carefull with the word, Infinite. Remember that your system resources have to generate that geometry. Lets see if we can take this apart in steps.
1. Planets are round- so even though you might think that a sphere isnt the best option, it still is. you just have to cut it in half and flatten it out a bit. leave some curvature to run into your horizon.
2. It's special effects, not real world representation- Try working with the idea of compositing your peice together in a third party compositor. Doing this you can keep the enviroment on a small scale and then create hdr images for your lighting or what not. Remember, if you adjust your camera right you can make an object very small look very large without loosing track of your scene. ps. set a second camera up to do this and use your persp cam to veiw the real thing.
3. keep looking at the real thing- while your working on this scene, always have a real photograph of a similar perspective. this will help you with your construction and adjustments within maya.
4. This isnt the only way- keep asking for other peoples opinions and reseach it all until you find one that works for you.
Hope this helped a little.
And after calming me down with some orange slices and some fetal spooning,
E.T. revealed to me his singular purpose.
--TOOL, 10,000 Days---