Eli5 how do we know what the core of distant planets are made of

Question

Eli5 how do we know what the core of distant planets are made of

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Answer 1

We can guess what it’s made of by discovering its type, mass and size.

Our earth is rather massive because of the iron in its core. If it was only rock, we would have lower gravity here. For instance, Mars is half of earth’s size, but has only 11 percent the mass of the earth and as a result about 38 percent the gravity. So, you find out the size and the mass of the planet you can guess if it has large metal core like our own planet, or is more rocky.

The size is rather straight forward, as we currently find most exoplanets using the transit method – the dip in a stars brightness as the planet passes between us and the star, the size is proportional to the dip. The mass we calculate by measuring the wobble of the star in its position as the planet orbits it.