Exploring this new star system you have been pretty lucky to find an Earth like planet, with plenty of Oxygen, both in gaseous and in chemically bonded state (water, rocks, etc.).
Just before you were opening that bottle of Chateau Centaurix 68 to toast this discover, your supercomputer starts beeping like crazy: due to gravitational perturbation, in some decades your new found planet is going to impact that blue Neptune-like giant planet over there.
You don't need a triple Ph.D. to guess that a blue giant planet is very rich in methane and other hydrocarbon gases, and you don't even dare deviating the orbit of the planets, as you know from kindergarden it is impossible with your means.
You bid farewell to your colonization dreams, but you start thinking that this is going to be a show that can benefit science: a bubble of Oxygen impacting a bigger bubble of Methane; quite a big 4th July firework!
What kind of observation equipment are you going to prepare, and where are you going to stand to observe it without being broiled?
Assuming the Earth as reference, its atmosphere mass is 5x10^21 g (source), of which roughly 1/5 is Oxygen, which gives about 6x10^19 mol of O2.
These can burn 3x10^19 mol of CH4, releasing about 3x10^22 kJ, which converts to 6x10^9 Megatons