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The exoplanet in question is Proxima Centauri b. How might the mid-to-upper-level atmosphere of that planet be dangerously radioactive and full of airborne nuclear reagents, without it being the doings of an intelligent species?

Edit. I am aware that Proxima b is subject to extremely powerful solar winds from its host star, which makes it unlikely to have an atmosphere. However, if it were to have an atmosphere, is it possible that the gases that make up that atmosphere could react with the protons, electrons, alpha particles, etc. that it's being constantly bombarded with to undergo nuclear reactions as a result?

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    $\begingroup$ In two words? Um-possible. $\endgroup$
    – Karl
    Nov 27, 2019 at 22:13
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    $\begingroup$ What you are saying is (puns aside) pretty much outside our understanding of what's possible naturally. Could it be the case that an intelligent species might have done something which accidentally contaminated the system in this way? (BTW Proxima centuri b is pretty much guaranteed to not have any atmosphere whatever). $\endgroup$
    – Escaped dental patient.
    Nov 27, 2019 at 23:02
  • $\begingroup$ Ionosphere? Air there is constantly bombarded by xray solar radiation. $\endgroup$
    – user6760
    Nov 28, 2019 at 0:10
  • $\begingroup$ @user6760 its not radioactive though, and you'd get more radiation exposure as you left the atmosphere. $\endgroup$
    – Starfish Prime
    Nov 28, 2019 at 9:17

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Your planet is kicking out huge quantities of radon gas.

https://en.wikipedia.org/wiki/Radon

Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through which thorium and uranium slowly decay into lead and various other short-lived radioactive elements; radon itself is the immediate decay product of radium.

Radon can be problematic here on earth, even with the very low qualities of thorium and uranium in the crust. On a planet with much more of these heavy elements in the crust there would a lot more radon generated. It is heavy and so it will not be lost. As on Earth it will preferentially accumulate in low areas because it is heavy, but wind could carry it upward.

Airborne particulate heavy metals.

Potential Human Health Effects of Uranium Mining, Processing, and Reclamation

Radon and its alpha-emitting radioactive decay products are generally the most important, but are not the only radionuclides of health concern associated with uranium mining and processing. Workers are also at risk from exposure to other radionuclides, including uranium itself, which undergo radioactive decay by alpha, beta, or gamma emission. In particular, radium-226 and its decay products (e.g., bismuth-214 and lead-214) present alpha and gamma radiation hazards to uranium miners and processors. Radiation exposures to the general population resulting from off-site releases of radionuclides (e.g., airborne radon decay products, airborne thorium-230 (230Th) or radium-226 (226Ra) particles, 226Ra in water supplies) present some risk...

If your world had enough uranium in the crust to make radon problematic, you could make volcanoes that spew uranium-containing ash high into the air. These particulates are long lived and can lodge in the lungs, and so are potentially more dangerous than radon. Volcanic ash particles can remain airbone for long periods and travel great distances. You could assert that the mid to upper atmosphere was contaminated by such particles.

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  • $\begingroup$ This was my first thought as well. On a youngish planet that had high concentration of heavy elements, you could have plenty of radon gas in the atmosphere. $\endgroup$
    – abestrange
    Nov 28, 2019 at 0:09
  • $\begingroup$ Ooh. I like this a lot. I might use it but I'll wait until if there are any other answers first. $\endgroup$
    – ZarHakkar
    Nov 28, 2019 at 0:45
  • $\begingroup$ Radon is dangerous to us because it can be concentrated in houses and other buildings where we spend extended periods of time and over the years it raises our risk of cancer. A planet with vast (read: unrealistic) quantities of thorium and uranium might be able to generate a lot of atmospheric radon, but even then you could trivially protect yourself with a mask and be fine unless you spent months breathing the atmosphere. I don't think that's quite the "dangerously radioactive" the OP was hoping for. $\endgroup$
    – Starfish Prime
    Nov 28, 2019 at 9:16
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Natural Nuclear Reactors

A planet with high concentrations of uranium or other suitable fissile materials could contain numerous natural fission reactors like the one in Oklo, Gambon.

Natural fission reactors

Perhaps these fission reactions happen when radioactive materials bubble up in lava flows and concentrate to the point where chain reactions kick off and they create radioactive waste. Then when the volcano erupts in full, the radioactive materials are blown into the stratosphere.

So, a highly volcanic world with large percentages of concentrated fissile material welling up from underground could be in a near constant state of having radioactive dust in the high stratosphere. Also, the presence of large quantities of radioactive material in the crust and mantle could be the reason why there is so much vulcanism, as the radioactive materials would add extra heat to the planet.

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