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. It is named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptune’s trident.

Some facts about Neptune

Diameter-  49,500km

Orbital period-  164.8yrs

Length of a Day-   16.1hrs

Axis tilt- 28 degrees

Distance from  the Sun- 30.1AU(4.5 billion km)

Moons- 14

Special features

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 is our solar system’s windiest world. Despite its great distance and low energy input from the Sun, Neptune’s winds can be three times stronger than Jupiter’s and nine times stronger than Earth’s. These winds whip clouds of frozen methane across the planet at speeds of more than 1,200 miles per hour (2,000 kilometers per hour). Even Earth’s most powerful winds hit only about 250 miles per hour (400 kilometers per hour).

In 1989 a large, oval-shaped storm in Neptune’s southern hemisphere dubbed the “Great Dark Spot” was large enough to contain the entire Earth. That storm has since disappeared, but new ones have appeared on different parts of the planet.

Natural satellites and Rings

Neptune has 14 known moons. Neptune’s largest moon Triton was discovered on October 10, 1846, by William Lassell, just 17 days after Johann Gottfried Galle discovered the planet. Since Neptune was named for the Roman god of the sea, its moons are named for various lesser sea gods and nymphs in Greek mythology.

Triton is the only large moon in the solar system that circles its planet in a direction opposite to the planet’s rotation (a retrograde orbit), which suggests that it may once have been an independent object that Neptune captured. Triton is extremely cold, with surface temperatures around minus 391 degrees Fahrenheit (minus 235 degrees Celsius). And yet, despite this deep freeze at Triton, Voyager 2 discovered geysers spewing icy material upward more than 5 miles (8 kilometers). Triton’s thin atmosphere, also discovered by Voyager, has been detected from Earth several times since, and is growing warmer, but scientists do not yet know why.

  Neptune has at least five main rings and four prominent ring arcs that we know of so far. Starting near the planet and moving outward, the main rings are named Galle, Leverrier, Lassell, Arago and Adams. The rings are thought to be relatively young and short-lived. Neptune’s ring system also has peculiar clumps of dust called arcs. Four prominent arcs named Liberté (Liberty), Egalité (Equality), Fraternité (Fraternity) and Courage are in the outermost ring, Adams. The arcs are strange because the laws of motion would predict that they would spread out evenly rather than stay clumped together. Scientists now think the gravitational effects of Galatea, a moon just inward from the ring, stabilizes these arcs.

Structure and Atmosphere

Neptune is one of two ice giants in the outer solar system (the other is Uranus). Most (80 percent or more) of the planet’s mass is made up of a hot dense fluid of “icy” materials—water, methane and ammonia—above a small, rocky core. Scientists think there might be an ocean of super hot water under Neptune’s cold clouds. It does not boil away because incredibly high pressure keeps it locked inside.

Neptune’s atmosphere is made up mostly of hydrogen and helium with just a little bit of methane. Neptune’s neighbor Uranus is a blue-green color due to such atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color.

Exploration

Voyager 2 is the only spacecraft that has visited Neptune. The spacecraft’s closest approach to the planet occurred on 25 August 1989. Because this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, similarly to what was done for Voyager 1’s encounter with Saturn and its moon Titan. The images relayed back to Earth from Voyager 2 became the basis of a 1989 PBS all-night program, Neptune All Night.

https://en.wikipedia.org/wiki/Neptune
https://solarsystem.nasa.gov/planets/neptune/overview/