Ever wondered what's below Earth, considering space surrounds us in every direction? It's a fascinating question that delves into our understanding of 'up' and 'down' in the vast cosmos. This article, originally published by The Conversation and contributed to Space.com's Expert Voices, unravels this concept, offering a unique perspective on cosmic orientation.
Let's start with a familiar image: the solar system. You've likely seen illustrations where planets orbit the sun in a relatively flat plane, all moving in the same direction. But what does 'down' even mean in this context?
Gravity, the Earth's invisible force, plays a pivotal role in our perception of 'down'. We experience it as the direction things fall, but this direction is relative to where you stand. Imagine pointing 'down' from North America; that same line, extended through the Earth, would be 'up' for someone in the southern Indian Ocean. Isn't that mind-bending?
Zooming out, we can define 'down' as being below the plane of the solar system, known as the ecliptic. By convention, planets orbit counterclockwise above this plane and clockwise below.
But here's where it gets controversial... Is there anything special about the direction 'down' relative to the ecliptic? To answer that, we need to broaden our scope even further.
Our solar system, with the sun at its center, is just one small part of the Milky Way galaxy, which contains about 100 billion stars. Each star, along with its planets, orbits the galaxy's center, similar to how planets orbit their stars, but on a much grander scale. Like planets, these stars tend to orbit within a plane called the galactic plane.
And this is the part most people miss... The galactic plane isn't aligned with our solar system's ecliptic; they're tilted at about a 60-degree angle.
Taking another step back, the Milky Way is part of the Local Group, a cluster of galaxies. These galaxies, too, tend to align within a plane called the supergalactic plane. This plane is nearly perpendicular to the galactic plane, with an angle of about 84.5 degrees between them.
So, how do these celestial bodies end up traveling in paths that are close to the same plane? It all comes down to their formation.
The Collapse of the Solar Nebula: The material that formed the sun and planets began as a vast cloud of gas and dust, the solar nebula. Gravity, though weak between individual particles, caused them to attract each other.
Initially, the particles moved slowly. Over time, this mutual gravitational attraction caused the cloud to collapse inward. Any slight rotation of the nebula, perhaps from a passing star, would have increased in speed as the cloud contracted, like a spinning figure skater pulling their arms in.
As the cloud shrank, particles interacted more, both through gravity and collisions. These interactions reoriented particles far from the overall rotation direction, causing them to settle into a plane. Imagine particles colliding and canceling out vertical motion, thus aligning their orbits. Eventually, the cloud flattened into a disc, and particles in similar orbits clumped together, forming the sun and planets.
Similar processes likely confined stars within the galactic plane and galaxies within the supergalactic plane.
Ultimately, the orientations of these planes—ecliptic, galactic, and supergalactic—are rooted in the initial, random rotation of the clouds from which they formed.
So, what is below the Earth? There's nothing particularly special about the direction we call 'down' relative to Earth, except that not much orbits the sun in that direction. Beyond that, you'll find other stars with their own planetary systems, each with different orientations. Venture even further, and you'll encounter galaxies with their own planes of rotation.
This question truly highlights the beauty of astronomy, which puts everything into perspective. If you asked a hundred people on your street, 'Which way is down?' they'd all point the same way. But consider asking people worldwide, or even intelligent life forms in other galaxies—they'd all have different answers.
What are your thoughts? Does this shift your perspective on 'up' and 'down'? Share your thoughts in the comments below!
Jeff Moersch is a Professor of Earth, Environmental, and Planetary Sciences at the University of Tennessee.
