To @pllim's first suggestion `solar_system.MERCURY`: I don't really see that as a half-way compromise in terms of ease of use, because that would require 20 characters to get mercury instead of one, and anyone needing multiple planets would need to import a bunch of extra variables to use the tools.  To @mhvk, I'm also not sure what `PLANETS.get(index, index)` is meant to do. 