Neptune is the 8th planet from the Sun, while Planet Nine is still thought to be hypothetical. Pluto was for a long time considered the ninth planet but is now considered a dwarf planet.
The ice giant Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. (Galileo had also recorded it as a fixed star during observations with his small telescope in 1612 and 1613). When Uranus didn't travel exactly as astronomers expected it to, a French mathematician, Urbain Joseph Le Verrier, proposed the position and mass of another as yet unknown planet that could cause the observed changes to Uranus' orbit. After being ignored by French astronomers, Le Verrier sent his predictions to Johann Gottfried Galle at the Berlin Observatory, who found Neptune on his first night of searching in 1846. Seventeen days later, its largest moon, Triton, was also discovered.
Clyde Tombaugh discovered Pluto in 1931. He was an astronomer from the United States. An astronomer is a scientist who studies stars and other objects in space. Venetia Burney named Pluto that same year, she being an 11-year-old girl from England.
Planet Nine was hypothesized when astronomers noticed that the orbits of several dwarf planets were highly oval in shape, rather than ellipses. Thus, astronomers thought that gravity from a planet the size of Neptune was pulling their orbits. The first appearance of Planet Nine was in an encyclopedia article about the Pioneer 10 and 11 probes featuring a diagram with a «tenth planet» (Pluto was the 9th at the time) and a «dead star» (https://www.youtube.com/watch?v=oZx6y06T27c).
Nearly 4.5 billion kilometers ( billion miles) from the Sun, Neptune orbits the Sun once every 165 years. It is invisible to the naked eye because of its extreme distance from Earth. The highly eccentric orbit of the dwarf planet Pluto brings Pluto inside Neptune's orbit for a 20-year period out of every 248 Earth years. The orbit of Planet Nine crosses the orbits of several other dwarf planets, which causes their orbits to be distorted. The dwarf planets orbit with several other minor planets in the Kuiper belt, similar to the asteroid belt but much farther away.
Pluto compared to Charon, the Moon, and the Earth.Neptune has a diameter of 49,244 kilometers. It has winds that travel at more than kilometers per hour. In 1989, Voyager 2 tracked a large, oval-shaped, dark storm in Neptune's southern hemisphere. This "Great Dark Spot" was large enough to contain the entire Earth, spun counterclockwise, and moved westward at almost 1,200 kilometers (750 miles) per hour. Subsequent images taken by the Hubble Space Telescope showed no sign of this Great Dark Spot, but did reveal the appearance and then fading of two other Great Dark Spots over the last decade. Neptune has known moons, six of which were discovered by Voyager .
Pluto is not very big at 2,374 kilometers, which is only a quarter as wide as the entire territory of the United States. Pluto is smaller than Earth's moon. This dwarf planet takes Earth years to go around the sun. If you lived on Pluto, you would have to wait Earth years to celebrate your first birthday. One day on Pluto is about 6 days on Earth. Pluto is about 30 times farther from the sun than Earth is. This dwarf planet has five moons. Its largest moon is named Charon. Charon (1,212 kilometers) is about half the size of Pluto. Pluto's four other moons are named Kerberos, Styx, Nix and Hydra. NASA decided to send a spacecraft to fly close to Pluto. The spacecraft’s name is New Horizons. It is only about the size of a piano. New Horizons launched in January 2006. In summer 2015, New Horizons flew by Pluto and its moons. It took almost 12 years to travel that far from Earth.
Pluto is not the only dwarf planet; there are others as well.
Ceres, unlike the other dwarf planets, orbits in the asteroid belt between Mars and Jupiter.
Chariklo is usually not considered a dwarf planet, but it is worth mentioning; it orbits the Sun between Saturn and Uranus. It was found to have two rings orbiting it.
Haumea has a set of rings orbiting it.
Makemake is a reddish-colored minor planet.
Eris orbits the Sun twice as far as Pluto. Once thought to be larger than Pluto at 2,600 kilometers, it is now smaller at 2,326 kilomters. It has one moon called Dysnomia.
The last dwarf planet is Sedna. No pictures have been taken of it so far, but scientists have confirmed it to have a red surface.
