Neptune

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

• a = 30 AU
• P = 160 yr

Neptune has eight moons, one of them is large: Triton.

Here is a picture of Neptune.

• Mass is 17 times the mass of the earth.
• Radius is 3.9 times the radius of the earth.

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.)

• Cold on the outside, hot on the inside (perhaps 7000 K).
  • Neptune is warmer at the surface than it would be if it were just absorbing sunlight and reemitting it.
  • One expects that the insides of planets got heated when the planets formed and then have not cooled off yet.
  • Decay of radioactive elements helps heat the centers of planets.
• A small rocky core (fluid).
• Then perhaps 80 % of the mass in H₂O and some N H₃, broken into ions (fluid).
• Then a cool atmosphere, mostly H with some He.
• Also, N H₃, which gives Neptune its pretty blue color.

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