Journey Through the Solar System: Planet Neptune

Neptune
The Voyager 2 spacecraft camera captured this view of Neptune in 1989. The dark spot in the center was the Great Dark Spot, which has since disappeared. Storms and other features come and go on the planet. NASA/JPL

The distant planet Neptune marks the beginning of our solar system's frontier. Beyond this gas/ice giant's orbit lies the realm of the Kuiper Belt, where places such as Pluto and Haumea orbit. Neptune was the last major planet discovered, and also the most distant gas giant to be explored by spacecraft. 

 

Neptune from Earth

Neptune and chart
Neptune is incredibly dim and small, too difficult to spot with the naked eye. This sample star chart shows how Neptune would appear through a telescope. Carolyn Collins Petersen

Like Uranus, Neptune is very dim and its distance makes it very difficult to spot with the naked eye. Modern-day astronomers can spot Neptune using a reasonably good backyard telescope and a chart showing them where it is. Any good desktop planetarium or digital app can point the way. 

Astronomers had actually spotted it through telescopes as early as Galileo's time ​but didn't realize what it was. But, because it moves so slowly in its orbit, no one detected its motion right away and thus it was probably thought to be a star. 

In the 1800s, people did notice that something was affecting the orbits of other planets. Various astronomers worked out the mathematics and suggested that a planet WAS further out from Uranus. So, it became the first mathematically predicted planet. Finally, in 1846, astronomer Johann Gottfried Galle discovered it using an observatory telescope.

 

Neptune by the Numbers

Neptune and Earth
A NASA graphic showing how large Neptune is compared to Earth. NASA

Neptune has the longest year of the gas/ice giant planets. That's due to its great distance from the Sun: 4.5 billion kilometers (on average). It takes 165 Earth years to make one trip around the Sun. Observers tracking this planet will notice that it seems to stay in the same constellation for years at a time. Neptune's orbit is quite elliptical, and sometimes takes it outside the orbit of Pluto!

This planet is very large; it measures more than 155,000 kilometers around at its equator. It's more than 17 times the mass of Earth and it could hold the equivalent of 57 Earth masses inside itself. 

As with the other gas giants, the massive atmosphere of Neptune is mostly gas with icy particles. At the top of the atmosphere, there's mostly hydrogen with a mixture of helium and a very small amount of methane.Temperatures range from quite chilly (below zero) to an incredibly warm 750 K in some of the upper layers.

Neptune from the Outside

Dark spots on Neptune
Neptune's upper atmosphere hosts constantly changing clouds and other features. This shows the atmosphere in visible light and with a blue filter to bring out details. NASA/ESA STSCI

Neptune is an incredibly lovely blue color. That's largely because of the tiny bit of methane in the atmosphere. The methane is what helps give Neptune its intense blue color. The molecules of this gas absorb red light, but let blue light pass through, and that's what observers notice first. Neptune has also been dubbed an "ice giant" due to the many frozen aerosols (icy particles) in its atmosphere and slushy mixes deeper inside.

The planet's upper atmosphere is host to an ever-changing array of clouds and other atmospheric disturbances. In 1989, the Voyager 2 mission flew by and gave scientists their first close-up look at the storms of Neptune. At the time, there were several of them, plus bands of high thin clouds. Those weather patterns come and go, much as similar patterns do on Earth. 

Neptune from the Inside

Neptune interior
This NASA cutaway of the interior of Neptune shows (1) the outer atmosphere where the clouds exist, (2) the lower atmosphere of hydrogen, helium, and methane; (3) the mantle, which is a mixture of water, ammonia, and methane, and (4) the rocky core. NASA/JPL

Not surprisingly, Neptune's interior structure is a lot like Uranus's. Things get interesting inside the mantle, where the mixture of water, ammonia, and methane is surprisingly warm and energetic. Some planetary scientists have suggested that at the lower part of the mantle, the pressure and temperature are so high that they force the creation of diamond crystals. If they exist, they would be raining down like hailstones. Of course, no one can actually get inside the planet to see this, but if they could, it would be a fascinating vision.  

Neptune Has Rings and Moons

Neptune's rings, as seen by Voyager 2. NASA/LPI

Although the rings of Neptune are thin and made of darkened ice particles and dust, they are not a recent discovery. The most substantial of the rings were detected in 1968 as starlight shone through the ring system and blocked some of the light. The Voyager 2 mission was the first to get good close-up images of the system. It found five main ring regions, some partially broken into "arcs" where ring material is thicker than in other places.

Neptune's moons are scattered among the rings or out in distant orbits. There are 14 known so far, most of the small and irregularly shaped. Many were discovered as the Voyager spacecraft swept past, although the largest one—Triton—can be seen from Earth through a good telescope. 

Neptune's Largest Moon: A Visit to Triton

Neptune's moon Triton
This Voyager 2 image shows the weird cantaloupe terrain of Triton, plus dark "smears" that are caused by plumes of nitrogen and dust from underneath the icy surface. NASA

Triton is quite an interesting place. First, it orbits Neptune in the opposite direction in a very elongated orbit. That indicates that it is likely a captured world, held in place by Neptune's gravity after forming somewhere else.

This moon's surface has weird-looking icy terrains. Some areas look like the skin of a cantaloupe and are mostly water ice. There are several ideas about why those regions exist, mostly having to do with motions inside Triton. 

Voyager 2 also caught sight of some strange smudges on the surface. They're made when nitrogen vents out from underneath the ice and leaves behind dust deposits. 

Exploration of Neptune

Voyager and Neptune
An artist's conception of Voyager 2 passing by Neptune in August, 1989. NASA/JPL

Neptune's distance makes it tough to study the planet from Earth, although modern telescopes are now fitted with specialized instruments to study it. Astronomers watch for changes in the atmosphere, particularly the comings and goings of clouds. In particular, the Hubble Space Telescope continues to focus its view to chart changes in the upper atmosphere. 

The only close-up studies of the planet were made by the Voyager 2 spacecraft. It swept past in late August ​1989 and returned images and data about the planet.