It’s only natural to wonder why things are the ways that they are. Take a look at our Solar System, for example.
A central bulge with planets, moons, and whatnot moving in a disc around it. Is this the way things have to be? My friend Rich, a chemist, asks:
It seems that all the objects in our solar system orbit the sun in nearly the same plane. Why is that? Why doesn’t the solar system have spherical symmetry?
In particular, Rich wants to know why our Solar System doesn’t look more like this:
Our Solar System is definitely not shaped like a sphere; it definitely is a bulge at the center with a disc on the outskirts. Well, I tried googling for bulge and disc and this was all I got.
But this shape, bulge + disc, is one of the most common in the Universe. In fact, we can look at scales much smaller than the Solar System, such as the gas giants, and what do we find?
A bulge at the center (the planet), with a disc — rings and many moons — orbiting it. Saturn’s rings are the most spectacular, but Jupiter, Uranus and Neptune all have them, and they’re all in the same plane as most of their massive moons.
But why look smaller than the Solar System? Why not look at galaxies too?
This famous “spiral” shape, when viewed edge-on, is clearly also just a bulge and a disc. So this shape that Rich is asking about is actually very, very common. How did we get stuck with this?
The bulge part is easy: gravity. All of us — planets, solar systems, and galaxies — form from gravitational collapse. You take a big blob of mass, you let gravity do its thing, and it collapses down as best it can. But these initial blobs are never perfect spheres. The stuff that forms galaxies looks like this.
While the stuff that forms stars looks like this.
In other words, the initial shape is not a perfect sphere. You have three dimensions to play with, and it’s always shortest in one dimension and longest in another. We have a fancy name for shapes like this — a triaxial ellipsoid — but it’s just a sphere that’s stretched and crunched like so.
In the diagram above, the blue direction is the shortest. Newton’s gravity is good enough to tell us that the shortest direction collapses the fastest, while the largest directions take longer to collapse down. But the galaxy, the solar system, and even the region where planets form are full of normal matter. What happens when you run two blobs of matter (or two halves of one blob of matter) into each other at high speeds?
Well, yes, they go “SPLAT!” But more importantly, they form a pancake-like shape. Now, if gravity were the only thing going on, all of this splatter along the other two axes (the ones that didn’t collapse first) would eventually be pulled in towards the center, which means we’d just get a bulge.
But there’s one more piece to the puzzle: things rotate. (In physics, we call this conservation of angular momentum.) Galaxies, star systems, and individual planets all rotate, because they all start with some rotational energy, and that remains. The stuff that doesn’t have enough rotational energy collapses inwards, and becomes part of the bulge. But the stuff that is rotating quickly enough stays on the outskirts, and forms the disc.
So everything — planets, solar systems, and galaxies — all form, initially, with this “bulge + disc” shape. No giant atoms.
“But wait a minute,” you might say. “What about elliptical galaxies? Aren’t they exactly what Rich is asking about?”
Ahh, ellipticals. Galaxies that look, well, like giant Bohr atoms.
They didn’t form this way! The way you make an elliptical galaxy is by taking two or more “normal” (i.e., spiral) galaxies and smacking them into each other. The stuff that results from this collision is elliptically shaped, as you can see for yourself.
We haven’t seen one yet, nor do we know if they’d be stable, but that’s what it would take. And now you know.