system
Given the following post, respond as if you're commenting on Reddit:
human
How do scientists determine what a planet core is made of ?
gpt
There are no direct methods, but there are several indirect methods that we can use. First off, it depends on the kind of planet you are studying.

Let’s start with gas giants like Jupiter. Our first step would be to look at the moons circling it, from their orbits we can get a really good guess at what the overall mass of Jupiter must be. Next, from regular observations we can estimate it’s size. This gives us a density estimate which puts us in the ballpark of what kind of materials we can expect.

Additionally, we can get measurements of its magnetic field and surface make-up from probes. Earth’s own magnetic field is created primarily because our liquid core flows and spins. As magnetic fields can arise from the movement of electrons, a good explanation for the existence of our magnetic field is a hot, liquid core undergoing convection. We can apply the same reasoning to many planets, if it has a strong magnetic field it is possible that it has a very active core and so we might initially expect things to be liquid down there.

For terrestrial planets there’s one more trick we can use: seismic activity. Since waves travel differently through solids and liquids we can guess at what kind of materials might be under the surface. This gets more useful if you have many monitoring stations in different areas to all track the same seismic event.

Finally we can begin to play a sort of detective game where we deduce elements and structure from what we know about our solar system, physical properties of the planets, and the principles of physics and chemistry. Building off all of this knowledge, one paper used Density Functional Theory, a result of quantum mechanics, to estimate the behavior of iron inside the Earth’s core. Using this analysis they concluded the temperature of the core should be within 5300K-5700K and that there is likely between 8%-13% oxygen in the outer core.

Scientists have done an amazing job using basic physical principles to extrapolate what might be happening in labs millions of miles away from their chalkboards.

Sources:

[1] Goldstein, classical mechanics ch. 3

[2] Geophysical Research Letters, Seismic anisotropy in the Earth's innermost inner core: Testing structural models against mineral physics predictions, B. Romanowicz, et al.

[3] Griffiths, introduction to electrodynamics ch. 7

[4] contemporary physics, temperature and composition of the earths core, D. Alfé, et al.