Well it all depends how low you want to go with android devices. Biggest problem for older devices are transparent materials, and atmosphere ring is transparent material.
So first step would be making ring mesh that extends from edge of planet (or bit below it) and use only this for simulating atmosphere. (less transparent pixels this way)
There is one nice shader somewhere here that does great job at atmosphere, if you make only ring around planet, that shader works quite well on newer android devices.
The lowest of low minimal shader would be ring with some gradient material (no texture gradient from nodes) with additive transparency, unlit, rough, and some vector variable to add hint of color.
I am working on android game in space that abuses planets and stars, and really it all depends on what are minimal hardware requirements you aim for. Shader can be from something as simple as i described above to very nice one that runs only on nvidia k1. My first prototype was done for PC, it had awful performance on android. Then I stripped down most of fancy shaders, and now i am slowly rebuilding it back to former glory, my game looks “almost” like original prototype from pc.