NEPTUNE

Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly more massive than its near-twin Uranus. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. The planet orbits the Sun once every 164.8 years at an average distance of 30.1 AU (4.5 billion km; 2.8 billion mi). It is named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptune's trident.
Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. After Bouvard's death, the position of Neptune was predicted from his observations, independently, by John Couch Adams and Urbain Le Verrier. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 13 known moons were located telescopically until the 20th century. The planet's distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989; Voyager 2 remains the only spacecraft to visit Neptune. The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar.
Like Jupiter and Saturn, Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, though it contains a higher proportion of "ices" such as water, ammonia and methane. However, similar to Uranus, its interior is primarily composed of ices and rock; Uranus and Neptune are normally considered "ice giants" to emphasise this distinction. Traces of methane in the outermost regions in part account for the planet's blue appearance.

Interesting Facts:

• Neptune is the Smallest of the Gas Giants:
  With an equatorial radius of only 24,764 km, Neptune is smaller than all the other gas giants in the Solar System: Jupiter, Saturn and Uranus. But here’s the funny thing: Neptune is actually more massive than Uranus by about 18%. Since it’s smaller but more massive, Neptune has a much higher density than Uranus. In fact, at 1.638 g/cm3, Neptune is the densest gas giant in the Solar System.
• Neptune’s Surface Gravity is Almost Earth-like:
  Neptune is a ball of gas and ice, probably with a rocky core. There’s no way you could actually stand on the surface of Neptune without just sinking in. However, if you could stand on the surface of Neptune, you would notice something amazing. The force of gravity pulling you down is almost exactly the same as the force of gravity you feel walking here on Earth.
• The Discovery of Neptune is Still a Controversy:
  The first person to have seen Neptune was likely Galileo, who marked it as a star in one of his drawings. However, since he did not identify it as a planet, he is not credited with the discovery. That credit goes to French mathematician Urbain Le Verrier and the English mathematician John Couch Adams, both of whom predicted that a new planet – known as Planet X – would be discovered in a specific region of the sky.
  When astronomer Johann Gottfried Galle actually found the planet in 1846, both mathematicians took credit for the discovery. English and French astronomers battled over who made the discovery first, and there are still defenders of each claim to this day. Today, the consensus among astronomers is that Le Verrier and Adams deserve equal credit for the discovery.
• Neptune has the Strongest Winds in the Solar System:
  Think a hurricane is scary? Imagine a hurricane with winds that go up to 2,100 km/hour. As you can probably imagine, scientists are puzzled how an icy cold planet like Neptune can get its cloud tops t0 move so fast. One idea is that the cold temperatures and the flow of fluid gasses in the planet’s atmosphere might reduce friction to the point that it’s easy to generate winds that move so quickly.
• Neptune is the Coldest Planet in the Solar System:
  At the top of its clouds, temperatures on Neptune can dip down to 51.7 Kelvin, or -221.45 degrees Celsius (-366.6 °F). That’s almost three times the coldest temperature ever recorded here on Earth (-89.2°C; -129°F), which means that an unprotected human being would flash freeze in a second! Pluto gets colder, experiencing temperatures as low as 33 K (-240 °C/-400 °F). But then again, Pluto isn’t a planet any more (remember?)
• Neptune has Rings:
  When people think of ring systems, Saturn is usually the planet that comes to mind. But would it surprise you to know that Neptune has a ring system as well? Unfortunately, it is rather difficult to observe compared to Saturn’s bright, bold ring; which is why it is not so well-recognized. In total, Neptune has five rings, all of which are named after astronomers who made important discoveries about Neptune – Galle, Le Verrier, Lassell, Arago, and Adams. These rings are composed of at least 20% dust (with some containing as much as 70%) which are micrometer-sized, similar to the particles that make up the rings of Jupiter. The rest of the ring materials consists of small rocks. The planet’s rings are difficult to see because they are dark, which is likely due to the presence of organic compounds that have been altered due to exposition to cosmic radiation. This is similar to the rings of Uranus, but very different than the icy rings around Saturn.