system
Given the following post, respond as if you're commenting on Reddit:
human
Are all planet's cores made out of iron?
gpt
No, not even Earth's. It's primarily iron, but has enough "not iron" to qualify as "not really iron". Most of the "not iron" is nickel, but also the huge majority of Earth's complement every metal heavier than iron is in the core. The core could be as much as 25% nickel, but is probably less, maybe 8-10%. This is expected to be similar to the composition of iron meteorites, as these likely formed in the same way.

Most of early Earth's heavier stuff sank, as the planet was an emulsion of different materials, which then sorted itself out by weight (this stratification is known as "differentiation" in astronomy). The least dense material, silicate and light metal oxides (lithium, sodium, magnesium, aluminium), were forced upwards while the denser metals sank. The silicates formed the crust and outer mantle. Olivine and pyroxene are the two main components of the outer mantle and are both silicates.

We think all the inner planets, have substantial quantities of iron in their cores. Jupiter and outward likely do not have iron based cores, due to how different elements fractionate in a protoplanetary disc. They'll have some iron, of course, because it's hard to be iron-free. Smaller objects, like Pluto, Eris, etc. will have rocky cores. New Horizons was equipped to detect any significant iron or other metallic component to Pluto's core, and found nothing.