Since Neptune takes so long to go around the sun once, seasons on the planet last for many Earth years. This Hubble Telescope picture shows the Southern Hemisphere of Neptune in spring, which began in the 1960s.

Neptune is the eighth planet from the sun, the fourth largest, and a gas planet. It is named after the Roman god of the sea. Neptune is four times the size of Earth, and its day lasts a little more than 16 hours. Its year is about 165 Earth years. Neptune's orbit is a perfect circle. Neptune was the last stop on spacecraft Voyager's epic trip through the solar system. From earth, Neptune appears to be an interesting blue world. Even so, this mysterious planet had many surprises for us.

The first big surprise was that the blue world has winds in its outer layer that move at hundreds of miles per hour. In fact, the winds on Neptune are the fastest in our solar system. Also, the planet has different-colored bands that are nearly as pronounced as those on Jupiter. The Voyager mission also sent back photos of "storms" on the planet, similar to the Great Red Spot on Jupiter. These storms have since disappeared, but it was still quite a surprise. The Voyager spacecraft also showed us that Neptune has a very thin ring system. These rings are so thin that they are invisible from Earth, but Voyager was able to send us images of the rings.

When Voyager 2 passed Neptune, one of the last things it photographed was the moon Triton. Scientists were amazed to see that this distant moon had a very unusual surface. The dark smudges on the lighter part of the moon are nitrogen geysers.

The biggest surprise of all was Neptune's moon, Triton. Neptune has at least 13 moons in its family, including active Triton. The images sent back from Voyager showed dark eruptions on the moon's surface that look a lot like geysers here on Earth. Scientists later determined that these "geysers" were actually eruptions of nitrogen from beneath the icy polar caps. Triton orbits Neptune in a backward direction called "retrograde." Eventually, scientists think, it will spiral close enough to Neptune to be torn apart by the planet's gravity. Future missions to Neptune will give us more information about this interesting moon. Neptune and Uranus are often called the twin giants because they are about the same size and mass, and both have very cold temperatures. Neptune's atmosphere is made up of helium, hydrogen, and methane. Methane absorbs red light and reflects green light, which is why Neptune appears blue.

In the midsection of Neptune, a storm rages at more than 500 miles per hour. It has been nicknamed the Great Dark Spot.

Neptune's plain appearance can mislead us into thinking it's a very calm world. When the Voyager spacecraft arrived, it discovered a dark blue area, nicknamed the Great Dark Spot, which has a storm with winds of more than 500 miles per hour at its center. And there are clouds in Neptune's atmosphere that reach speeds of 2,000 km (1,250 miles) per hour. Astronomers looked for Neptune before they ever saw it — based on the unexpected movement of Uranus. You need very good binoculars or a telescope to see Neptune. You would probably look for it somewhere before Pluto. But you might be wrong. Neptune and Pluto occasionally change places as the farthest planet from the sun because of Pluto's very elliptical orbit.