The concept of 'down' is relative and depends on one's location. On Earth, 'down' points towards the planet's center, a direction that would be 'up' for someone on the opposite side of the globe. In a broader cosmic sense, 'down' can refer to directions relative to specific planes of celestial bodies.
Cosmic Planes
- Ecliptic Plane: This is the plane in which planets in our solar system orbit the Sun. From 'above' this plane, planets appear to orbit counterclockwise; from 'below,' they appear clockwise.
- Galactic Plane: Our solar system, including the Sun and its planets, orbits the center of the Milky Way galaxy. The stars and associated planets within the Milky Way orbit close to this plane, which is angled about 60 degrees relative to the ecliptic plane.
- Supergalactic Plane: The Milky Way is part of the Local Group of galaxies. Most of these galaxies also lie within a common plane, known as the supergalactic plane, which is nearly perpendicular (about 84.5 degrees) to the galactic plane.
Formation of Flat Orbits
The alignment of celestial bodies within these planes is attributed to their formation process. The Sun and planets originated from a vast cloud of gas and dust called the solar nebula.
- Gravitational Collapse: Due to mutual gravitational attraction, the solar nebula began to contract.
- Rotation and Flattening: The nebula possessed a slight initial rotation. As it collapsed, this rotation intensified, similar to a spinning ice skater drawing their arms inward. Interactions between particles, driven by gravity and collisions, caused them to lose vertical motion and reorient their orbits into a disc shape.
- Planetary Formation: Over time, particles within this disc clumped together, eventually forming the Sun and the planets.
Similar processes are believed to have confined stars within the Milky Way's galactic plane and galaxies within the Local Group's supergalactic plane. The specific orientation of each plane reflects the initial, random rotation direction of the respective gas and dust clouds from which they formed.
Therefore, there is no inherent 'down' direction in space beyond these defined planes. Traveling far enough in any direction will lead to other stellar systems and galaxies with their own unique orbital orientations.