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Because it is impossible to penetrate to the centers of planets, scientists must probe with experiments and simulations. Three techniques have provided insights: diamond-anvil cells, shock-wave experiments and molecular simulations. Each tries to create tiny worlds in which matter is compressed to high pressures where extreme transformations take place. To date, these methods have shown that Neptune must be filled with methane that dissociates to form hydrocarbon chains as pressure and temperature increase with depth, and finally there are likely to be diamonds in the planet's core. Experiments have shown how hydrogen can become a metallic fluid at the extreme pressures and temperatures that may characterize most of Jupiter's interior. Finally, it appears that Earth's core may be almost twice as hot as we thought—as hot as the surface of the sun.
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