[ ♪ Intro ].

When you see a map of the solar system, it looks like all of the planets are orbiting the sun in a flat, two-dimensional plane, but that’s not just to save paper. While there is a little wiggle room, the planets really are orbiting on mostly the same level.

And we’ve actually known that since before telescopes existed, just by charting where the planets and the sun appear in the sky. But why are their orbits so flat? Well, it all comes down to how the solar system formed.

Our stellar neighborhood started out as a giant, roughly spherical cloud of gas and dust particles. And that cloud had a very tiny amount of spin. Then, for some reason, it started collapsing.

Astronomers aren’t 100% sure what triggered that collapse, but most will say it was probably a nearby supernova. Regardless, gravity caused everything in that cloud to pull itself together. And as the blob got smaller, it started spinning faster.

This is because of a principle called conservation of angular momentum. Basically, an object’s angular momentum describes its tendency to stay rotating. How much angular momentum that object has depends on two things: how its mass is distributed, and how fast it’s spinning.

If either of those properties changes, the other has to compensate so that the total angular momentum stays the same, or is conserved. So as that giant space cloud got smaller, it had to start spinning faster. And eventually, that spinning generated so much force that it flattened the cloud into a disk shape.

It’s kind of like how spinning a ball of dough gets you a pizza crust. We don’t just know this through math, either. We can actually observe these disks forming around young stars in our galaxy!

We’re literally witnessing solar systems being born! In our solar system, as time went on, some particles of dust and gas in that disk kept colliding and sticking to one another, which formed larger and larger space rocks. Most only got enough mass to make potato-shaped asteroids, but others eventually became Earth and the other seven planets.

And since they're all formed out of the same flat, swirling disk of material, all the planets in our solar system orbit in the same plane, and, as a bonus, in the same direction around the sun! Now, there are plenty of smaller objects that have really inclined, or angled, orbits, like Pluto, asteroids, and comets. These all probably started out with flatter orbits, but got kicked into more extreme positions by Jupiter and Neptune as the planets moved into their current places.

Then again, those planets also just flung a lot of objects out of our solar system entirely. So, Pluto and its lot could have had it a lot worse. It might seem weird or even unique, but the fact that our planets orbit on the same plane is pretty normal, and it happens in lots of other star systems, too.

But, hey, our solar system has a lot of other weird stuff to make up for it. Like us. Thanks for asking, and thanks for watching this episode of SciShow!

If you want to learn more about our solar system and the rest of the universe, you can check out our channel for all things space, over at youtube.com/scishowspace. [ ♪ Outro ].