First you need a lot of ram, and then a few hires earth maps like: surface map,specular map, available at earthobservatory in 8K resolution.Then search the web for good looking hires clouds map, also 8K.Then i made a sphere and scale it at 127.6, since the earth ecuator length is about 12760 Km. This sfere will have the surface map shader.Then i duplicate it and scale it 100.2%.This will be the clouds sphere.This enlarges the sphere so that the cloud tops are around 15 miles above the surface (relatively speaking, that is... the actual scale is 1/100th that in these models).After that i duplicated this sphere also and scale it 101.7%...from the original sphere to make the earth's athmosphere. This expands the sphere so that it extends about 120 miles or so (relative to the scale) which is where the atmosphere thins out to nothing(This isn't extremely precise).Plus, the atmosphere always has a different look depending on the Sun's illumination angle and a myraid other factors that just can't be simulated in 3D. Some photos of the Earth from shuttle orbital altitudes show an extremely prominet limb, while others don't appear to have much of a fuzzy edge at all.).I added also a bump map to earth and the clouds layer. The last sphere i made a gradient transparency with the help of facing ratio atribute, and i added some glow.I use one directional light with 2.5 Intensity, to give a little crisp falloff.To avoid overblowning i twicked out the diffuse of the shaders.First render it's creap, but i spend about 4 hours tweaking this.And by the way i used a machine (because of the hires maps) with 4G of ram and 2xdual core proc(this is from the office).At home, where i have single proc and 1G ram it just crashes....(I have some closeups but i'm not sure if i can post them...firm policy)