Neptune is the the planet that is almost the farthest from the sun. (Pluto is the farthest, but it is exceptional in its properties.) Neptune orbits the sun with Neptune has eight moons, one of them is large: Triton.

Here is a picture of Neptune.

What is its density, compared to earth (approximately)?

We get a density somewhere between rock and ice. The density suggests that Neptune was made of rock and ice, plus some gas at the top.

We also know a little more about how the mass is distributed, based on the details of Neptune's gravitational field.

This suggests a model. (See note on molecules.)

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Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA