What thickness of atmosphere, and strength of magnetosphere, would I need for a planet to have a deadly daytime and a hospitable nighttime?

I'm wanting a planet in which humans can survive on as long as they come outside at night. The Sun's UV light is lethal, and so no human can come out during the day; but the night is non-lethal.

The Sun in question is a main sequence G-type star, about 4.6/4.7 billion years old; and the orbit is close enough to an AU to say, essentially, an identical orbit to Earth. Not the actual solar system, but very similar.

What would have to happen to magentosphere, ozone layer, air density and composition, etc?

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    $\begingroup$ Just to clarify - you want a breathable atmosphere when the humans (and other animals?) come out at night? Because if you do, there has to be an ecosystem of plants that can survive exposure to the radiation (of whatever type) to keep renewing the oxygen levels. $\endgroup$ Commented Apr 14, 2019 at 2:08
  • $\begingroup$ @KerrAvon2055 A breathable atmosphere for humans, yes. The humans are non-native to the planet; the native life has evolved to be fine in the planet's conditions, but the atmosphere is breathable for humans. $\endgroup$
    – Ushumgallu
    Commented Apr 14, 2019 at 2:26
  • $\begingroup$ How/which kind of deadly are we talking about? For example, one could argue Sahara desert has strong solar radiation and uninhabitable during daytime. $\endgroup$
    – Chenxi GE
    Commented Apr 19, 2019 at 21:17

3 Answers 3


Mars: no magnetosphere required

Just to clarify concepts, the Earth's magnetosphere does not prevent sunrays from being "lethally radioactive" during daytime, but it does protect the whole atmosphere from being eroded, including the ozone in the stratosphere.

Ozone prevents UV rays from reaching the surface and provoking major mutations on DNA sequences. Such mutations do not kill us instantaneously but give us cancer and prevent DNA to be copied adequately to the offspring, so all multicellular life (with long DNA sequences) becomes unviable in few generations.

What you are looking for is a mix between the Earth and Mars. It seems that Mars was a hospitable place some billion years ago, with a warm atmosphere that could hold liquid water and microbial life. However, for several billion years, the solar wind eroded the martian atmosphere so Mars is now a desert. During this process, there must have been an era in which the conditions for life started to disappear slowly.

Your planet could be situated in a similar era, when ozone in the atmosphere is being depleted at a dangerous rate so it is unsafe to expose oneself to sunlight.

PD. I forgot to say that Mars --the same as Venus or the Moon-- does not have a true magnetosphere.

The nature of the interaction between an unmagnetized planet and the supersonic solar wind is determined principally by the electrical conductivity of the body. If conducting paths exist across the planet’s interior or ionosphere, then electric currents flow through the body and into the solar wind where they create forces that slow and divert the incident flow. The diverted solar wind flows around a region that is similar to a planetary magnetosphere (p. 523 in the link).

  • $\begingroup$ So why did the solar wind strip Mars of its atmosphere but not Venus? $\endgroup$ Commented Apr 20, 2019 at 1:52
  • $\begingroup$ @eyeballfrog Venus is much more massive and can retain its atmosphere better due to its steeper gravity well. Read throug this en.m.wikipedia.org/wiki/Atmospheric_escape for a comprehensive answer. $\endgroup$ Commented Apr 20, 2019 at 15:57

It could be the magnet property’s of the planet are weaker and the ozone is not existent, otherwise similar atmosphere to breath, it just allows dangerous levels of radiation through that plants have adapted to

  • $\begingroup$ But, specifically, how much weaker? What's the right balance? $\endgroup$
    – Ushumgallu
    Commented Apr 14, 2019 at 18:50

For radioactivity (gamma rays) you need a neutron star. A normal star will not produce such high energy radiation.

A closer thing you can get with a 'normal' star is more neutrons colliding with molecules in the atmosphere and producing radioactive isotopes (similar to our Carbon 14, but that is not too radioactive due to a long half life).

Tritium is something which can be produced in similar ways. And an oxygen-rich atmosphere can capture the tritium atoms and rain them down as water. A sun which swallowed a lot of fissionable material from a nearby supernova could produce the needed neutrons. However, half life is 12 days, so it would be radioactive at night as well - except maybe in some places where the water rains down during the day but not at night.

Best would probably be to go without radioactivity and just with uv rays (radiation). That is quite easy: Just make the air thinner and the weather less humid (no oceans and no clouds during the day). A more extrem version of our deserts. Plants may survive off ground water and nightly rainfall. But those are quite harmless for humans: Clothing and sun blocker are enough to protect us from even very extreme levels.

Or we go with simple heat: Make the planet close enough to the sun, dry enough, and the atmosphere thin enough that daylight temperatures are above 100 Celsius. Add some wind for the heating effect. Plants adapted by storing their water underground. Humans could only survive this for more than a few minutes without heavy temperature controlled clothing - basically like a diving suit with an air conditioning system. Rain in the evening cools things down very quickly.

  • $\begingroup$ Yeah, I meant uv rays but worded it bad. But I want to know like, just how thin, and how dry? how much of the ozone should be gone? stuff like that $\endgroup$
    – Ushumgallu
    Commented Apr 20, 2019 at 3:15
  • $\begingroup$ "For radioactivity (gamma rays) you need a neutron star. A normal star will not produce such high energy radiation." You seem to confuse the concepts of decay and emission in the electromagnetic spectrum. $\endgroup$ Commented Apr 20, 2019 at 16:01
  • $\begingroup$ @Ushumgallu: The ozone layer is overhyped. It has very little to do with radiation levels. Remove all the ozone completely and we'll just evolve a shade darker. As to radiation otherwise: The Sahara desert, as someone else mentioned, is already quite deadly and leaving someone there without preparation is likely to kill them in a day or two. Make the planet close enough to the sun or adding more atmosphere so that the heat is twenty degrees higher during the day will be enough to make it unsurvivable without cooling equipment or cover. $\endgroup$ Commented Apr 20, 2019 at 20:45
  • $\begingroup$ @TheDyingOfLight: Gamma rays are a product of nuclear processes. Not as energetic as beta radiation (electrons) or alpha radiation (protons), but still part of the concept of radioactivity - and electromagnetism. $\endgroup$ Commented Apr 20, 2019 at 20:46

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