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Comment by duxup | Hacker News Reader

Earth's magnetic field comes from a molten core / currents from it right? (someone correct me if I'm wrong, this isn't my area of expertise)

Is an exoplanet that far out likely to have a molten core / maintain a magnetic field in some other way?

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It's very close to its star, the comparison to Neptune is based on size, not its orbit.

DOH my misunderstanding.

That leads me to the following question: is earth the onnly planet in our system which has a molten core or do they all have molten cores?

I think all the rocky planets (Mercury, Mars, Venus) probably have molten cores, though it's pretty uncertain. The best evidence is for Mars.

https://newatlas.com/mars-gravity-map/42430/

Mercury probably has one from insolation and tidal forces from the sun. Venus might still have one thanks to its thermal insulation blanket. Earth and Moon flex each other with tidal forces, and Earth also has a much thinner insulation blanket. Mars might still have one, but its moons don't have much tidal influence, and its blanket is even thinner than Earth's.

So if Mars still has one, chances are good that all the inner planets also have one. They would all be warmed some by radioactive decay, too.

Io (vulcanism) and Ganymede (magnetic field) probably have layers of molten rock/metal in their cores, from tidal forces with Jupiter. Icy moons Europa and Callisto probably have a liquid water layer for the same reason.

I may be wrong, but insolation probably isn't a substantial factor in keeping cores molten because of how small the energy input is. Venus does have extremely active volcanoes. There's suspicion that it might have a stronger magnetic field if it were spinning faster (it's rotational period is currently longer than its year).

Earth's inner core is solid according to Wikipedia

The metal part is in fact partially liquid and partially solid depending on how deep in you are. The increasing pressure makes the liquid go to solid state.

It is the rotation of this solid part inside of the liquid part that generates magnetic field

What keeps it rotating?

Inertia, presumably? What keeps the rest of the Earth rotating?

How does pressure affect melting points? I thought the core was like ~6000 C and the melting point of iron is like ~1538 C.

My brain is seg-faulting trying to figure out why pressure would make something need to be hotter to melt. I know melted things generally take more space than solid things (except ice, right?). But why does that have to be? Doesn't pressure affect volume already?

https://en.m.wikipedia.org/wiki/Phase_diagram

This YouTube video explains a bit more, and also 'how we know all this': https://youtu.be/muWrmfXpivY

It is in French (sorry) but it is subtitled and his channel has many very clear explanations about a diverse range of STEM subjects.

Estimates I've seen are that Earth's core temps are attributably roughly 50-50 to latent gravitational heat of formation, and to radioactive decay of heavy elements (incidentally producing virtually all helium found on the planet).

Any sufficiently large rocky planet or gas giant is likely to have a similarly molten core, the latter likely within a metallic hydrogen core.

(Highly speculative, see e.g., https://www.universetoday.com/47966/jupiters-core/)

Asteroids and ice planetoids probably not: insufficient initial heat, radiative heat losses.

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