The universe features large-scale alignments of celestial bodies, from planetary systems to clusters of galaxies, that orbit within relatively flat planes. These structures, including the solar system's ecliptic plane, the Milky Way's galactic plane, and the Local Group's supergalactic plane, are understood to have formed through a process of gravitational collapse and intensified rotation of initial gas and dust clouds. The specific orientation of each plane is determined by the original rotational direction of its parent cloud.
Relative Directions in Space
The concept of "down" is dependent on one's location. In a cosmic context, directional references often relate to the specific planes within which celestial bodies orbit. This means there is no universal "down" in space; instead, directions are relative to these defined planes.
Major Cosmic Planes
Several significant planes define the alignment of objects across different scales in the universe:
- Ecliptic Plane: This is the approximate plane within which the planets of our solar system orbit the Sun. When viewed from 'above' this plane, planets appear to orbit in a counterclockwise direction.
- Galactic Plane: Our solar system, along with billions of other stars and their planetary systems, orbits the center of the Milky Way galaxy close to this distinct plane. The galactic plane is angled at approximately 60 degrees relative to the ecliptic plane.
- Supergalactic Plane: The Milky Way is part of a cluster of galaxies known as the Local Group. Most of these galaxies are observed to align largely within a common plane. This supergalactic plane is oriented nearly perpendicularly to the galactic plane, with an angular difference of about 84.5 degrees.
Formation of Flat Orbital Structures
The flattened configurations observed in solar systems, galaxies, and galaxy clusters are attributed to their formation processes, which begin with the collapse of diffuse clouds of gas and dust.
For the solar system, this process started with a vast cloud called the solar nebula:
- Gravitational Collapse: Due to mutual gravitational attraction, the particles within the solar nebula began to contract, causing the cloud to draw inward.
- Rotation and Flattening: The nebula possessed an initial, slight rotation. As it collapsed, this rotation intensified, analogous to a spinning ice skater pulling their arms inward to increase speed. Interactions and collisions between particles within the shrinking nebula caused them to lose vertical motion and reorient their orbits. This process gradually transformed the irregular cloud into a disc shape.
- Formation of Bodies: Over time, particles within this newly formed disc clumped together, leading to the formation of the Sun and the planets.
Similar principles are believed to apply to the alignment of stars and their systems within the Milky Way's galactic plane and the arrangement of galaxies within the Local Group's supergalactic plane. The particular orientation of each plane is linked to the initial random rotation direction of the primordial cloud from which it originated.