Brad Thompson

Since you said your goal is to get this VDB into another application for rendering, I'll give you a heads up on two problems you might run into, just so you are aware.

1- The color gradients that you see in planet atmospheres come from the light angle. So if you bake the colors into your VDB, you are essentially baking the lighting. Generally, it's better to use a shader to create the colors, like the Jon Parker one mentioned above, or the built-in Karma Atmosphere LOP, which gives you more than just a sky. If you pull back far enough, you'll see that it's a full spherical planet atmosphere.

2- VDB resolution. The Earth's atmosphere is very thin. It's hard to get enough resolution using square voxels to represent a thin spherical shell. This is another reason that doing color & density in your shader is important. Depending on your shot, you might get away with it, but if your camera is going to be close to the atmosphere and your not going for a stylized look, you'll probably start to see the voxels if you bake color and density into a VDB. The solution to both of these problems is to make sure your target rendering application has a good atmosphere shader.