Eli5 how do we know what the core of distant planets are made of
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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.
We kind of don’t. We can determine the combination of elements by comparing a planet’s mass to their radius. But that’s like knowing someone’s fitness level by their BMI.
Science can infer a lot with spectral analysis and our understanding of elemental properties at specific temperatures and pressures. We can also determine how active or inactive their cores are (and even how long ago they cooled!) But we don’t know everything. Case in point: gas giants.
As I’m sure you’re aware, Jupiter isn’t just a giant ball of storms and clouds. Somewhere in the depths there’s something… solid. What exactly lies at the “surface” of Jupiter? We don’t know. What lies at the core of Jupiter? Even more of a mystery.
We can’t see that far down, and the properties of Jupiter’s atmosphere are difficult to recreate. Suffice to say, there’s likely to be all kinds of exotic matter and interactions going on. Like metal hydrogen. If you can imagine what that’s like. I can’t. Sounds weird… and explody.
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