Exoplanets
This article talks about common types of exoplanets discovered, and why those are probably not the most common planets, just the easiest to find.
I'll throw in another reason not mentioned there: we're finding a lot of super-Earths, but it is more likely to have lots of little planets in a solar system. That's because it takes less mass, so each of them consumes a smaller amount of available resources, leaving plenty for other planets to form. You're more likely to see just a few big planets farther out, where they have time and room to sweep up lots of material without bashing into anything, and less likely to have it blown away by solar winds. More small things: fewer large things is a robust pattern throughout nature.
I'll throw in another reason not mentioned there: we're finding a lot of super-Earths, but it is more likely to have lots of little planets in a solar system. That's because it takes less mass, so each of them consumes a smaller amount of available resources, leaving plenty for other planets to form. You're more likely to see just a few big planets farther out, where they have time and room to sweep up lots of material without bashing into anything, and less likely to have it blown away by solar winds. More small things: fewer large things is a robust pattern throughout nature.