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I'm currently working on a comic-series set in a solar-system that isn't ours, and i'm reworking most of my planets.

One of them, a cold ocean-planet below freezing temperatures that orbits its k-class host-star barely outside of the frost-line, has been nagging me for quite a while, and i've already annoyed this site with a question regarding it.

It's about 0.95 times as massive as earth, has 0.89 times its radius and an average temperature of -86 °c, (though these values can change if they allow for a better answer).

Previously, i wanted the planet to still remain habitable for humans, but i want to change that. Because making it decidedly deadlier could make the story much more interesting.

I plan for the planet to have a high-altitude, white cloud-cover, concealing a deep-red (and most likely very toxic) atmosphere, above red oceans (that don't necessarily need to be made from water), with enormous winds.

I've come across some explanations for how these colors could arise. Like iron-oxides in water, colored microbes, or replacing both the oceans and atmosphere with Bromine, but there are little to no other sources i can find as to how plausible these options are, and i also aim for a planet that is both deadly to unprotected humans, but still able to be built-on to some degree (like pressurized ground-stations).

So my question would be: What chemicals could realistically cause a planet to have these features?

Ps: If it's possible for this type of deadly ocean-planet to exist, but the chemical make-up would cause it to have different a different coloration than the desired Red, i'd still be open to hear those possibilities.

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  • $\begingroup$ just make it anoxic, that is a low oxygen atmosphere, that will keep people from breathing on it, color can be provided by microbes. earth was anoxic for much of its history. $\endgroup$
    – John
    Commented Oct 6, 2023 at 21:42
  • $\begingroup$ There are some very potent dyes that produce colors in very low concentrations that could be produced by organisms and explain the ocean, but the atmosphere is really difficult...strongly-colored gases tend to be highly reactive, unstable, or both, and not likely to stay below white cloud cover. Does it have to be the atmosphere itself which is colored? Would, say, fog colored by spores carrying strong dye substances work? (With the side effect of surfaces tending to get coated with said substances...) $\endgroup$
    – Christopher James Huff
    Commented Oct 9, 2023 at 17:33

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Carbon-iron planets could have oceans of iron pentacarbonyl[1]. Which is very nasty (for humans) volatile red liquid in the -20 to 100°C range at 1 atm. To get white clouds, atmosphere could contain sulfur dioxide. It is likely that it reacts with the carbonyl resulting in array of toxic organosulfur chemicals.

[1] https://en.wikipedia.org/wiki/Iron_planet

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  • $\begingroup$ Honestly, i might've found the solution to the question on my own in the mean-time, but i really like this answer, and it might fit another planet of the same system very well. I'll still keep this question open for the next few days just to see what other answers might be possible, but this one's in the lead. $\endgroup$
    – NimRad
    Commented Oct 9, 2023 at 19:38
  • $\begingroup$ I believe if your planet had planetary collisions, like what happened with Mercury, only more so, you could have lots of iron, nickel, and sulfur, and get the carbon and oxygen from the planet's atmosphere or from meteors/comets. That would be good enough to create iron pentacarbonyl, nickel pentacarbonyl, your sulfur compounds, and maybe some dicobalt octacarbonyl. $\endgroup$
    – Neil Iyer
    Commented Oct 9, 2023 at 19:56
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One chemical compound comes to my mind: Carbon monoxide.

Its boiling point is -191.5 °C so it may stay gaseous at this temperature. The atmosphere does not have to be completely out of that stuff to be deadly to humans. Even a concentration of less than 1% is lethal enough. Regarding your oceans, I don't think you would have a liquid ocean at that temperature. Take as an example the article "Does salt water expand as much as fresh water does when it freezes?", Source. In this article look at the graph "%NaCl by weight". The lowest melting point (Eutectic point) of sea-water is achieved at 23.3% NaCl by weight, lowering melting point to just -21.1 °C. More phases in the graph here.

To make your ocean liquid at this stage would require lots of deep sea geothermal vents.

Hypothetical chlorine planets can have a liquid ocean with a eutectic temperature just below -70 °C with 25% by weight of hydrochloric acid in water according to this wikipedia phase diagram.

If you don't want to use exotic compounds, take Titan as an example. The main oxygen-based compound - water - is solid, so the whole atmospheric cycle is based on carbon compounds. It is similar in many respects to a carbon planet. Carbon monoxide is in trace amounts but you can increase it by volcanism. The planet may be younger than Earth, hence more active, yet not active enough to melt all the oceans and become habitable. Methane and other hydrocarbons are colorless. However, they may react with solar radiation and polymerize into tholins. These tholins stain the atmosphere into the orange opaque color associated to Titan.

EDIT To increase percentage of Bromine the planet must have collected a higher ratio of stellar dust from exploding massive stars. (sources of Nucleosynthesis) It may increase the occurrence of elements up to Rubidium, but I can't tell how to raise Bromine levels enough to make oceans.

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  • $\begingroup$ Thank you for the answer! and, i didn't realize that i forgot this in the question, but the oceans absolutely do not need to be made from water. Just any liquid that might produce the needed effects, and might be realistically able to naturally from an ocean on a planet. Hence why i referenced the fact that i thought of making the whole ocean and atmosphere out of Bromine :p $\endgroup$
    – NimRad
    Commented Oct 7, 2023 at 17:25
  • $\begingroup$ An ocean at -60C might happen to be of propane or H2S for example, apparently they are liquid at these temperatures. Although this set of conditions makes the planet "toxic" in MoO terms, at difficulty level 5 of 6 for colonization, and not "dead" (L3), thus I say the humanity would ignore it for habitation for a lot longer than expected. $\endgroup$
    – Vesper
    Commented Oct 9, 2023 at 7:09
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    $\begingroup$ Bromine is about a million times less common in space than oxygen. Even chlorine isn't exactly commonplace, but is hundreds of times more abundant. Getting an ocean of bromine looks exceptionally unlikely, and bears all the hallmarks of Precursor Race Messing About For No Adequately Explained Reason. PRMAFNAER is of course exceptionally common in scifi, and so I wouldn't expect anyone to notice or care if you use it. $\endgroup$
    – Starfish Prime
    Commented Oct 9, 2023 at 10:29
  • $\begingroup$ It might be possible to create an ocean made of another liquid, with some dilute bromine in there, but that would be exceptionally difficult to do. Also, you need to make sure that the liquid wouldn't react with the bromine. Dilute chlorine oceans may be more manageable, but still rare. $\endgroup$
    – Neil Iyer
    Commented Oct 9, 2023 at 13:40
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If you're willing to delve a bit into unrealism, one of your best bets is Chromyl Chloride. It's blood red, it has a melting point of -96C which is a little bit below your average temperature, and it is extremely toxic to humans.

I'm not really sure how you'd get a full planet of it, or how stable it actually would be, though.

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    $\begingroup$ Just to clarify, chromyl chloride is not stable anywhere light, water, or organic compounds are present, and decomposes when it comes into contact with them. $\endgroup$
    – Neil Iyer
    Commented Oct 9, 2023 at 13:35

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