The next object we find in this series as we move out far beyond Uranus is the planet Neptune. Since 2006 when Pluto lost its status as a fully-fledged planet, Neptune assumed the role of the last planet in our Solar System. It is also the last of the gas giants and measures 30,760 miles across. It is a very long way out from the Sun with an average distance of 2.8 billion miles. Because of this distance, it takes Neptune over 164 Earth years to make one orbit of the Sun. However, its rotation speed is fast at just over 16 hours. This results in a bulge appearing at its equator.
Like the other gas giants, we can only observe the upper atmosphere using visible light. The planet appears blue in colour. This is not due to water but to the 3% of methane present which absorbs red light from the Sun and reflects blue light back. Hydrogen makes up 79% of the atmosphere with helium adding another 18%. There are some other gases also present but they are there in only trace amounts. The temperature at the top of the clouds is a very cold minus 200º Celsius.
Neptune’s structure is similar to the other gas giants. We believe it to have a solid core made of rock and ice surrounded by a layer of methane, ammonia and water in the form of ice. Finally, this is surrounded by a gaseous atmosphere.
Being located so far from the Sun you might expect Neptune to be quite a docile world. But this is not the case. Neptune is subject to violent storms and extremely strong winds. Part of the reason for this is probably not the little heat and light that does reach the planet from the Sun, but that there may be some additional heat being generated deep inside the planet.
Jupiter has its Great Red Spot and Neptune can offer its Great Dark Spot. This was a distinct feature in the atmosphere indicating a very violent storm. However, unlike the Spot on Jupiter, this one has disappeared. It was observed by the Voyager II space-craft as it flew by the planet. But some years later when the Hubble Space Telescope focused on the planet, it was nowhere to be seen.
Neptune has natural satellites in orbit around it and at present, thirteen have been detected. Triton is its largest moon and is bigger than Pluto which provides more evidence why Pluto is now regarded as only a dwarf planet.
Neptune also has a system of rings. They appear as four distinct bands. There is a debris field between two of them. However, compared with the rings of Saturn they are less significant. We are now at the outer reaches of the solar system but we are far from finished with surprises.
John Pullen 