Ever wondered what's really below us, considering space stretches out in every direction? It's a question that sparks the imagination, and today, we're diving deep into the cosmos to find some answers! This exploration comes from the 'Curious Kids' series, where experts tackle questions from inquisitive minds like yours. Got a burning question? Send it to CuriousKidsUS@theconversation.com!
Our query comes from Purvi, a 17-year-old from India. She asks: What's below Earth?
If you've ever glanced at illustrations of our solar system, you've probably noticed something: the planets all orbit the Sun in roughly the same plane, moving in the same direction. But what lies above and below that plane? And why aren't the planets' orbits scattered every which way?
I'm a planetary scientist, and my job involves working with robotic spacecraft, like rovers and orbiters. We send these out to explore our solar system, and understanding the 3D map of our space neighborhood is crucial for us.
So, which way is 'down'?
Gravity plays a huge role in how we perceive 'up' and 'down'. Things fall towards the ground, but that direction depends on your location. Imagine you're in North America and point 'down'. If you could draw a line straight through the Earth from your fingertip, it would point 'up' for someone on a boat in the southern Indian Ocean.
In a broader sense, 'down' could be defined as being below the plane of the solar system, known as the ecliptic. By convention, we see planets orbiting counterclockwise above the plane and clockwise below it.
Even More Flavors of 'Down'
But is there anything special about the direction of 'down' relative to the ecliptic? To answer that, we need to zoom out even further. Our solar system is centered on the Sun, which is just one of about 100 billion stars in our galaxy, the Milky Way.
These stars, along with their planets, orbit the center of the Milky Way, just like planets orbit their stars, but on a much longer timescale. And just as the planets in our solar system aren't in random orbits, stars in the Milky Way orbit the galactic center close to a plane, called the galactic plane.
Here's where it gets interesting: This galactic plane isn't aligned with our solar system's ecliptic. The angle between the two planes is about 60 degrees.
Let's go one step further. The Milky Way is part of a cluster of galaxies called the Local Group. And you guessed it – these galaxies mostly fall within another plane, called the supergalactic plane. The supergalactic plane is almost perpendicular to the galactic plane, with an angle between the two planes of about 84.5 degrees.
So, how do these celestial bodies end up traveling along paths that are close to the same plane? It all goes back to how they formed.
The Collapse of the Solar Nebula
The material that would become the Sun and planets started as a vast cloud of gas and dust called the solar nebula. Every particle had a tiny amount of mass, and because of gravity, they were attracted to each other, albeit weakly.
The particles in the solar nebula started slowly. But over time, the mutual attraction caused the cloud to shrink. There was also some slight rotation to the solar nebula, perhaps due to a passing star. As the cloud collapsed, this rotation sped up, just like a spinning figure skater.
As the cloud shrank, particles got closer and had more interactions due to gravity and collisions. These interactions caused particles in tilted orbits to reorient their orbits. For example, if a particle coming down through the orbital plane slammed into a particle coming up, the interaction would cancel out the vertical motion and reorient the orbits into the plane.
Eventually, the cloud collapsed into a disc shape. Particles in similar orbits started clumping together, forming the Sun and planets.
On much larger scales, similar interactions likely confined most of the stars in the Milky Way into the galactic plane and most of the galaxies in the Local Group into the supergalactic plane.
The orientations of the ecliptic, galactic, and supergalactic planes all stem from the initial random rotation of the clouds they formed from.
So, what's really below the Earth?
There's nothing special about the direction we define as 'down' relative to Earth, other than the fact that there's not much orbiting the Sun in that direction.
If you go far enough, you'll find other stars with their own planetary systems orbiting in different orientations. And if you go even farther, you might encounter other galaxies with their own planes of rotation.
This question highlights one of the most fascinating aspects of astronomy: perspective. If you asked a hundred people on your street, 'Which way is down?' they'd all point the same way. But imagine asking that question to people all over the Earth, or to intelligent life forms in other planetary systems or galaxies. They'd all point in different directions.
What are your thoughts? Do you find the concept of 'down' relative to the universe as mind-bending as I do? Share your thoughts in the comments below! And, if you're a curious kid (or an adult with a curious mind), send your questions to CuriousKidsUS@theconversation.com. Don't forget to include your name, age, and city. Let's keep exploring the mysteries of the universe together!
