Chemical reactions we are used to here on Earth are hardly possible in most of interstellar space due to extremely low temperature and density (though some pretty complex molecules are found there, that Wikipedia article listing

gas-phase ion chemistry (often driven by cosmic rays) and surface chemistry on cosmic dust

among possible formation mechanisms). On the other extreme, close to a star or other energy source, chemical reactions also make little sense because they would be overtaken by other processes with energy much higher than that of chemical bonds. This is why astronomers call all elements except hydrogen and helium metals.

Somewhere in between lies our home, where schoolchildren study all the diversity:

combination, decomposition, single-replacement, double-replacement, and combustion.

Obviously, all of these types of chemical reactions are possible on any other Earth-like planet. However, what about other types of planets?

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    $\begingroup$ You might be interested in the Kinetic Database for Astrochemistry, which provides chemical networks for a range of astrophysical environments. $\endgroup$
    – lucas
    Oct 26, 2022 at 10:34

1 Answer 1


Yes! It just depends on this main factors that affects whether a reaction will take place or not:

Gibbs free energy $G$, is a determiner of whether a reaction takes place spontaneously or not. So if on a planet the $G$ is negative, then the reaction is spontaneous, if it is positive, then it is not spontaneous.

Now, some reactions like you mentioned, combustion for example specifically requires oxygen to react with the compound, so some planets without certain elements or compounds in their atmosphere won't be able to have reactions that require external elements/compounds to take place.

However, majority of reactions can take place, provided that the proper compounds/elements are available of course, through exothermic (reaction releases heat) or endothermic reactions (reaction absorbs heat). It all comes down to the amount on so-called "energy" (mainly thermal energy) in the atmosphere that the reaction can take in if the reaction has a positive $G$.

You can also read this question on how solar winds affect the atmospheric composition and density of planets, per the OP's comments below.

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    $\begingroup$ This is a good start, but if my layman's understanding of Gibbs energy is correct, it only imposes a limit on the temperature of the environment, and only a lower limit. Could there be more requirements besides minimum temperature (and the presence of reactants which you also mention)? Are all reactions possible on, say, Mercury, or maybe any molecule heavier than water will be shred into atoms by the solar wind and blown away? $\endgroup$ Oct 31, 2022 at 11:06
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    $\begingroup$ @AlexSerenko hmm good question! I'll look into it soon $\endgroup$
    – DialFrost
    Oct 31, 2022 at 11:29

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