I wonder how much the Van Allen radiation belts are a contributor to the Fermi paradox, i.e. how much they contributed to providing a suitable place for life to originate and flourish, and how rare they are.
The belts themselves are an effect of Earth's magnetic field, which I believe is particularly strong because of flow within the Earth's liquid iron-nickel outer core. (I had long believed that the spinning of the inner core was the primary contributor but given a surface-level skim of the literature that doesn't seem to be the case; convection seems to be more of a driver.)
I think perhaps many otherwise similar planets don't have a liquid iron core, so they may not have the strong radiation belts that shield life from the solar wind. Of course I am not sure what fraction of otherwise-similar planets have liquid iron cores, but Mars for example does not seem to. It is probably a function of the size of a planet (governing the pressure distribution in the interior), the ratio of iron to other elements, the temperature field (a function of the amount of radiogenic elements in the planet and its age), and perhaps other factors. Other planets may not be hot enough to have a liquid iron core at the right pressures, or be too massive (too much pressure) at the right temperatures, etc.
The composition of a planet's atmosphere has to do with the RMS velocity of gas molecules at a given planetary temperature. When this velocity exceeds the escape velocity of the planet, that gas is lost to space.
But there is one more factor. In the absence of a magnetic field, gas molecules can dissociate from being hit with the particles from the solar wind. E.g., water can dissociate into oxygen and hydrogen, and hydrogen having a relatively high RMS velocity readily leaks out to space. The remaining oxygen is too reactive to remain and then forms carbonates in rocks and carbon dioxide in the atmosphere. This is, from what I read, the explanation for the atmospheres of both Mars and Venus, which have only a small to non-existent magnetic field.
So yes, a magnetic field seems to be essential to holding a life-friendly atmosphere.
I think that solar radiation isn't a direct danger to life, as it is quickly blocked by the surface of oceans and land. If an atmosphere turns out to be a major factor in the development of life, lack of a field could be a bigger impact. That said, atmospheric stripping like what happened to marks isnt sure bet. Venus has no internal Dynamo, but a massive atmosphere, despite 5X the solar radiation.
I've read that the relative contribution of planetary magnetic fields is overstated relative to atmospheres. The thickness of Earth's is the same mass as a 10 meter-high column of liquid water; not much radiation gets through that much shielding, magnetic field or no. (I think it's solely muons?)
I'm pretty sure that the belts are a requirement for some types of life to originate and survive. Along with Jupiter helping protect us, our location in the galaxy, etc.
If abiogenesis occurred in the thermal vents deep under the ocean I believe that could have happened without the radiation protection as the water would be more than adequate.
Sure, but small amounts of radiation are beneficial. And those early organisms would eventually ahve to move to the shallows and land and deal with all the masked radiation at some point. It's all speculation, we really have no idea whether it was vents-first or not.
> And those early organisms would eventually [have] to move to the shallows and land and deal with all the masked radiation at some point.
Do they, though? Why is land the requirement? What's keeping life from, say, evolving to live deep underground? Or in the deep ocean? Both those places are heavily shielded from radiation, and organisms there wouldn't be affected much at all by not having a magnetosphere. Extremophiles on Earth get by just fine hanging around thermal vents, for instance. (Edit: this was mentioned above and I didn't see it - sorry for repeating.)
I think part of the problem with the Fermi paradox is that our base assumptions about what life needs are possibly a bit off. Maybe the fact that we have what we have is, well, quirky, and the fact that we evolved as living creatures that crawl around on the outside of our planet and need really fussy little temperatures to survive is just plain weird in comparison to the rest of the universe.
"Life as we know it" is a lot tougher criterion to meet than "life," I suspect.
Life may be abundant. Intelligent life with technological civilizations is probably not. It took 4 billion years on earth. That’s 1/3 the age of the universe.
These are all fair questions, and to go further, life may not even have required light at all- there are chemoautotrophs living deep in the rock that never see light.
I was going to say "obviously, nothing I said above would apply to life as we don't know it, like on the surface of a neutron star".
The belts themselves are an effect of Earth's magnetic field, which I believe is particularly strong because of flow within the Earth's liquid iron-nickel outer core. (I had long believed that the spinning of the inner core was the primary contributor but given a surface-level skim of the literature that doesn't seem to be the case; convection seems to be more of a driver.)
I think perhaps many otherwise similar planets don't have a liquid iron core, so they may not have the strong radiation belts that shield life from the solar wind. Of course I am not sure what fraction of otherwise-similar planets have liquid iron cores, but Mars for example does not seem to. It is probably a function of the size of a planet (governing the pressure distribution in the interior), the ratio of iron to other elements, the temperature field (a function of the amount of radiogenic elements in the planet and its age), and perhaps other factors. Other planets may not be hot enough to have a liquid iron core at the right pressures, or be too massive (too much pressure) at the right temperatures, etc.