The farthest planet from the Sun, Neptune, is an Ice Giant that likely formed closer to our host star before shifting to the outer solar system around 4 billion years ago. At an average distance of 4.5 billion km (2.8 billion miles), it is the only planet in our solar system not visible to the naked eye. At such distance, it takes the lonely photons approximately 4 hours to travel from the Sun to Neptune 🙂

The blue giant was the first planet whose existence was predicted by mathematical calculations. A series of irregularities observed in Uranus’ orbit, in fact, led 19th century astronomers to believe that the influence of an undiscovered planet could be the reason behind these anomalies.

Among these was French astronomer Urbain Le Verrier, who in 1845 began his calculations to determine the position of the new planet. It was then in 1846 that the German astronomer Johann Galle, using those predictions, was finally able to point a 9.6-inch telescope at Neptune, to observe it in all its beauty.

And Le Verrier? Well, he passed into history as “the discoverer of a planet with the point of his pen” and, once aware about the correctness of his calculations, he claimed the right to choose the new world’s name: Neptune, the Roman god of the sea.

The name was a clear reference to the planet’s vivid blue tint, that at the time was thought to be due to liquid water floating on the planet’s surface. However, while there is in fact a mix of water, ammonia, and methane ices on the surface (hence the appellative Ice Giant), Neptune’s blue colour is the effect of the small amount of methane present in its atmosphere, which absorbs red light and reflects blue light outward.

The atmosphere, mostly made up of hydrogen and helium, is characterised by extreme winds that can reach up 2,400 km/h (1,500 mph), by far the fastest in our Solar System. Approximately three times stronger than Jupiter’s and nine times stronger than Earth’s, these winds stir clouds of frozen methane across the planet.

Credit: NASA ®

Neptune’s rotation axis is tilted 28 degrees, similar to that of our planet, meaning the Ice Giant too experiences four distinct seasons, although each lasts a little over 40 years. This is because Neptune takes around 165 Earth years to complete an orbit around the Sun, a Neptune year. A day on the blue planet, on the other hand, is relatively short at about 16 hours.

Like its neighbour Uranus, Neptune too has its own ring system, which is made up of 5 main rings and four prominent ring arcs and, according to NASA, it is believed to be relatively young. At the time of this article, 14 moons have been discovered orbiting around Neptune, the largest being Triton, discovered by British astronomer William Lassell* just 17 days after Galle spotted the planet for the first time.

Triton is quite interesting for scientists as it is the only moon in the Solar System to orbit its planet in a direction opposite to the planet’s rotation; this characteristic is called a retrograde orbit and may be explained by the fact that Triton once was an independent object that was then captured by Neptune’s gravity.

Also, due to its distance from the Sun, Triton is extremely cold, with surface temperatures reaching low peaks of -235 degrees C (-391 degrees F), but its thin atmosphere is getting warmer, leaving scientist without an exaplanation to this day.

*Lassell was a Liverpool businessman who made a fortune in the brewery business and he used this fortune to finance his hobby and his innovative telescopes, including the one used to discover Triton. It is then safe to say that we owe the discovery of this moon to beer!