How can we tell what the core of a planet millions of light years away is made of, yet we’re just now finding a 4.8 quintillion pound mass of metal under the surface of our own moon?

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How can we tell what the core of a planet millions of light years away is made of, yet we’re just now finding a 4.8 quintillion pound mass of metal under the surface of our own moon?

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Well, I mean you can use binoculars to determine the exact gender and breed of bird eating at your bird feeder in your back yard but still not find your car keys in your own house. One has nothing to do with the other.

Part of how we guess the core of far away planets is the lens effect they have on electromagnetic waves.

The most common of these is light and we can see how gravity bends light as the planet passes stars that are further behind it from us.
The more mass of the planet, the more it bends light. With this we can start to work out the density of the planet (how much gravity it exerts) and so if it is likely made up of hydrogen gas (not very dense/heavy) or of something denser like iron or nickel.

This is only a guess – we know the average density of these stars, not that they have a large deposit of X at a certain Spot.

The moon being closer, we can look at where deposits might be from taking much more accurate measurements of gravity across the surface. We’ve known for a long time that moon was of a certain average density and with more certainty than far away planets.

We cannot tell anything about cores of planets million LY away. Most known exoplanets are within hundreds of LY, and we know little about them, only the orbital parameters.

Stars are a whole different matter. We really can tell a lot of things about the innards of stars, no matter how far away they are. This is because stars emit spectra depending on their elemental compositions.

We can’t tell that about distant planets – we can make some decent guesses, but that’s about it.