For the purposes of this thread, fire is defined as rapid combustion that produces a hot, incandescent plasma. This does not included nuclear reactions such as those found in stars.
So is fire limited to earth-like planets, with a similar oxygen atmospere? Or are there other, non-oxygen dependent combustion reactions that produce a similar incandescent plasma?
Are we the only lifeform that sits around a campfire and pokes sticks into the embers while humming Desperado?
No, but trust me, you really don’t want to know about the others!
This is an interesting question. Fire, as we know it, is an oxidizing process.
One type of “non-oxygen dependent combustion reactions that produce a similar incandescent plasma” would be those nuclear reactions you excepted in your OP. But I do wonder if something like fire can arise in non-oxygen atmospheres.
I should mention that typical fire–the kind in a campfire, for example–doesn’t create plasma. Combustion produces incandescent gases and suspensions of small incandescent solid particles (like soot which makes flames yellow). But the temperatures involved aren’t high enough to make a plasma.
The vast majority of atoms in the universe, 98+%, are hydrogen. Much of the rest is fire-inert helium.
That leaves precious few atoms to do any oxidizing. Together, oxygen, sulfur and fluorine make up less than 0.06% of all atoms.
So yes, fire is rare in the universe.
But oxygen isn’t the only gas that can “oxidize.” Chlorine is another example, IIRC.
Also, the oxidizer doesn’t have to be the atmospheric gas. If you were on a planet with a hydrogen or methane atmosphere, you can burn oxygen from a tank.
Fire must account for a very tiny fraction of the universe, excepting nuclear reactions as in the OP. Very few stars have chemical compounds inside, except for the smallest ones. Since creating the elements that would combust together involves much more energy than the combustion per se, there must not be much of either going on, for such stars to be so cool and dim. Practically all the mass of Earth is not fire, and this is a planet where we know fire exists. It’s hard to imagine fire on any of the other Solar planets. Certainly black holes and neutron stars wouldn’t have fire on them, nor would the mass of thin dust and gas from which stars form.
In fact, could the universe even be a part per million fire? Or per billion?
Yes, but only because we’re the only lifeform that has evolved to Desperado. The bird-creatures of Xulicon V have the same melody, but the title of the song is "Hesperornis.
Regarding other atmospheres, I mentioned not too long ago in another thread that a lot of 1930s and 1940s science fiction talked about life in chlorine astmospheres, apparently based on the vivid Chemistry lab demo of a candkle burning in a chl;orine atmosphere.
I’m sure there are other possibilities out there. Certainly you don’t need to be in gaseous oxygen – magnesium flares burn underwater, and you can always pack an ocidizer in your fuel, as with gunpowder. But I doubt if most of these are common enough to make a plausible system. As others pointed out in that other thread, it’s not really all that likely to have a world with a chlorine atmosphere.
This question occurred to me while watching the fires in California. How odd it must sound to the inhabitants of Tau Seti 3, upon learning that the homes of Earthlings frequently turn into hot gas and disappear.
Well, heck, it seems weird to us here on earth when the Big Sun rises every thirty years and your houses sublimate. But I guess, since it’s predictable, you don’t see it as weird.
Only to find out your homeowner’s insurance covers evaporation, but not sublimation. Damn you, Allstate!
Fire can only exist when chemicals aren’t in equilibrium. On Earth, this happens because plants continually pump O2 into the atmosphere, and continually create non-oxidized organic compounds. If all the plants died, those organic compounds would combine with the free O2 fairly quickly, some of those reactions would be “fire”. After all the atmospheric O2 is used up, then no more fire is possible.
Even things like metallic magnesium burning underwater won’t happen any more, because where does that metallic magnesium come from? As far as I know there aren’t any natural processes that create metallic magnesium.
Oxygen is actually a pretty common element in the universe. Sure, hydrogen and helium are the big ones, but carbon, oxygen and nitrogen are the next most common. The only trouble is that oxygen will naturally combine with other elements to form the lowest energy compounds. So you don’t find O2 out there in the unverse, you find H20 and CO2.
Any planet that has animal life would require some form of autotrophic life first, and that autotrophic life, and the biggest source of energy for that autotrophic life is likely to be a nearby star. So you have photosynthesis which creates chemical disequilibrium, and when you have chemical disequilibrium you’ve got the potential for fire. So any planet with animal life is almost certainly going to have fire. Sure, there could be planets with only anaerobic life, but those anaerobic life forms won’t be animals, anaerobic respiration isn’t energetic enough for animal life. Eukaryotes only evolved on Earth many hundreds of millions of years after the evolution of photosynthetic bacteria.
If we lived on Jupiter with its methane atmosphere, we would consider oxygen a fuel. Or any other strongly oxidizing agent.
>Are we the only lifeform that sits around a campfire and pokes sticks into the embers while humming Desperado?
According to a paper by physicist Max Tegmark, there are an infinite number of planets where the life forms do this, and an infinite number where they hum Desperado backwards, and an infinite number whose sticks are purple, and so forth. But they’re typically very far away.