Or - okay so I've got a planet selected and the "SCANNING..." icon just sits and spins forever.:
DNA, it's not that hard.
The signals on the tuning chart show you what's left to discover in the system. On the far left are signal sources, then things like conflict zones, then moons, different types of planets, and finally gas giants will show up further right.
Let's say you are zoomed out and tuned to signal sources, but there is a planet at the signal source pulse. First tune to the planet, discover it, then tune to the signal sources, then discover them. There are occasions where signals will overlay each other and make it a little difficult to zoom in, and in those cases you just have to fiddle with it.
By the time you've fully scanned a couple of systems, you will be able to do it with your eyes closed.
As for the spinning scan meter, I think it just is what it is - a constant, passive scan of the body.