<p>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.</p>

<p>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.</p>

<p>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 <em>making</em> it.</p>

CC BY-SA 4.0
2019-10-19T19:22:29.703Z
Ohh I see. So oxygen is necessary in order to start a fire. Not just a flammable gas itself like methane. Thanks sir!
draw134
CC BY-SA 4.0
2019-10-20T00:17:23.770Z
@Vince an oxidizing agent is require to *maintain a fire* as well. Oxygen alone doesn't burn, fuel alone doesn't burn. Only a mixture of a fuel and an oxidizer can burn. When either one is absent, fire ceases. That's one way to stop a fire, displace the oxygen with CO2 or Halon or some other gas.
uhoh
CC BY-SA 4.0
2019-10-20T00:18:34.747Z
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.
uhoh
CC BY-SA 4.0
2019-10-20T03:56:18.277Z
What are other oxidizing agents?
Randy Zeitman
CC BY-SA 4.0
2019-10-20T04:00:27.257Z
I recently saw an image of oxygen burning in an atmosphere of flammable gas, however, I forget what the gas was.
CJ Dennis
CC BY-SA 4.0
2019-10-20T07:10:22.257Z
@RandyZeitman chorine, florine, bleach and lots more besides.  Most oxidsing agents are pretty nasty, (and oxygen is too, but we've evolved to cope)
James K
CC BY-SA 4.0
2019-10-20T10:32:01.910Z
Interestingly, in Neptune or Titan you could make a gas stove work by feeding it with oxygen bottles instead of gas bottles.
Pere
CC BY-SA 4.0
2019-10-20T15:48:46.463Z
@CJDennis I think you are remembering [this video](https://www.youtube.com/watch?v=8jmX-TUQkx4) where it is an oxygen flame in a propane atmosphere
fyrepenguin
CC BY-SA 4.0
2019-10-20T17:13:53.493Z
@JamesK I never thought of it that way, with Oxygen being comparable to Flourine or Chlorine. I've learned something new (or thought something new? IDK).
rydwolf
CC BY-SA 4.0
2019-10-21T15:45:22.877Z
@Pere  Asimov  used this in one of his "Mysteries" a lifetime ago :-)
Carl Witthoft
CC BY-SA 4.0
2019-10-19T19:21:09.960Z
2019-10-19T19:21:09.960Z
James K
<p>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?</p>

CC BY-SA 4.0
2019-10-19T18:45:04.800Z
Most likely the flame would be extinguished due a lack of oxidizing agent.
BillDOe
CC BY-SA 4.0
2019-10-19T19:16:55.807Z
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
draw134
CC BY-SA 4.0
2019-10-20T00:15:29.943Z
@Vince your right, cryogenic, liquified ammonia has even been used as a fuel to [fly the X-15 into space](https://space.stackexchange.com/q/22758/12102)! But it also carried [cryogenic, liquified oxygen](https://en.wikipedia.org/wiki/North_American_X-15#Propulsion) (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.
uhoh
CC BY-SA 4.0
2019-10-21T03:46:27.380Z
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...
Rich
CC BY-SA 4.0
2019-10-21T15:09:41.333Z
Besides the good answer you have, I'll add that if a planet did contain flammable gas AND an oxidizing agent then it would have already burned.  For example, a small meteorite would act as the trigger.
James
CC BY-SA 4.0
2019-10-21T15:44:34.620Z
There's a nice Asimov's Mysteries  story about this situation.    Anyway, the point is that you can't get flame aka oxidation without two agents -- not to mention that you can't "throw fire" -- you can only throw energy in the form of hot reagents or photons.
Carl Witthoft
CC BY-SA 4.0
2019-10-21T20:59:40.710Z
You might consider reading up on the chemistry of reduction/oxidation reactions. Burning is oxidation, and oxidation *by definition* requires that you move some electrons around. Moving electrons implies that they *start* somewhere and then *end up* somewhere else. So the thing to figure out when you're trying to burn a big pile of hydrogen is: where are the necessary electrons coming from? On Earth when we burn hydrogen the electrons come along with the oxygen, but there is no oxygen on Neptune.
Eric Lippert
CC BY-SA 4.0
2019-10-24T04:12:36.243Z
This has been asked in Worldbuilding StackExchange, but nevertheless i believe valuable here too.
Max0815
CC BY-SA 4.0
2019-10-25T01:19:11.883Z
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](https://en.wikipedia.org/wiki/Flame)."
Mazura
CC BY-SA 4.0
2019-10-25T01:22:45.627Z
Thank you for your answers sirs !!
draw134
<planet><solar-system><space><gas>
Starting a fire in a cold planet that was full of flammable gas
draw134
# Starting a fire in a cold planet that was full of flammable gas

(Asked by: username_0 on 2019-10-19)

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?

**Tags:** planet, solar-system, space, gas
> **Comments:**
> **username_1**: Most likely the flame would be extinguished due a lack of oxidizing agent.  
> **username_0**: 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  
> **username_2**: @Vince your right, cryogenic, liquified ammonia has even been used as a fuel to [fly the X-15 into space](https://space.stackexchange.com/q/22758/12102)! But it also carried [cryogenic, liquified oxygen](https://en.wikipedia.org/wiki/North_American_X-15#Propulsion) (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.  
> **username_3**: 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...  
> **username_4**: Besides the good answer you have, I'll add that if a planet did contain flammable gas AND an oxidizing agent then it would have already burned.  For example, a small meteorite would act as the trigger.  
> **username_5**: There's a nice Asimov's Mysteries  story about this situation.    Anyway, the point is that you can't get flame aka oxidation without two agents -- not to mention that you can't "throw fire" -- you can only throw energy in the form of hot reagents or photons.  
> **username_6**: You might consider reading up on the chemistry of reduction/oxidation reactions. Burning is oxidation, and oxidation *by definition* requires that you move some electrons around. Moving electrons implies that they *start* somewhere and then *end up* somewhere else. So the thing to figure out when you're trying to burn a big pile of hydrogen is: where are the necessary electrons coming from? On Earth when we burn hydrogen the electrons come along with the oxygen, but there is no oxygen on Neptune.  
> **username_7**: This has been asked in Worldbuilding StackExchange, but nevertheless i believe valuable here too.  
> **username_8**: 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](https://en.wikipedia.org/wiki/Flame)."  
> **username_0**: Thank you for your answers sirs !!  

---

## 1 answers

### Answer by username_9 on 2019-10-19. Score: 29 | Accepted answer

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.

> **Comments:**
> **username_0**: Ohh I see. So oxygen is necessary in order to start a fire. Not just a flammable gas itself like methane. Thanks sir!  
> **username_2**: @Vince an oxidizing agent is require to *maintain a fire* as well. Oxygen alone doesn't burn, fuel alone doesn't burn. Only a mixture of a fuel and an oxidizer can burn. When either one is absent, fire ceases. That's one way to stop a fire, displace the oxygen with CO2 or Halon or some other gas.  
> **username_2**: 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.  
> **username_10**: What are other oxidizing agents?  
> **username_11**: I recently saw an image of oxygen burning in an atmosphere of flammable gas, however, I forget what the gas was.  
> **username_9**: @RandyZeitman chorine, florine, bleach and lots more besides.  Most oxidsing agents are pretty nasty, (and oxygen is too, but we've evolved to cope)  
> **username_12**: Interestingly, in Neptune or Titan you could make a gas stove work by feeding it with oxygen bottles instead of gas bottles.  
> **username_13**: @CJDennis I think you are remembering [this video](https://www.youtube.com/watch?v=8jmX-TUQkx4) where it is an oxygen flame in a propane atmosphere  
> **username_14**: @JamesK I never thought of it that way, with Oxygen being comparable to Flourine or Chlorine. I've learned something new (or thought something new? IDK).  
> **username_5**: @Pere  Asimov  used this in one of his "Mysteries" a lifetime ago :-)  

---

