predominantly composed of icy bodies that formed around the system during the early stages of its evolution. Scientists believe that these bodies were pushed out there by the outer planets, specifically Saturn, Uranus, and Neptune, during their migration.

According to the established picture, the cloud consists of two parts. The outer part forms a gigantic sphere with a radius of approximately 100,000 astronomical units, meaning it is located 100,000 times farther from the Sun than Earth. For comparison, Pluto's distance from the Sun is about 30-50 astronomical units.

There is also an inner part of the Oort Cloud, which remains at a distance of roughly 1,000 to 10,000 astronomical units. Interestingly, comets from this region are less frequent than those from the outer sphere, as this part is more influenced by the gas giants, which tend to push away incoming small bodies.

It was previously thought that this inner Oort Cloud was a relatively flat disk-like or even ring structure located approximately in the plane of the ecliptic, which is the same plane in which the planets orbit around the Sun. Recently, a team of astronomers from the USA, Czech Republic, and Argentina modeled the formation of this structure and stated that it is not actually disk-like or ring-shaped, but rather spiral. The researchers shared their findings in an article published on the Cornell University preprint server (USA).

They explained that the entire scatter must have been inevitably influenced by the gravitational pull of our Milky Way galaxy. At distances of 1,000 astronomical units from the Sun, this influence becomes significant and begins to compete with the gravity of the star and planets. According to the researchers, it is the effect of galactic gravity, combined with the influence of nearby stars, that shaped the outer part of the cloud into a spherical form.

Calculations suggest that the galaxy slowly deformed the inner part, but over the 4.6 billion years of the system's existence, it should also have left its mark. Astronomers modeled the dynamics of the orbits of 34,000 objects within this structure throughout the existence of the Solar System and ultimately obtained a spiral with two arms that extends over 15,000 astronomical units. The researchers concluded that this "galactic" appearance of the inner Oort Cloud began to manifest within the first hundreds of millions of years after the formation of the Solar System.

Moreover, the galaxy has tilted this spiral: calculations showed that it is positioned at an angle of about 30 degrees to the plane of the ecliptic. It is worth noting that the ecliptic plane itself, which is the plane of our Solar System, is tilted at 60 degrees relative to the plane of the Milky Way disk. As a result, our miniature "galaxy" is now nearly perpendicular to the actual Galaxy.

The researchers pointed out that the orbits of certain long-period comets and asteroids indirectly hint at the "spiral nature" of the inner Oort Cloud. In particular, they were intrigued by the 300-kilometer (541132) Leleakuhonua, which is located over 2,600 astronomical units from the Sun and completes one orbit around our star in 39,000 years. It is suspected that its orbit has undergone significant changes.

Directly detecting or "seeing" the Oort Cloud is extremely challenging: it requires finding a large number of objects located there. However, there is another option — attempting to capture the thermal radiation of dust within this cloud. Unfortunately, this radiation is weak, and even with modern space observatories, the chances of detecting it are slim.