system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know things like what the inside of the earth is made up of, or what a planet like Neptune is made up of, if we haven't been able to get there and find out for sure?
gpt
Well, in the case of Earth, we have a lot of sources of info:

We have extensive exposure of upper mantle in ophiolitic complexes; so there's that.

Then there are several exotic magmatic intrusions such as lamprophyres and kimberlites which are derived from deep structural levels. These at times carry chunks and crystals of the stuff they cross on the way up (xenoliths and xenocrysts).

For the parts we have no samples from (lower mantle and core), we have geophysical data: the magnetic field, density, wave transmission properties through seismic data.