Neptune

A size comparison of Neptune and Earth

Neptune is said to be the eighth and farthest known planet from the Sun. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth. Compared to its fellow ice giant Uranus, Neptune is slightly more massive, but denser and smaller. Being composed primarily of gases and liquids, it has no well-defined solid surface, and orbits the Sun once every 164.8 years at an orbital distance of 30.1 astronomical units (4.5 billion kilometres; 2.8 billion miles). It is named after the Roman god of the sea and has the astronomical symbol ♆, representing Neptune's trident.

Status

From its discovery in 1846 until the discovery of Pluto in 1930, Neptune was the farthest known planet. When Pluto was discovered, it was considered a planet, and Neptune thus became the second-farthest known planet, except for a 20-year period between 1979 and 1999 when Pluto's elliptical orbit brought it closer than Neptune to the Sun, making Neptune the ninth planet from the Sun during this period. The increasingly accurate estimations of Pluto's mass from ten times that of Earth's to far less than that of the Moon and the discovery of the Kuiper Belt in 1992 led many astronomers to debate whether Pluto should be considered a planet or as part of the Kuiper belt. In 2006, the International Astronomical Union defined the word "planet" for the first time, reclassifying Pluto as a "dwarf planet" and making Neptune once again the outermost-known planet in the Solar System.

Moons

For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their moons. Shown in this image are Neptune and its moons: Triton, Galatea, Naiad, Thalassa, Despina, Proteus, and Larissa An annotated picture of Neptune's many moons as captured by the James Webb Space Telescope. The bright blue diffraction star is Triton, Neptune's largest moon. Neptune has 16 known moons. Triton is the largest Neptunian moon, accounting for more than 99.5% of the mass in orbit around Neptune, and is the only one massive enough to be spheroidal.

Naming

Most languages today use some variant of the name "Neptune" for the planet. In Chinese, Vietnamese, Japanese, and Korean, the planet's name was translated as "sea king star" (海王星). In Mongolian, Neptune is called Dalain van (Далайн ван), reflecting its namesake god's role as the ruler of the sea. In modern Greek, the planet is called Poseidon (Ποσειδώνας, Poseidonas), the Greek counterpart of Neptune. In Hebrew, Rahab (רהב), from a Biblical sea monster mentioned in the Book of Psalms, was selected in a vote managed by the Academy of the Hebrew Language in 2009 as the official name for the planet, even though the existing Latin term Neptun (נפטון) is commonly used. In Māori, the planet is called Tangaroa, named after the Māori god of the sea. In Nahuatl, the planet is called Tlāloccītlalli, named after the rain god Tlāloc. In Thai, Neptune is referred to by the Westernised name Dao Nepchun/Nepjun (ดาวเนปจูน) but is also called Dao Ket (ดาวเกตุ, lit. 'star of Ketu'), after Ketu (केतु), the descending lunar node, who plays a role in Hindu astrology. In Malay, the name Waruna, after the Hindu god of seas, is attested as far back as the 1970s, but was eventually superseded by the Latinate equivalents Neptun (in Malaysian) or Neptunus (in Indonesian).

See Also

Pisces

Sedna

Kuiper Belt

Gas Giant

Uranus