Skip to main content
Physics LibreTexts

7.32 Neptune

Neptune is so far away from the Earth that not much was known about it until Voyager 2 visited the planet in 1989. A second consequence of its distance from the Sun is a long orbital period, in accordance with Kepler's Laws. Neptune takes 165 of our years to complete one Neptunian year, meaning humans have not yet witnessed one complete orbital cycle since its discovery. Although it is the smallest of the gas giant planets, it still dwarfs the Earth, with 60 times the volume. The blue color whereby it derives its name is due to methane in the atmosphere, which absorbs longer wavelengths of light. 

The average distance between Neptune and the Sun is 4.50 billion km (about 30.1 AU), and it completes an orbit on average every 164.79 years, subject to a variability of around ?0.1 years. Click here for original source URL


Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Click here for original source URL.

Neptune's largest satellite, Triton, was discovered just one month after the discovery of the planet itself. Neptune's rings, however, were not detected until 1981. Scientists were looking at a star as it passed behind the planet, hoping it would give them some information about the atmosphere. Surprisingly, the star's light blinked just before and after Neptune blocked it, revealing what appeared to be partial rings, or ring arcs. 

Before Voyager reached Neptune, that was about all we knew about the farthest gas giant planet. Voyager's journey brought it closer to Neptune than any other planet since leaving the Earth. It was also the spacecraft's last planetary encounter before leaving the Solar System forever.

The picture shows the?Great Dark Spot?and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible. Click here for original source URL.

Voyager discovered Neptune's odd magnetic field, which is tilted and offset even more than Uranus's. The magnetic field was used to measure Neptune's rotation rate accurately, something nearly impossible to do from Earth. This is because the only visible features — clouds — move faster and slower at different latitudes and heights, and do not represent the rotation of the entire planet. Voyager also found that Neptune's rings do go all the way around the planet, but they are clumpy, giving the appearance of partial rings. Six new satellites were also found, most of them small and dark, with irregular shapes.

The Great Dark Spot, as imaged by Voyager 2. Click here for original source URL.

Although Neptune gets only 3% of the sunlight that Jupiter receives, it still has extremely energetic weather systems. Voyager returned images of the Great Dark Spot, a storm system similar to Jupiter's Great Red Spot. Neptune also has other storm systems; dark and bright spots that zip quickly around the planet; bright clouds high in its atmosphere, like cirrus clouds on Earth; and the fastest winds in the solar system — up to 2,000 km/hr (1,300 mph)! Recently, the Hubble Space Telescope has detected seasonal changes in the brightness and the amount of clouds on Neptune. Neptune's weather conditions also change rapidly. By 1994, the Great Dark Spot had disappeared, as opposed to Jupiter's Great Red Spot, which has persisted over centuries of observation. Compare all this activity to Uranus, which appears almost featureless. Somehow, Neptune sustains severe weather with just a tiny fraction of the Sun's energy that the other planets receive. Scientists are still puzzled by this mystery.

Neptune and Uranus are an interesting case in comparative planetology. The two gas giants are almost twins: they are nearly the same size, color, and composition, they have similarly oriented magnetic fields, and they are at the same temperature (although Neptune is almost half again as far from the Sun as Uranus). The few differences that exist between the two planets raise some perplexing issues. Uranus's unusually tilted rotation axis is thought to be due to a large impact in its history. It would not be impossible for Neptune to escape such a collision. But the reason for Neptune's singularly fierce weather, and Uranus's lack thereof, is still unknown.