I have a planet in my game and it is next to the sun (the only light source in my project.) The side that is facing the sun is brighter and the side that is facing away is darker, as expected. But, if you look at the dark side of the sphere, the bottom is darker than the top by a noticeable amount both in the editor and in game. The sun is to the SIDE of the sphere so this does not make sense. Is this part of the lighting settings? If so how can I turn it off?

TL; DR

In environment lighting settings, "Ground" refers to "up" and "Sky" refers to down, the "Ground color" colors all mesh faces pointing down, while up does the opposite, you should change the environment lighting settings.

Explanation

The color multiplication is happening inside your shader, presumably the Standard Shader (it's the one you marked at the top of your material).

This shader interacts with Unity's rendering pipeline to render things like shadows and ambient light, in your case, you are explicitly setting the "Sky Color", using the gradient option of the environment light (check your photo - environment lighting).

"Sky" and "Ground" refer to "Up" and "Down" in the scene, or Vector3(0, 1, 0) and Vector3(0, -1, 0) (check https://docs.unity3d.com/ScriptReference/Vector3-up.html). (In outer space, there's no concept of "up" or "down").

The standard shader interacts with these settings and makes your "Sky Color" for an example, a multiplier of the color of all the faces of your mesh that are pointing up (technically, the points of the mesh which have their normal pointing up), your "Ground Color" is darker, so when it multiplies the color of your planet by it, the result is darker (color values closer to 0 are darker, and usually go from 0.0 to 1.0 in shaders, when a color is multiplied by white, or 1.0, 1.0, 1.0, the result is the same color, when it's multiplied by a color close to black, like 0.1, 0.1, 0.1, the result color is close to black)

(Normals are vectors pointing from the points in your mesh)

You could:

• Change the environment color to your skybox, unity will compute the ambient color based on the color of your skybox
• Change the environment color to a single color (not a gradient) and make it the color you like.
• Make "Sky Color" and "Ground Color" the same color, as in your environment, the outer space, there's no "ground" and "sky".
• Write your own shader, that shades the planet as you like, using unity CG (HLSL) / shader lab (depends on your rendering pipeline).