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I am currently creating a planet with an atmosphere rich in noble gasses, especially neon, xenon, and argon. It's composition is roughly about this:

  • Nitrogen (N2) - 61.5%
  • Oxygen (O2) - 21%
  • Neon (Ne) - 15.5%
  • Xenon (Xe) - 1%
  • Water Vapor (H2O) - 0.5%
  • Argon (Ar) - 0.479%
  • Carbon Dioxide (CO2) - 0.02%
  • Trace - the rest

The atmospheric pressure is 0.98 atm. Most of the elements here are quite easy to get (nitrogen, water, CO2, and oxygen are common in the universe) and argon comes from the radioactive decay of potassium in the crust. However, neon and xenon are a problem.

My solution to get neon in the atmosphere was the planet to originally be a gas giant with some trace amounts of neon. The star then went into its T-Tauri phase and blew off most of the hydrogen and helium from the gas giant early in the planet's history. Neon, being heavier stayed in the gas giant. Then, a major collision moved the planet into the habitable zone and the rest of the hydrogen and helium was lost. Eventually, the rock and water from the formed a core, mantle and crust.

This works for neon since it is quite common in the universe. However, xenon is much much rarer, so how could xenon accumulate in the planet in the first place?

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  • $\begingroup$ To clarify how rare xenon is: about 30 to 40 tons of xenon are produced annually, by repeated fractional distillation of liquid air. Xenon is about 50 times more expensive than neon, and about 1000 times more expensive than argon. (But about 35 times less expensive than gold, by weight.) $\endgroup$
    – AlexP
    Commented Sep 2, 2023 at 19:35
  • $\begingroup$ Also, in the earth's atmosphere xenon is at a concentration at 0.08ppmv. $\endgroup$
    – Neil Iyer
    Commented Sep 4, 2023 at 1:40

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It can occur, if your planet forms in an stellar graveyard

Xenon, unlike most other noble gases like argon or neon, isn't formed by classical stellar nucleosynthesis, as synthesizing of elements above Iron-56/Nickel-60 is dramatically throttled, as the reaction becomes endothermic. That is to say, that doesn't mean that no heavy elements are produced in stellar nucleosynthesis. It is just that it is a pain in the a** to produce these heavy nuclei for stars. Hence most nuclei above Fe-56 are produced after a supernova, either by fast/slow neutron capture (in case of lead,gold and uranium), or by radioactive decay of the nuclei formed by the supernova.

Xenon is formed mainly by the radioactive decay of radioactive isotopes like uranium, which are formed in the aftermath of a supernova, or directly by slow neutron capture.

However, it is tricky for a planetary atmosphere to be enriched with xenon, as it would imply that your planet contains a considerable amount of radioactive isotopes, or that your planet was basically once a hot Jupiter being stripped of its atmosphere, losing lighter gases, while leaving behind heavier gases, and by sheer luck, managed to migrate outwards to a farther orbit.

The latter seems a bit plausible, considering we have observed a few real life examples of this "atmosphere stripping" process, but it still doesn't explain how the lighter gases like nitrogen or neon remained. Maybe it managed to regain back the neon and nitrogen by asteroids and stuff, but it is definitely hard to get a b***load of neon and nitrogen on a planet that was just freshly stripped of its light gases.

So, to sum up, it is possible to have an planetary atmosphere enriched in xenon, but it would be extremely hard to form considering the fact that its formation requires a b***tload of radioactive isotopes or being stripped of its atmosphere. However, since this is your story, I am sure you can find a way to fix the problem and somehow manage to get a xenon enriched atmosphere.

Happy Worldbuilding =D

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  • $\begingroup$ The uranium decay thing could actually work. My planet is richer in uranium than usual, so from the fission of U-238 and Pu-244, the long-lived Xe-136 would form, which could be released by geochemical processes, leading to an atmosphere rich in xenon. (In a unique layer full of radioactive elements between my core and mantle, I started with ≈0.1% Pu-244 from supernovae, which releases about ≈6.90e+41 atoms of xenon and i started with 10% U-238, which releases a larger number - from calculations on this source) $\endgroup$
    – Neil Iyer
    Commented Sep 3, 2023 at 15:50
  • $\begingroup$ Atmosphere stripping occurs when the velocity of molecules exceeds escape velocity. Heavier molecules at the same temperature move slower, so just heat up the planet until all the light stuff you don't want is gone. $\endgroup$
    – Monty Wild ♦
    Commented Sep 4, 2023 at 1:02
  • $\begingroup$ That could work, but remember, I still want light gasses like neon, nitrogen, and oxygen in my atmosphere. $\endgroup$
    – Neil Iyer
    Commented Sep 4, 2023 at 1:32

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