Andrew Kramer's awesome Orb plugin makes making planets really easily. The plugin allows you to take texture maps, bump maps and even illumination maps and create realistic planets. I've gone so far as to publish an entire series of tutorials creating all the planets in the solar system ( https://youtube.com/playlist?list=PL58-kYtQUHvObfVpn5viKl3lLvCU-c7Ue
When Andrew launched the plugin, he also shared a tutorial to recreate Earth and during that tutorial he briefly talked through how to make an atmosphere. I could never remember the steps taken so eventually decided to rewatch his tutorial and note the settings and then save these settings as a preset. And seeing as I was doing that, I thought it's only right to share it with you too.