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What would happen to a cold planet that has a large amount of flammable gas, like Neptune, if we throw some fire into it? Will it burn, or would the flame be extinguished due to the cold?

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    $\begingroup$ Most likely the flame would be extinguished due a lack of oxidizing agent. $\endgroup$ – BillDOe Oct 19 at 18:45
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    $\begingroup$ what do you mean by that sir? Can you explain to me in the answer? And as far as i know ammonia and methane and hydrogen are flamamble $\endgroup$ – Vince Oct 19 at 19:16
  • $\begingroup$ @Vince your right, cryogenic, liquified ammonia has even been used as a fuel to fly the X-15 into space! But it also carried cryogenic, liquified oxygen (LOX) as well, and that's the basis of JamesK's answer. The chemical process of oxidation (burning) doesn't require actual oxygen, but it does require some oxidizing agent, and oxygen is a relatively good one. Put a small glass jar over a candle and it burns for a few seconds then goes out. Why? no fresh supply of oxidizer. $\endgroup$ – uhoh Oct 20 at 0:15
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    $\begingroup$ There are such things as monopropellants that burn (e.g. decompose exothermically) without a separate oxidiser - all the ones I can think of are liquids, require a catalyst, or both. You could have an atmosphere of FOOF, but it would need to be very cold... $\endgroup$ – Rich Oct 21 at 3:46
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    $\begingroup$ Ammonia, methane and hydrogen are flammable gasses, but combustion (fire, aka: oxidation) requires fuel and an oxidizer. That's why the candle goes out when you put the lid on; it quickly uses up all the remaining oxidizer (oxygen), while there's plenty of fuel left (wax). - The fuel must be vaporized (or hot enough to vaporize) or be a gas : "In this [gaseous] state they can then readily react with oxygen in the air, which gives off enough heat in the subsequent exothermic reaction to vaporize yet more fuel, thus sustaining a consistent flame." $\endgroup$ – Mazura Oct 25 at 1:19
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The outer parts of Neptune are mostly hydrogen and helium. There are small amounts of other gases such as methane, ammonia and water vapour. However, there is no oxygen at all.

If you took some of Neptune's outer layer back to earth and mixed it with our air, it could burn. Even very cold hydrogen can burn (it soon heats up!) This couldn't happen on Neptune, because a fire needs both fuel and oxygen to burn.

It is very unlikely that any planet would have large amounts of both fuel and oxygen in its atmosphere. Oxygen is very reactive and will react with any flammable gases to produce (mostly) water and carbon dioxide. Oxygen is not stable in a hydrogen atmosphere over a period of millions of years. If we find oxygen in a planet's atmosphere, we can be fairly sure that something on the planet is making it.

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    $\begingroup$ Ohh I see. So oxygen is necessary in order to start a fire. Not just a flammable gas itself like methane. Thanks sir! $\endgroup$ – Vince Oct 19 at 19:22
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    $\begingroup$ nit alert: the chemical process of oxidation (burning) doesn't require actual oxygen, just some oxidizing agent, and eponymous oxygen happens to be a relatively good (and popular) one. $\endgroup$ – uhoh Oct 20 at 0:18
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    $\begingroup$ @RandyZeitman chorine, florine, bleach and lots more besides. Most oxidsing agents are pretty nasty, (and oxygen is too, but we've evolved to cope) $\endgroup$ – James K Oct 20 at 7:10
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    $\begingroup$ Interestingly, in Neptune or Titan you could make a gas stove work by feeding it with oxygen bottles instead of gas bottles. $\endgroup$ – Pere Oct 20 at 10:32
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    $\begingroup$ @CJDennis I think you are remembering this video where it is an oxygen flame in a propane atmosphere $\endgroup$ – fyrepenguin Oct 20 at 15:48

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