The concept of 'down' is a fascinating cosmic riddle. What lies beneath Earth when space surrounds us in every direction?
When you visualize the solar system, you'll notice a pattern: planets orbit the sun in a relatively flat plane, all traveling in the same direction. But what determines 'down' in this vast universe?
Gravity plays a pivotal role in our perception of up and down. Objects fall downward, but this direction is relative to your position. Stand in North America and point down; extend that line through Earth, and you'll find it points up for someone in the southern Indian Ocean. This concept of 'down' is further expanded in the context of the solar system.
In the grand scheme of things, 'down' can be defined as below the plane of the solar system, known as the ecliptic. By convention, we say planets orbit counterclockwise when viewed from above this plane and clockwise from below. But the story doesn't end there...
And here's where it gets intriguing: The concept of 'down' extends far beyond our solar system. Our sun is just one of roughly 100 billion stars in the Milky Way galaxy, and each star, along with its planets, orbits the galactic center. Similar to our solar system, these stars orbit close to a plane, known as the galactic plane. But here's the twist: this plane is not aligned with the ecliptic; they form an angle of approximately 60 degrees.
The Milky Way is part of a larger cluster called the Local Group, and yes, you guessed it, these galaxies generally align with yet another plane, the supergalactic plane. The supergalactic plane is nearly perpendicular to the galactic plane, with an angle of about 84.5 degrees between them. These alignments are not coincidental; they are a result of how these celestial bodies formed.
The solar system's origins lie in the solar nebula, a vast cloud of gas and dust. Each particle in this nebula had mass and thus exerted gravitational attraction, albeit weak. Over time, these particles started moving closer, shrinking the cloud. A slight rotation in the nebula, possibly due to a passing star's gravity, caused it to collapse, increasing its spin, much like a figure skater spinning faster when drawing their arms in.
As the cloud contracted, particles interacted more frequently, affecting their orbits. These interactions realigned orbits that were initially tilted, eventually forming a disc. This process led to the formation of the sun and planets we know today.
On a grander scale, similar interactions likely confined most stars in the Milky Way to the galactic plane and most galaxies in the Local Group to the supergalactic plane. The orientations of these planes are a result of the initial random rotation of the clouds from which they formed.
So, what's below Earth? Well, there's nothing inherently special about the direction we call 'down' relative to Earth. If you travel far enough in that direction, you'll find stars with unique planetary systems and galaxies with their own rotational planes. This perspective is a beautiful reminder that our perception of 'down' is relative and varies across the universe.
A thought to ponder: If you ask people around the world, 'Which way is down?', you'll get diverse answers. But what if you posed this question to intelligent life forms in other planetary systems or galaxies? How would their perspectives differ from ours? The universe, it seems, is full of surprises and varying viewpoints.
