The Big Blue
With Pluto no longer classified as a planet, the honor of the most distant planet in the Solar System goes to Neptune. What’s interesting is that even though Pluto is further away from the Sun than Neptune, there are periods when Pluto actually orbits closer to the Sun, because its orbital path is very elliptical. Neptune is the first planet to be discovered, or to be more exact, its existence was predicted by mathematical calculations, rather than observation of the sky. Astronomers were actually studying Uranus, and when it turned out it wasn’t travelling along the expected orbit, they suspected that the existence of another planet and its gravitational pull were responsible for such behavior. They calculated the possible position of a planet, and soon they found Neptune.
At a distance of 4,5 billion kilometers from the Sun, Neptune orbits the Sun once every 165 years. Since its discovery in 1864, it has completed its orbit only once. Although the smallest planet of all four gas giants, with an equatorial radius of 24,764 kilometers, it is the most dense. Compared to Earth, it has 17 times its mass, and is approximately 4 times larger. The most interesting fact is that the gravity on Neptune is about the closest thing you can find to Earth gravity, within the boundaries of the Solar System. If you could actually stand on its surface, the gravity pull would be only 17% stronger.
Neptune owes its recognizable blue color to yet unidentified compound found in its atmosphere, which is mostly made up of hydrogen and helium, and methane, which absorbs red light. Despite its great distance from the Sun, and receiving very little light and heat that helps drive its atmosphere, Neptune’s winds are the fastest in the Solar System, reaching speeds of about 2,400 kilometers per hour, compared to which the hurricanes on Earth seem like a light summer breeze. The Great Dark Spot which can be seen on its surface is actually a storm about the size of the Earth. Like the other three gaseous planets, it has rings, but Neptune’s rings are not complete, and they are more similar to arcs, with some of them fading away rapidly.
Neptune has 13 moons, named after gods and nymphs from Greek mythology, just as Neptune was named after the Roman god of the sea. Triton is the only spherical moon, and is unique due to the fact that it circles around the planet in a direction that’s opposite to the planet’s rotation. This is commonly known as retrograde orbit, which suggests that Triton was a dwarf planet once, that got pulled into Neptune’s orbit by its gravity. The gravitational pull is bringing Triton closer to the planet, which means that in a few million years, Triton could be pulled apart by Neptune’s gravity. It is even colder than Neptune, with temperatures on its surface reaching as low as minus 235 degrees Celsius.
The only spacecraft to visit Neptune was Voyager 2 spacecraft, which did a flyby in 1989, passing at around 3,000 kilometers of the planet’s north pole, during which it captured some amazing photographs of the planet’s surface, its atmosphere, moons and rings. There are no firm plans to send any more spacecrafts to research Neptune, although NASA is planning on a new mission called the Neptune Orbiter, in the near future.