Question: How do you know what is in the cores of other planets?

Lidunka Vocadlo answered on 18 Jun 2015:

@Florence great question! Well one thing we do have data for is something called Moment of Inertia and that gives us the approximate distribution of mass within the planet. It tells us that for the terrestrial planets (mercury, venus, mars) the mass is all concentrated at the centre. Knowledge of the physics of self-compression, together with knowledge of the size of the planets tells us approximately what the pressure profile is throughout the planet. Together with an understanding of the cosmic abundances of all the elements, we can then match these data with materials. We also have evidence from meteorites as to what planetesimals are made of. And the answer is that all the terrestrial planets are pretty earth-like but with differences. Mercury has a much bigger core, venus is much hotter, mars has little atmosphere. Roughly the same method is used for the gas giants, although the very centres are slightly more speculative.