Depending on what results you hope to achieve (for example how close to the planets you want to get), this can be fairly simple.
Terragen supports multiple planets, but all beyond the first/base one get created with their atmosphere and texturing nodes *inside* the Planet node. So that's the first thing to be aware of. So create as many planets as you want to visualize in a single scene, then you work on placing them. You can try to use real-world distances but it will probably be challenging due to the immense scales involved. If you do, you'll probably need to greatly increase the radius of the Background Sphere (which, incidentally, uses a negative radius because it's inside out, so to increase it you want "bigger" negative values).
You'll also want to adjust your planets to their real-world equivalent sizes. As I mentioned, things are measured in Meters in Terragen. So find out the radiuses of the planets you want to model and set those values in Terragen. Rename the Planet nodes to indicate which planet is which for easy reference.
After that it's a fairly simple matter of sourcing the appropriate texture maps - displacement or bump map for height (for non-gas planets), and color map, those are the only two necessary.
Apply the Displacement/Bump map to the planet surface using an Image Map Shader set to Spherical Projection, with the "position" set the same as your Planet's position. Adjust displacement amplitude as-needed. To be accurate, you probably want to use an Amplitude/Multiplication value that is equal to the highest point in the respective planetary body you are trying to visualize. So for Earth it's around 6500 meters, for example.
Apply color using an Image Map Shader as well, same projection and position as with Displacement.
The atmospheres of each need to be adjusted, of course. That will have to be done by eye as there are no easily translated measurements or other specifications for any of the non-Earth atmospheres, as far as I know.
That should get you started. Hopefully I haven't missed anything.
- Oshyan