Discovery by Prediction

In the early 19th century, astronomers observed slight irregularities in the orbit of Uranus. Using Kepler’s equations, they hypothesized the existence of an eighth planet beyond Uranus’ orbit. In 1846, Neptune was discovered less than a degree away from its expected position, confirming the hypothesis. Today, scientists continue to carefully examine the orbits of planets and comets in the solar system in search of any other hidden planets.

The Smaller, Stormier Ice Giant

Neptune is an ice giant, slightly smaller in size but larger in mass than Uranus. Unlike Uranus’ solid blue-green, Neptune’s royal blue is occasionally broken up by clouds. In 1989, the camera system on Voyager 2 discovered a large storm called “The Great Dark Spot.” Unlike Jupiter’s centuries-old Great Red Spot, Neptune’s storm faded away within a few years, but was followed shortly by another large dark storm. Since then, there have been many disappearing/reappearing acts by similar storms around Neptune.

The Moons of Neptune

While most of Neptune’s moons orbit in the same direction as the planet's rotation, the largest moon, Triton, orbits in the opposite direction! When Voyager 2 captured images of Triton in 1989, it revealed a fascinating icy world with geysers even larger than those found at Yellowstone National Park erupting black material from the surface. Interestingly, Triton’s composition and orbit suggest that it was not formed as a moon of Neptune, but rather was captured from the outer solar system.

University of Arizona Legacy

The founder of the Lunar and Planetary Laboratory, Gerard P. Kuiper, discovered Neptune’s moon Nereid in 1949 and another group of scientists at LPL identified the first signatures of the moon, Larissa. The camera system team on the Voyager 2 mission was led by LPL Professor Bradford Smith.


MASS: 1.0x10^26 kg 17 times Earth
DIAMETER: 49,244 km 4 times Earth
SURFACE GRAVITY: 11.2 m/s2 14% greater than Earth
AVG. DISTANCE TO SUN: 4.5 billion miles 30 au

The largest of Neptune's 14 known moons, Titan has a criss crossing surface terrain that is thought to overlie a thick crust of water ice and a subsurface ocean.