The universe appears vast and random, yet a striking pattern emerges when observing planetary orbits: they align on roughly the same plane, like a celestial pancake. This begs the question: what lies “below” Earth if space extends in every direction? The answer isn’t simple, and it requires understanding the nested structures governing the cosmos – from our solar system to galactic superclusters.
Defining ‘Down’ in a 3D Universe
Our intuition dictates “down” as the direction gravity pulls us, but this is relative. Standing in North America, “down” points toward the Earth’s core, while someone in the Southern Indian Ocean would experience “down” in the opposite direction. Extrapolating this, astronomers define “below” as the region beneath the ecliptic, the plane in which planets orbit the sun.
However, this is just one layer. The solar system itself orbits within the Milky Way galaxy, confined to a galactic plane. This plane is tilted about 60 degrees relative to the ecliptic, meaning our “down” isn’t aligned with the galaxy’s overall structure.
Galactic and Supergalactic Planes: A Hierarchy of Discs
The Milky Way isn’t alone; it resides within the Local Group of galaxies, which are themselves clustered in a supergalactic plane, almost perpendicular to the galactic plane (at an 84.5-degree angle). This reveals a hierarchy of flattened structures extending across cosmic scales. The question of what’s “below” Earth ultimately depends on how far you zoom out.
From Nebulae to Discs: How These Planes Formed
The reason for these aligned planes lies in the way these structures formed. The sun and planets originated from a collapsing cloud of gas and dust, the solar nebula. This nebula possessed a slight initial rotation; as it shrank under gravity, that rotation accelerated.
Particles within the nebula collided and interacted, gradually flattening the cloud into a disc. Oblique orbits were canceled out through repeated collisions, forcing everything into a single plane. The sun and planets formed within this flattened disc. The same principle applies, on larger scales, to the stars within the Milky Way and the galaxies within the Local Group.
What Lies Beyond?
Ultimately, there’s nothing inherently special about the direction we call “down.” It’s merely a consequence of the physics governing how structures form. Beyond our solar system, other stars orbit in different orientations. And beyond those stars lie other galaxies, each with its own plane of rotation.
The universe doesn’t care about our sense of direction. The cosmic hierarchy of discs is a consequence of gravity and rotation, not a purposeful design. The next time you look up at the night sky, remember that “down” is relative, and space extends equally in all directions.
