Rendering atmosphere for a planet

Let me premise my question by saying that I am fully aware that rendering atmosphere with any degree of accuracy is hard to do. I have done research and it looks daunting to say the least. I am aware of more than a few sources that explain how to render atmospheric scattering with different degrees of realism and right now I am not going to spend time trying to understand any of that … yet.

I am building a space-flight simulator. I’m more interested in working on orbital and atmospheric flight physics right now than in making things look graphically realistic … again … for now. I’ll work on making things look better later, after I make a good physics framework I can work with.

So I am looking for a very simple (it can be ugly and unrealistic) way to display some sort of atmosphere. From orbit I’d like to see a thin layer with a “gradient” that blends from blue to black around the planet. As the space vehicle enters the atmosphere the gradient changes to a brighter and brighter blue until it goes away and it becomes uniform blue (inside the atmosphere I’d just get rid of it and just paint the sky blue … I don’t even care about clouds for now).

I’m really just looking for a “placeholder” in my code so that first off, I can give the idea of the presence of some sort of atmospheric layer and second I can put in place the functions or framework for handling a skeletal atmosphere engine so that when later on I will work on making the atmosphere look realistic I can work on that section of the code independently from everything else.

Does anyone have any “tricks” to make something like this happen? is it possible maybe to build a “fuzzy” sphere or a sphere with a gradient of some sort that I can wrap around my planet? Any idea of how I would do that in OpenGL?

I hope my question makes sense.

Thank you!