Have you ever wondered what lies beneath the Earth, especially when space surrounds us in every direction? It's a mind-boggling question that leads us on an incredible journey through the cosmos.
When we look at illustrations or models of our solar system, we notice a fascinating pattern: all the planets orbit the Sun in a relatively flat plane, moving in the same direction. But which way is 'down'?
Earth's gravity plays a crucial role in our perception of up and down. Things fall towards the ground, but this direction varies depending on our location. Imagine standing in North America and pointing downward. If you extend a line from your fingertip through the Earth, it would point 'up' for someone on a boat in the southern Indian Ocean.
On a larger scale, 'down' could be defined as the direction below the plane of our solar system, known as the ecliptic. Conventionally, we say that the planets orbit counterclockwise above this plane and clockwise below it.
But here's where it gets controversial... Is there a special significance to the direction of 'down' relative to the ecliptic? To answer this, we need to zoom out even further.
Our solar system, centered around the Sun, is just one of about 100 billion stars in our galaxy, the Milky Way. Each of these stars, along with their planets, orbits the center of the Milky Way, much like the planets orbit their stars, but on a much longer timescale. And just as the planets in our solar system follow a specific plane, the stars in the Milky Way also orbit close to a plane called the galactic plane.
This galactic plane is not aligned with our solar system's ecliptic; in fact, the angle between them is about 60 degrees.
Taking a step back, the Milky Way is part of a cluster of galaxies known as the Local Group. These galaxies, too, mostly fall within another plane called the supergalactic plane, which is almost perpendicular to the galactic plane, with an angle of about 84.5 degrees between them.
So, why do these celestial bodies travel along paths so close to the same plane? It all comes down to how they formed in the first place.
The material that eventually formed the Sun and the planets of our solar system began as a vast, diffuse cloud of gas and dust called the solar nebula. Every particle within this nebula had a tiny mass, and thus, a gravitational force that attracted other particles, albeit very weakly.
Over time, this mutual attraction caused the cloud to collapse inward, shrinking. There was also a slight overall rotation to the nebula, possibly due to the gravitational pull of a passing star. As the cloud collapsed, this rotation sped up, much like a figure skater spinning faster as they draw their arms in.
As the cloud continued to shrink, the particles grew closer, and their interactions increased, affecting their motion due to gravity and collisions. These interactions caused particles in orbits tilted far from the direction of the overall rotation to reorient their orbits into the plane.
For instance, if a particle moving down through the orbital plane collided with a particle moving up, the interaction would cancel out their vertical motion and realign their orbits into the plane.
Eventually, the amorphous cloud of particles collapsed into a disc shape, and particles in similar orbits began clumping together, forming the Sun and the planets we know today.
On a larger scale, similar interactions likely confined most of the stars in the Milky Way to the galactic plane and most of the galaxies in the Local Group to the supergalactic plane.
The orientations of these planes all trace back to the initial random rotation direction of the clouds from which they formed.
So, what's below the Earth? There's nothing particularly special about the direction we define as 'down' relative to the Earth, except that there aren't many objects orbiting the Sun in that direction.
If you travel far enough in that direction, you'll encounter other stars with their own planetary systems orbiting in completely different orientations. And if you go even farther, you might find other galaxies with their own planes of rotation.
This question highlights the beauty of astronomy, putting everything into perspective. If you asked people from different parts of the Earth, or even intelligent life forms from other planetary systems or galaxies, 'Which way is down?', they'd all point in different directions.
It's a reminder that our perspective is just one among many, and the universe is vast and full of wonders waiting to be explored.
Jeff Moersch, Professor of Earth, Environmental, and Planetary Sciences at the University of Tennessee, invites you to share your thoughts and continue the conversation in the comments below.
