Will asteroid impacts, have an effect on a planets rotation and atmosphere
e.g. Atmosphere will be filled with dust and the sun will be blocked out, causing a mass extinction if the planet has life
Will we be able to customise a planet's Atmosphere
e.g.
45% Methane
25% Carbon Monoxide
15% Fluorine
10% Chlorine
5% Neon
Mabey we could choose from a Periodic Table of Elements, and combine different gases and choose a percentage.
That atmosphere ain't gonna happen:
First of all, Chlorine and Fluorine are insanely reactive. Basically, they will vanish from a planetary atmosphere very quickly unless there's life producing them.
Neon can in principle remain in a planetary atmosphere and is fairly common in the universe, but it does not appear to exist in significant concentrations in most planetary atmospheres:
http://en.wikipedia.org/wiki/Neon#OccurrenceSo, you have a methane / carbon monoxide atmosphere. Your atmosphere contains no hydrogen or helium, so it must be either small enough or hot enough to not retain them. However, there's also no nitrogen, and any planet which lost its nitrogen would also lose its methane. Nitrogen also cannot freeze out at temperatures at which methane is liquid. Therefore, the nitrogen must be in some other form, such as ammonia and related compounds, which froze out. I may therefore conclude that your planet had a primordial atmosphere rich in hydrogen, which was later lost, and a surface temperature below 195 K.
You may also want to check how the chemical equilibrium works out for a mixture of CO2, CO, and CH4.