The thing that always irritated me about the planets that the crew stumbled across mostly seem to just be there. They're rarely mentioned as having their own Sun, or being part of a solar system, or even having any moons. Same with asteroids they just stumble over.
I'm not saying there are never rogue planets, asteroids, and comets roaming the galaxy; it's just that the Enterprise ran across a helluva lot of them.
Examples? I think most of the planets were implied to be in solar systems with suns and other planets in them. Do we really need dialogue like "Mr. Chekov, plot a course to Quadrant 1, Sector 294, Gamma Triangula System, Planet IV. Oh and when we arrive make sure the sun is visible on the viewscreen. I like a good sun shot when entering orbit."