Overview and Exploration

  • Name: Neptune – Poseidon: God of the sea.
  • Viewing: Need a telescope; Galileo saw it in 1613 and thought it was a star (whose position somehow changed...); Predicted from perturbations of Uranus' motion, and identified as a planet in 1846.
  • Orbit: Radius about 30 AU, almost circular [eccentricity 0.01, period 165 years]; sometimes [for example between 1979 and 1999] actually more distant than Pluto.
  • Planet: A gas ball 4 times the Earth's size [17 times the mass, 1.7 times the density of water], a little smaller and more massive than Uranus.
  • Exploration: Visited only by Voyager 2 (in 1989), which now is more than 90 AU from the Sun, leaving the Solar System; Some teams are planning future missions to Neptune and Triton.


  • Appearance: Bluish from methane (more than Uranus), cloud bands and spots; the strongest atmospheric circulation of all the gas giants!
  • Rotation: The atmosphere rotates once every 17.3 hrs (the interior rotates faster, but the planet is not very flattened); tilted 30° with respect to the orbit (seasons).
  • Surface features: More cloud features than Uranus, because it is heated from below, like Jupiter and Saturn; Great Dark Spot (seen by Voyager, gone by 1994).
  • [Interior: Molten rocky core, Earth-sized; The rest is a mix of water, ammonia, methane.]

  Moons and Rings

  • Moons: At least 14 (the last one was reported in 2013); The largest one is Triton (larger than Pluto), on a retrograde orbit, with active ice volcanoes, and the only one with a known atmosphere, other than Saturn's Titan; Nereid is medium-sized, the other ones are small.
  • Rings: 3 narrow ones, 1 wide one, made of microscopic dust; The outermost ring (Adams) consists of 5 short arcs, that are kept from dispersing by the nearby moon Galatea.

page by luca bombelli <bombelli at>, modified 15 jul 2013