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I have a hypothetical world in the works and I have a rough idea of what it might be like.

My idea is that the planet was life-supporting with a widely varied biosphere. The magnetosphere failed and let particles in the incoming solar wind into the biosphere.

My question is: How might a planet quickly (relatively speaking) lose its magnetosphere? Would said loss of the magnetosphere let high amounts of radiation reach the surface? What other adverse effects might this have on my planet?

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    $\begingroup$ I am not sure I get your problem. Earth core is cooling down for 4.5 billion years, yet we are alive and well (sort of). $\endgroup$ – L.Dutch Apr 30 at 12:47
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    $\begingroup$ Is ‘radiation, like normal planets do’ an acceptable answer or are you looking for something a bit faster? $\endgroup$ – Joe Bloggs Apr 30 at 12:47
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    $\begingroup$ How fast do you need it to cool? Because every planet terrestrial planet cools and solidifies eventually due to the extinction of radioactive decay followed by black body radiation. $\endgroup$ – Ash Apr 30 at 12:51
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    $\begingroup$ The magnetic field of a planet does not do much to stop "radiation" from hitting the surface. That's mainly the job of the atmosphere. $\endgroup$ – AlexP Apr 30 at 12:55
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    $\begingroup$ @Ash: Very quickly as in many millions of years... The word "quick" has quite different meanings in geology and history. $\endgroup$ – AlexP Apr 30 at 13:00
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The mantle eventually solidified, causing the magnetic field to fail and let in radiation from the star it orbits.

Eventually is a very, very long time scale for this to happen naturally. A planet is more likely to suffer a catastrophic asteroid impact on these times scales (themselves very rare events) that would wipe out almost all life. Political developments (i.e. war) and improper environmental management (e.g. climate change denial) could wipe out a civilization far faster - hundreds of years perhaps. On the other hand the very slow loss of the magnetic field is something that could be adapted to and ultimately a technological civilization able to survive long enough to worry about this problem would possibly be able to fix the problem by generating some kind of EMF shield itself. You're looking at a K1 or K2 civilization on these time scales. Worst case scenario - you go underground and live in large atmospherically sealed environments. Or if you have the tech (K1 or K2) you could just forget the messy planet thing and live is large space stations. Planets are for young civilizations, not mature ones.

Incidentally we have few example planets to go on for life (one) and few examples of non-life bearing (as far as we know) planets, so it's by no means a foregone conclusion that a planet without a magnetic field or even an atmosphere will not support life. Life does not necessarily require a surface environment to evolve on or continue existing on. For example it is conjectured that life might exist below the ice of Europa in a liquid ocean.

How might the inner parts of a terrestrial planet cool and solidify without initially killings off life?

Exactly as they do now.

As AlexP points out the time between the magnetic field stopping and their being significant impact to life would be on the order of millions of years at least. So "initially" - no problem.

The loss of the Ozone layer does not prevent life from existing. For us it would be catastrophic (at the rate we're screwing up I think "will be" is closer to the truth) because we lack (and will do on the relevant timescale) the technology to adapt quickly enough on a mass scale to drastic climate changes - that's a different issue from yours.

This might be different if the planet had an extremely strong magnetic field (which brings it's own problems) and was close to a star with an extremely strong particle wind. But this combination presents a number of problems for making a life sustaining planet in the first place. It seems unlikely.

The core supplies almost no power to the surface compared to the Sun so the climate would be unaffected in any realistic scenario.

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    $\begingroup$ @StepheneG We aren't confident in how the magnetic field is being generated let alone how much change would be required to lose it permanently. I think it would be plausible to assume that the core would not need to completely solidify in order to loose the magnetic field without which no life can survive, at least, most life would not survive. $\endgroup$ – Rob Apr 30 at 14:22
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A planet orbiting a Population I or II star would necessarily be light on metals and radioactives, compared to a Population III star like Sol. This makes them lower density and gives them very little in the way of heat reserves, they cool very quickly, geologically speaking and become geological dead. The problem is that such a world generally doesn't have much of a magnetosphere to begin with.

An alternative possibility would be a very salty Water World where a strong magnetic field generated by ocean circulation collapses due to a demineralisation of the oceans. The thing about this is that the evolution of life is the most probable cause, simple organisms change the chemistry of the ocean turning existing soluble minerals from the ocean into insoluble compounds that settle out of the water column and build up on the seabed. This process also generally involves out-gassing of lighter elements like Chlorine to reduce the solubility of elements like Calcium in the ocean.

The collapse of the magnetosphere on any world is the beginning of the end for the atmosphere and also for surface liquid water, this starts at the top of the atmosphere with the lightest gases, even with a powerful magnetosphere Earth loses Hydrogen and Helium to the solar winds Venus loses gases as heavy as Oxygen. The Ozone Layer will be the first victim, ionising radiation will energise and break up the unstable Ozone, then unfiltered UV will start to sterilise the planet. The stellar winds will continue to strip the atmosphere through a combination of particle collisions, magnetic disruption and ionisation reactions. Stripping off the whole atmosphere and then all the water on any world will take geological ages, plural, but unless something magically restarts the magnetosphere it's only going to end one way.

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1. Nonconductive core. Your planet is nonconductive - maybe alkanes all the way down. If you have minimal conduction it will be easier for your magnetosphere to disappear. No conduction means no resistive heating.

2. No radionuclides. This goes with #1 - your planet is a lightweight. It has no radioactive decay and so no inner heating.

3. Evaporative cooling. In with the alkanes composing your planets innards is a lot of hydrogen. No helium because of #2 but lots of hydrogen. This light molecule makes its way to the surface and leaves the planet into space, carrying away its heat.

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The strength of the Earth's magnetic field temporarily drops to near zero during a reversal of the magnetic poles. Unless you need the loss to be permanent, then the loss can be perfectly natural and normal on an Earth-like planet.

Reversal occurrences are statistically random, with some periods lasting as little as 200 years. There have been 183 reversals over the last 83 million years. The latest, the Brunhes–Matuyama reversal, occurred 780,000 years ago, and may have happened very quickly, within a human lifetime.

Wikipedia

On the reduction of field strength during one such occurrence:

What is remarkable is the speed of the reversal: "The field geometry of reversed polarity, with field lines pointing into the opposite direction when compared to today's configuration, lasted for only about 440 years, and it was associated with a field strength that was only one quarter of today's field," explains Norbert Nowaczyk. "The actual polarity changes lasted only 250 years. In terms of geological time scales, that is very fast." During this period, the field was even weaker, with only 5% of today's field strength. As a consequence, Earth nearly completely lost its protection shield against hard cosmic rays, leading to a significantly increased radiation exposure.

Ice age polarity reversal was global event: Extremely brief reversal of geomagnetic field, climate variability, and super volcano

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