Everyone knows about the old-school TOS "alien planet" complete with simplistic (sometimes one-colored) atmosphere, styrofoam rocks, standard "class M" vegetation, and the hollow "choir echo" in the distant background. This is a classic Trek trademark called "low-budget set design that can be re-purposed at every show." Just change the sky lighting color, airbrush the rocks and vegetation to a different tint, and voilà - we are now on planet X8324J!

We hardly ever see the sun, so presumably it's all overcast. From a photographic perspective, this means the light all around the landing party should be diffused and therefore harsh shadows should disappear. To the person behind the camera filming the scene, this should be obvious and straight-forward.

But of course, this wouldn't be Star Trek if that were the case. This is obviously a technical production issue.

Let's take a look at some alien worlds which apparently look all remarkably similar. Pay close attention to the shadows and the various light sources in the scene which keep the crew nicely lit, but the background sky implies something completely different.

Since we're talking about aliens worlds, however, I guess I'll grudgingly accept the potential catch-all explanation that there may be two or more suns which cast light from different angles. What a cop-out.