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If a planet's atmosphere consisted of 50% oxygen and 50% a flammable gas (e.g. methane) at 1 atm, could you put the entire atmosphere on fire by lighting a single candle?

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A minor note, but for maximum burnage, you'd want 75% Oxygen. 3 O2 + 1 CH4 -> 1 CO2 + 2 H2O.

An atmosphere of that composition is impossible because chemistry happens. Oxygen and Methane combine, so it's unlikely that large concentrations of both would ever happen.

Oxygen is quite reactive and so it tends to only exist in planet's atmospheres in special circumstances, such as photosynthesis, and Oxygen only came about in significant amounts in our atmosphere after the Iron was filtered out of the ocean and after the Methane was reduced to trace amounts.

But in theory, yes, if you had a planet with an Oxygen Methane atmosphere and there was no source of fire on that planet. No volcanoes or lightning, which is also, probably unlikely, then one match is all it would take, or one spark. Once lite, the entire atmosphere would go up like lighting a stove. I'm not sure how fast it would move, and that's a kind of interesting question, probably a couple hundred MPH. Maybe the speed of sound. I'm not sure.

But such a theoretical flame, 1 ATM, you could see the fire move across the planet from orbit, even a high orbit like our Moon. It would be interesting to see from a safe orbit.

Another interesting question is how long it would take to burn out. I think it would burn around the planet fairly quickly, in maybe a day or two days max, creating abundant CO2 and Watervapor. Eventually the wetness of the atmosphere would put it out, but most of the O2 and CH4 would probably burn first. 90% or a little more if I was to guess.

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