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I'm designing an iron planet which migrated into its habitable zone. Basic research told me iron planets cool off too quickly to have a magnetic field, so initially I was looking for ways native life could tolerate the resulting environment. However, I recently learned that Mercury, which I'm basing its core-mass ratio off of, actually has a weak magnetosphere, which is likely generated by an internal dynamo.

From this I have reasoned that, if Mercury had more mass and rotated much faster (i.e. the planet I'm trying to make), it might have a stronger magnetosphere that could last for the same amount of time. It would probably be weaker than Earth's, but still provide some protection to the atmosphere rather than none. However, I'm not sure how much more massive I can make a planet with this kind of interior structure before it's too rigid for convection currents to occur.

What is the maximum mass I can give my planet before an internal dynamo becomes impossible?

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    $\begingroup$ Please note than the bulk of the protection against harmful radiation coming from the Sun is provided by the atmosphere, and not by the magnetic field of Earth. The magnetic field helps a little, but not so much as to be noticed if it goes aways for a relatively short (geologically speaking) amount of time. It actually does go away for relatively short (geologically speaking) amounts of time quite often (geologically speaking), and this never had any harmful effects on living things. The importance of the magnetic field lays in protecting the atmosphere itself over looong amounts of time. $\endgroup$
    – AlexP
    Commented Nov 22, 2023 at 21:49
  • $\begingroup$ Ah, right! I should rephrase my question a bit, then. $\endgroup$
    – Thoth
    Commented Nov 22, 2023 at 23:43
  • $\begingroup$ By the way, I'm aware that a planet can have an atmospherically induced magnetic field like Venus. However, I wanted my planet to be relatively cold and have a few light gases like O2 as part of its atmospheric makeup, so I'd rather not go with that option. $\endgroup$
    – Thoth
    Commented Nov 23, 2023 at 5:12

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Earth is a planet with an iron core, along with other elements of course. It isn't really possible to get a planet that is pure iron, or pure anything else for that matter.

So, we then get to the issue of maximum mass. It is thought that the earth's core is so hot because of the insulating effect of the crust, along with residual heat, friction effects and radioactive decay, so the larger a planet is, the more likely it is to have a molten core. It's the small planets that are more likely to have cooled sufficiently to solidify. Another factor that is thought to assist with keeping the planet's core molten is having a large moon, where the tidal stresses contribute to heating the world, especially if the world's rotation is not tidally locked to the moon.

However, if you make a planet large enough, it will begin to accumulate hydrogen and helium, and with a sufficient mass of hydrogem, the hydrogen may begin to fuse, and then you'll have something from a gas-giant to a brown dwarf or even a star, in which a molten iron core ranges from irrelevant to impossible.

However, Jupiter has a particularly strong magnetic field, and stars have magnetic fields too, so it's more the case of "How big does a body need to be to begin to have a magnetic field?" than "What's the largest size that a body with a magnetic field can have?".

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    $\begingroup$ I'm going off of a looser definition of an "iron planet" rather than one made entirely out of pure iron. Like I said, its composition is based off of Mercury's, being roughly 70% metals (iron, nickel, etc) and 30% silicate rock. Also, I'm not sure if finding the minimum size will be very helpful as I'm trying to give my planet a relatively strong magnetosphere, at minimum stronger than Mercury's. Whatever said minimum is will probably be too weak for my interests. $\endgroup$
    – Thoth
    Commented Nov 23, 2023 at 5:04
  • $\begingroup$ @Thoth In effect, once you get a big enough planet, especially one with a sizeable moon, the only upper limit to size is when it turns into a gas giant or a star rather than being the rocky, tectonically-active planet you're after. $\endgroup$
    – Monty Wild ♦
    Commented Nov 23, 2023 at 5:12
  • $\begingroup$ I take it I shouldn't worry about a "maximum mass" and that the Wikipedia entry for Iron Planets (and the source it cited, which I can't seem to access) was wrong about them not having a magnetic field? $\endgroup$
    – Thoth
    Commented Nov 23, 2023 at 5:18
  • $\begingroup$ @Thoth If you have a good look at all the astronomical objects in the universe, the bigger they are, the more magnetic they tend to be. It's the smaller planets and moons that tend to be solid all the way through and non-magnetic. $\endgroup$
    – Monty Wild ♦
    Commented Nov 23, 2023 at 5:21
  • $\begingroup$ @Thoth, en.wikipedia.org/wiki/Iron_planet saying that they cool quickly is for small planets. The bigger they are, the slower they cool. $\endgroup$
    – Monty Wild ♦
    Commented Nov 23, 2023 at 6:08

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