Consider an Earth-like planet in a solar system much like ours that doesn’t have any magnetic field (or at least a very weak magnetic field) because of a lack of metals like Nickel beneath the surface. The magnetic field that currently surrounds our Earth protects us from a solar barrage of high-energy protons and electrons that the sun blows our way.

Suppose we move this unprotected planet a little bit farther than $1 \space\text{AU}$ from its sun (identical to ours) so that the extra energy from bombarding particles doesn’t evaporate all of the water and destroy the atmosphere.

What kind of life would you expect to evolve on this planet? It would almost certainly still be based on the same building blocks (carbs, lipids, amino acids, nucleic acids) because those evolve spontaneously from the chemicals present on early Earth. But what difficulties would this high-energy-particle-bombardment pose for the evolution of early life, and what kinds of adaptations would likely appear to overcome them?

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    $\begingroup$ Mars is a bit further than 1 AU and, not having a magnetic field, has said goodbye to all his water and atmosphere... $\endgroup$
    – L.Dutch
    Jul 16, 2020 at 6:28
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    $\begingroup$ Extra energy from bombarding particles would not evaporate all of the water and destroy the atmosphere. At least immediatly. This "extra" is very small. It can prevent to accumulate free oxygen thou. It was a very, very slow process. $\endgroup$
    – ksbes
    Jul 16, 2020 at 9:04
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    $\begingroup$ How would a planet not have a magnetic field? Is this reality one where iron is not at the bottom of the fusion decay chain? Are aliens going around with giant degaussers? $\endgroup$ Jul 16, 2020 at 12:26
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    $\begingroup$ @Harper-ReinstateMonica As mentioned by L.Dutch, Mars has no magnetic field. Earth has one because its core is molten, but Mars is frozen straight through. Same goes for Luna--no magnetic field. $\endgroup$ Jul 16, 2020 at 13:04
  • $\begingroup$ No magnetosphere = no Van Allen radiation belts = really, really, really bad sunburns. $\endgroup$
    – JBH
    Jul 16, 2020 at 15:26

2 Answers 2


It would likely be primarily aquatic and simple.

It turns out that both water and ice are excellent at blocking radiation. Paranicas et al. 2002 found that one meter of ice can provide an attenuation of up to six orders of magnitude for high-energy protons and electrons. The same holds for heavier particles - remember, water is decent shielding for radioactive waste from nuclear reactors. In addition to high-energy particles, water also does a good job of blocking ultraviolet light (Tedetti & Semprere 2007).

Doglioni et al. 2016 argue using the above that when Earth's magnetic field was weaker, the oceans (or, in a Snowball Earth scenario, ice) would have provided enough protection for microbes to thrive in the oceans while still photosynthesizing, as shorter-wavelength light is attenuated less than long-wavelength light. I imagine that the same would be true in your scenario, with basic lifeforms surviving mostly in the oceans.

I agree with other folks that there is the issue of the ablation of the atmosphere and oceans. I suspect this could be mitigated by 1) cooler temperatures, 2) a higher surface gravity, and 3) a parent star with minimal ultraviolet emission, all three of which would make atmospheric escape more difficult. All the same, this won't necessarily help the planet retain air or water over geologic timescales, i.e. billions of years. I would be surprised if you couldn't construct a scenario where the planet hangs on to both long enough for primitive life to form, like the microbes I mentioned above, but complex life would be quite tricky - there wouldn't be time for it to form.

  • $\begingroup$ If we strip the atmosphere away isn’t the steady temperature of the Earth on the order of -100 degrees c, both providing ice and locking liquid water away from the surface? I vaguely remember climate science lectures that went through the relevant equations but they were a whiiile back now. $\endgroup$
    – Joe Bloggs
    Jul 17, 2020 at 12:49
  • $\begingroup$ @JoeBloggs I think the effective temperature works out to around -20$^{\circ}$ Celsius, so yes, it would in general be quite colder sans atmosphere, but I'm trying to talk about the period before the atmosphere is entirely lost - beyond that, life will have some difficult hanging on. $\endgroup$
    – HDE 226868
    Jul 17, 2020 at 14:31

First of all: lack of metals would affect life much more than solar radiation. To the point when life would not be possible. There were a period on Earth, before the oxygen catastrophe, when all oceans were red-brown or dark-green (there is no single major theory), due to rust. The Ocean was full of dissolved iron and that was the base of "The Life Soup" where early life formed and developed. Without enough concentrations of iron that would not be possible. But Mars has quite a lot of metals and still has no strong magnetic field - so same can be with our Non-Magnetic-Earth.

Second of all: most radiation (75 μSv/h) comes from "cosmic rays", not from the Sun. So there is little sence to reposition a planet. Sun non-EM radiation can be fully shielded with millimeters of aluminium. Thick Earth's atmosphere would be enough to not be afraid of Sun.

For reference: "standart background radiation" is about 10-20 μSv/h. Dangerous levels begin with >40 μSv/h - for humans, who want to leave up to 80 years old more or less healthy

So what would be the consequences?

Radiation would not prevent appearnce and spread of life. Early Earth were more radiated than modern Earth due to higher geological activity. Even now newly-formed or active geological areas can have background radiation levels at open air 10 times more then normal background - up to 100 μSv/h. Which is more than cosmic radiation. I have personaly grew up in such an area (there were a lot of granite and basalt rocks and cliffs around) and can assure you - life is more than comfortable in there (of cause it has consequences for human health on long term - canser rate is double or triple of that for the "main land").

But that a short-term perspective. On long term thing are much worse.

At first our planet had no oxygen. Than some microorganisms appeared that inveted oxygen production as a weapon against others. Yes, at first oxygen was poisonous for life. And then for billions of year oxygen very slowly accumulated on Earth. But that is the problem. Since oxygen is much lighter than then-main component of air - CO2, it were accumulated mostly in high atmosphere. And without any magnetic field solar wind would just blow it away. It does so even now (mostly for hydrogen - but about that later) but at many orders slower rate.

So Non-Magnetic-Earth would most likely be unable to accumulate enough oxygen and there would be no oxygen-based life. For now we (I at least) do not know any complex anaerobic organisms. But that doesn't mean that this is impossible. It is a pure speculation field. You have some freedom of "invention" here.

This process of "blowing away" also consern such a vital substance as water. Water vapor also lighter than CO2. It dissolves to ozone and hydrogen due to radiation in upper atmosphere. And hydrogen is much more prone to be blown away than any other element (since it is the lightest). It means that water would also be slowly lost. But, unlike early oxygen, there were (and is) so much water on Earth, that that would not affect early stages of life much. Mars had water for very long period of time - and it was loosing it at times higher rate due to low gravity. With Earth's gravity it still would had lots of free water.

So Non-Magnetic-Earth would have visibly less water than modern Earth at the same age, but it still would be enough of it to support life.

About UV - that is not that large problem on planet scale. If we would somehow remove all UV protection from atmosphere now - it would not kill life on surface. It will not even kill the humanity. Biosphere would change, of cause, adapt. People and animals would become darker (to protect from UV) or lighter (to reflect it). More animals (and may be even humans) would become nocturnal. But at global geological scale there would be nothing dramatic.

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    $\begingroup$ What's the mkSv/h unit? I'm seeing mSv/year and μSv/h from sources like radiation-dosimetry.org/… and sciencedirect.com/topics/…. $\endgroup$
    – npostavs
    Jul 16, 2020 at 16:02
  • $\begingroup$ I think the large majority of the early free oxygen pretty much just instantly bonded with most anything it encountered rather than escaping out of the gravity well. $\endgroup$ Jul 16, 2020 at 18:18
  • $\begingroup$ I suspect it's a terribly confusing way to write μSv/h. If you can't write greek letters, it's preferable to just use uSv/h. $\endgroup$
    – AI0867
    Jul 17, 2020 at 8:07
  • $\begingroup$ @AI0867, thanks for comment! I've corrected that $\endgroup$
    – ksbes
    Jul 17, 2020 at 8:58
  • $\begingroup$ @MichaelRichardson, the thing is that in early atmosphere (just like in modern one) there were nothing for oxygen to bound to. And it was more than twice lighter than earely atmosphere. So if it made it's way to open "air" - it went all the way up (to the starts!) $\endgroup$
    – ksbes
    Jul 17, 2020 at 9:05

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