Neptune is the farthest planet in the Solar System. It is an ice giant almost identical to Uranus in mass, size and composition. It appears as a tiny blue dot in the telescopes at Henize. It has been visited by only one spacecraft as Voyager 2 passed by in 1989.

The discovery of Neptune was a triumph of science. It was determined that the orbit of Uranus was not exactly as expected. Sometimes Uranus was ahead in its orbit, sometimes it lagged. Calculations by the French mathematician Urbain Le Verrier predicted the position of the mystery planet. Those calculations were sent to the Berlin Observatory and on the evening of Sep 23, 1846, Johanne Galle found Neptune within an hour. The planet is named for the Roman god of the sea./

In 2011, Henize celebrated the one year anniversary of the discovery of Neptune. One Neptunian year, of course. Could you imagine a school year on Neptune?

Unlike its twin, Uranus, Neptune has active and visible weather patterns. The Great Dark Spot on Neptune is driven by winds of up to 2,100 km/h. Due to its distance from the Sun, the cloud tops of Neptune are only -218° C. Methane in the upper atmosphere gives Neptune its blue color.

he gravity of Neptune affects the objects in the Kuiper Belt beyond Neptune. The dwarf planet Pluto, for example, is locked into a 2:3 orbital resonance with Neptune, so even though Pluto crosses Neptune's obit, they can never collide.

We continue to discover objects in the Kuiper Belt. Plutoids such as Eris, Sedna, and Makemake are all leftovers from the formation of our Solar System when it formed from a cloud of gas and dust 4.5 billion years ago.

  • Neptune
  • Neptune Symbol
Orbit Specifications
Semi-major axis 4.46 x 109 km
30.07 AU
Eccentricity 0.087
Orbital Period 164.79 yr
Sidereal rotation 16.11 h
Physical Characteristics Specifications
Mean radius 24,622 km
3.883 Earths
Mass 1.0243 x 1026 kg
17.147 Earths
Mean density 1.638 g/cm3
Surface gravity 11.15 m/s2
1.14 g
Axial tilt 28.32°