Almost all objects orbiting the sun live in a certain plane, the ecliptic plane. However, a recent analysis of long-period comets shows a second home, a so-called “empty ecliptic”. And it can be populated with comets that are pulled there by none other than the gravitational pull of the Milky Way.
When solar systems like ours first form, they compress from a loose and irregular cloud of gas and dust into a rapidly spinning, thin, flat disk. All planets, asteroids and comets form on this disk. Once this disk disappears (either through its material accumulating on planets or being blown up by eruptions from the young Sun), all objects retain this original orbit preference, called the ecliptic plane.
You can see this effect today by observing the sun, moon, and all planets moving on roughly the same track in the sky.
The comets are a different story. While they initially formed along with everyone else on the ecliptic plane, they forced their gravitational interactions with the outer planets into other, essentially random, orbits around the sun. That is why comets penetrate our sky from all possible directions.
However, according to new research in the Astronomical Journal, the distribution of comets in our solar system may not be too random. This analysis revealed that long-period comets also tend to prefer a second level of the solar system.
This plane, known as the "empty ecliptic" because it started out barren and was later filled with comets, is at a very precise angle to the original ecliptic plane. The ecliptic itself is at 60 degrees relative to the plane of our Milky Way galaxy (in other words, our solar system is tilted 60 degrees relative to the galaxy) while the empty ecliptic sits at 60 degrees in the opposite direction.
This led the researchers to the conclusion that the environment of our solar system – namely the gravitational differences that surround us – plays a role in the shifting and shaping of the orbits of long-period comets.
However, these results are nowhere near dry. Comets have all kinds of orbits, with a general vague preference of hanging in either the normal or empty ecliptic, but even then the most preferred location is not precisely on either. Understanding why this is the case will help astronomers better understand how the galaxy itself influenced the evolution of our solar system.