Neptune, the eighth planet from the Sun, is thirty times further out from the Sun than the Earth. It is so far that it takes Neptune 165 years to go around the Sun once. Like Uranus, Neptune is what we call an ice giant. That is because it contains a thick atmosphere of hydrogen and helium covering a core composed of rock and a lot of hot ice. How can ice be hot? Deep down in Neptune, like inside any planet, the pressure is incredibly high – the weight of everything above is squashing down. This pressure stops ice from melting, although it can be slushy, and means that it can actually be hot. Inside Neptune it can be thousands of degrees. In the rocky core the temperature may reach 9,000 F and the pressure is millions of times the pressure that we feel standing on the surface of the Earth. Neptune is about seventeen times the mass of the Earth and about 4 times the size, which is why we also call it a “giant” planet.
At this distance from the Sun then sunlight is almost a thousand times fainter than at the Earth, and everything at the surface is very, very cold. Its bluish color is because of small amounts of the gas methane in its atmosphere, which makes a haze that reflects blue light.
Neptune has quite a lot of activity in its atmosphere, with great storms and clouds. One of these lasted for a few years and was called the Great Dark Spot. It was about 8000 miles across and flew across the planet at breakneck speeds.
Rings and moons
Like the other giant planets Neptune has rings made out of tiny pieces of material orbiting around it. They are very dark and faint. Neptune also has 13 moons that we have found so far. Most of these moons are small, but the largest is much bigger and is called Triton. Triton is very unusual. It is almost a quarter the size of the Earth, and it orbits around Neptune in the opposite direction to any other large moon in the solar system. So while we can think of Neptune spinning in a clockwise direction, while Triton orbits around Neptune in a counterclockwise direction. Because of this Triton is actually slowly moving closer to Neptune, and in about a billion years it will crash into the giant planet.
Think about it
Why is Triton going around Neptune this way, and why is it going to eventually crash into Neptune? Triton is probably an object that used to be much further out in the Solar System – in the Kuiper Belt, which is a region of space filled with similar icy objects, including the famous dwarf planet we all know and love: Pluto. Somehow Triton came close enough to Neptune for gravity to pull it in and capture it. Because Triton goes around Neptune in the opposite direction to the way Neptune spins then the tides that Triton makes on Neptune – just like the Moon makes tides in the oceans on Earth – in turn pull against Triton. The combined pulling from Neptune in one direction and its moon in the other means that gradually Triton is being pulled in towards its planet.
Triton has a tiny, very thin atmosphere, and its surface is a mix of frozen nitrogen, water, and carbon dioxide – it is a very cold place! Despite this, there are jets or “geysers” (pronounced “guy-sers”) of nitrogen gas that erupt from Tritons surface.
Image of Triton taken by the Voyager 2 spacecraft
The only spacecraft to visit Neptune was the robotic Voyager 2, that sped past taking pictures and recording data in 1989 before heading out of the solar system altogether. So there might be many more mysteries to uncover in this distant corner of the solar system!