I am building a habitable terrestrial 'super-earth' type planet. Ideally, my lifeforms should be primarily carbon-based and their biochemistry relatively similar to Terran biochemistry, but with slight modifications given their different environment (Atmospheric composition, Orbital characteristics and physical characteristics of their planet etc.) I am having difficulty determining exactly how the different environment would constrain/expand the possible biochemistry of my organisms, examples: what amino acids or amino acid analogues would be favoured in protein synthesis, what molecule/molecules would be most suited to transport oxygen around the organism, what compounds could work as Nucleic acid analogues given the different atmospheric conditions.

Basically, how to determine whether the organism would be able to exist and what biochemistry would "work" (be able to function) if various atmospheric conditions were altered such as the Ph of the oceans and/or addition of toxic compounds to atmosphere or biosphere. Although a have quite a bit of knowledge when it comes to Terran biochemistry and biology (so please feel free to explain in as comprehensive/complex a way as possible) I am struggling specifically with this topic of 'Xenobiochemistry'. I will explain the relevant characteristics of my planet below:

My planet's characteristics:

The planet is a super-earth type terrestrial planet with 1.5 times the gravity of earth, 1.48 times the radius and 3.15 times the mass. It is more volcanically active than earth and its atmosphere has relatively high concentrations of sulphuric molecules, CO2 and methane as a result (Exact numbers below.) The planet orbits a stable close-binary pair of dwarf stars at a distance of 0.412 AU. The planet's atmosphere is pressured around 1.7 Atm and its surface temperature is around 30 C (90 F) (Due in part to the greenhouse effect of the many volcanic compounds in the atmosphere and the increased bond albedo of the planets surface.)

Its precise atmospheric composition is as follows:

73.69% Molecular Nitrogen

24.50% Molecular Oxygen

1.50% Argon

1.00% Ammonia

0.05% Carbon Dioxide

0.05% Water vapour

0.05% Hydrogen sulphide

0.02% Sulphur Dioxide and Trioxide

0.02% Methane

The Ammonia in the atmosphere is due to its use as an early biochemical defence mechanism in unicellular organisms in the early oceans.

My organisms:

Biological building block: Primarily carbon-based.

Biological solvent: Ammonia-Water mixture (2-3% ammonia)

Nucleic acid/Nucleic acid equivalent: ?

Protein structure: ?

Oxygen transport: ?

Energy storage: ?

Cell structure: ?

Also, please just ask for whatever other characteristics you would need to answer the question, as I have calculated many, many of my planets qualities and features.

this is a long description, but I really hope someone can help me understand how to determine which biochemistries work in which alien world's, such as this one. thanks in advance. (:

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    $\begingroup$ Hello Curly, welcome to Worldbuilding. That hard-science tag is actually appropriate for your question. Congratulations! Almost no one uses it correctly the first time. Unfortunately, as @Slarty points out, you're asking a question that's so far beyond our understanding of science today that the tag must be ignored to give you an answer. You might be straining at the proverbial gnat. Looking at protein structures to build your world's version of an elephant is a bit like looking at the chemistry of concrete to build a skyscraper. It's probably not a necessary detail for a story. $\endgroup$
    – JBH
    Oct 27, 2020 at 17:53
  • $\begingroup$ I can't help much with the hard science (detailing an entire evolutionary path on a planet with differing chemistry is a bit beyond my reach) but you could start your research by looking at extremophiles: en.wikipedia.org/wiki/Extremophile but as Slarty points out, there are 500+ known amino acids of which just 20 appear in our genetic code. $\endgroup$
    – Tim
    Oct 27, 2020 at 18:55
  • 1
    $\begingroup$ I think the most important thing @JBH says here is "It's probably not a necessary detail for a story". This is so complex that the less you say and the more you leave to the imagination, the better, IMO. Any number of famous sci-fi writers have post-docs in physical sciences and all of them steer away from detailing any imagined technology beyond the "shape, smell, name" level of description. $\endgroup$ Oct 27, 2020 at 18:59
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    $\begingroup$ Scary, as I believe the presence of that much ammonia will invalidate ATP-ADP chemistry as the fundamental cellular energy source (ATP activity shuts down completely at less than 0.5% ammonia). If cells do not run on ATP, then DNA is invalidated. Virtually al cellular proteins are invalidated. *****EVERYTHING***** changes. $\endgroup$
    – user79911
    Oct 28, 2020 at 0:15

2 Answers 2


I think it would be very difficult to provide much of an answer as the scope of organic chemistry is vast beyond imagination and 99.999% of it lies in the dark beyond the chemistry that we are "familiar" with.

Humanity is not really fully up to speed with the exact detail of how our own biochemistry works, although much is known, much remains to be discovered. And when it comes to the vast array of plants and animals the scope is even wider.

As an illustration of the level of complexity look here: https://www.sigmaaldrich.com/technical-documents/articles/biology/interactive-metabolic-pathways-map.html Follow the link and have a look around and remember that this is just a fragment of the full thing. What would happen if we mixed this up a bit and swapped out a few methyl groups for hydroxyl groups how would that pan out?

In fact beyond the twenty or so common amino acids, many hundred are known to exist, perhaps a few thousand, but how many of those have been investigated or characterized in any detail? I suggest not that many. And the scope of the possible amino acids is much greater than that by many orders of magnitude. It would be easy to come up with a likely formula for an amino acid that was unknown to science.

On top of that amino acids are optically active so always come as twin possible enantiomers. But if any of the rest of the groups attached to the amino acid also contain chiral centres then the number of isomers is doubled for each one and each might well behave differently in a complex biological reaction.

Then there are bases and sugars and even more variety and possibility 99.999% of which will be unknown to science. And the list goes on as an alien biochemistry need not be limited to any of these three groups of compounds.

I say all this to highlight the immensity of the question that you are asking. I doubt very much that it is answerable beyond a few unsubstantiated generalities.

  • $\begingroup$ "Humanity is not really fully up to speed with the exact detail of how our own biochemistry works" +1 $\endgroup$
    – JBH
    Oct 27, 2020 at 17:49
  • $\begingroup$ @Slarty TFTR. I understand the immensity of the question proposed and the immensity of unknown biochemistry compared with the small sliver known to Terran biology. I mean, even just adding two extra nucleotides to DNA bumps possible amino-acids from 20 to 172. Perhaps I should have narrowed the criteria a bit. I am primarily looking for loose guidelines to possible alts to Terran biology. If I knew how to predict whether or not something would be soluble in my ammonia-water mixture or be unaffected by my atmospherics, I would be able to derive the rough features of my biosphere from that. $\endgroup$ Oct 27, 2020 at 17:59
  • $\begingroup$ I'm thinking amides and amines might be of interest $\endgroup$
    – Slarty
    Oct 28, 2020 at 0:07


Yes, yes. Panspermia. The same basic critters colonized all places in the universe where there is life, possibly with help. So you can start with the basic Earthlife biochem and riff on it. Given a fundamental setup of your colonizing organisms, imagine the selective pressure your environment would produce and then use your biochem knowledge to invent adapations that could evolve.

It is kind of like haiku vs free verse. Free verse risks being lame because you have no limitiations. Haiku constrains you which can also be liberating. Panspermia lets you can be creative within the confines of actual biology.

It occurs to me that must also be what you want. If you have fake made up chemistry it is just that. It can keep company with faster than light speed ships and other fake stuff. But if you riff on earth biochem then the biochem nerds who grok the molecules (your people) will be digging what you did, because they will understand. Or they will object on grounds of steric hindrance, and that is when you open the wine.

  • $\begingroup$ Panspermia is great! I’m using it as a reason that the life on my world is not so different to earth life, and it makes stuff much, much simpler $\endgroup$
    – Katze
    Nov 14, 2021 at 8:53

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