...and not more like a moth-infested light bulb?
Someone who is like a brother to me asked me this question recently (well, I may have rephrased it slightly). I thought it would be a good idea to write up my response as a blog post.
As it turns out -- in a manner of speaking -- the planets actually resemble both. To understand what I mean by this, let's think about the formation of a star like the Sun and its Solar System of planets.
Most stars begin as a huge cloud of gas and dust. Due to gravity, every molecule in the cloud tugs a little on every other molecule, but on average, the tugs mostly balance out. Except on the edge of the cloud, that is. You're right, I never said the cloud had an edge. But, face it, if it didn't have an edge, then this cloud we're talking about is the whole Universe, not a star; and that's an entirely different prospect. So, this cloud has boundaries, and you will respect them.
If you were a molecule on the edge of this cloud, because all of the molecules are on one side of you, and none are on the other, you will feel a gravitational pull toward the center of the cloud. It's just like standing on the surface of the Earth. But, there is no "surface" on the edge of this cloud. So, instead of feeling your weight supported by the solid surface, you begin to fall through the cloud. If you were to dig a hole under your feet through the center of the Earth and out the other side, you could experience the same sensation as this molecule (legal disclaimer: please do not dig a hole from your backyard through the center of the Earth and then jump in; if you do, then @wwwheniheard claims no responsibility for what may, literally, befall you). If you do that on Earth, you'll fall and fall, picking up speed until you reach the center of the Earth. When you reach the center of the Earth, because you are now falling VERY quickly, you'll zoom right on past. As soon as you do, however, because there is now more mass behind you than in front for you, you start slowing down. If we assume the Earth is perfectly uniform, then you'll come to a stop just as you reach the surface at the antipode (this is an awesome word to drop into a casual conversation: Google it). If you don't grab hold of the ledge, the next thing you'll see is the tiny mouth of your cave fading in the distance as you again fall through the Earth back to your starting point.
Of course, if you did this on Earth, most of the time, the other end of the tunnel would be under water, and the ocean would flood into the hole from the other side. As you fell, you'd eventually crash into this massive flood, and would very quickly cease to fall, and cease to live. Of course, even if you were lucky enough to live in a place where there is land at your antipode, then air molecules would rush in to fill the void you've created. As you fall, you'll encounter all of these air molecules. And each one you bumped into would slow you down just a little bit. This is called air drag. Skydivers are very thankful for this aspect of physics.
Now, back to the cloud of gas and dust. If you are very lucky, though it may take a while, you will fall all the way to the other edge (oh, did I not mention we are assuming it is a spherical cloud? Yeah, I'm a physicist, that's all we know), just as you did in the Earth example. But, the cloud is very big and you have very many molecular friends. You're going to run into one of them, and then another, and then another. And, just as in the Earth case, when you reach the other side, you'll find that you can't quite make it to the edge. So, you now are trapped inside the cloud, closer to then center than where you started. Now, imagine this happening to EVERY molecule in the cloud, and you can see that, eventually, we'll start to have a crowd near the center.
That's when things start getting interesting....
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