The Planet Neptune

The history of physics from ancient times to the modern day, focusing on space and time. Neptune is the furthest planet from the Sun, and was discovered by German astronomer Johann Galle and French mathematician Urbain Le Verrier in 1846. Probes flew past Neptune in the 1980s.

Last updated on 5th June 2017 by Dr Helen Klus

1. Characteristics of Neptune

Neptune is the furthest planet from the Sun, orbiting at about thirty AU (one AU is the distance between the Earth and the Sun)[1]. German astronomer Johann Galle discovered Neptune in 1846, following calculations made by French mathematician Urbain Le Verrier[2]. British astronomer William Lassell discovered its largest moon, Triton, two and a half weeks later[3].

Photograph of Neptune.

Neptune, image taken by Voyager 2. Image credit: NASA/JPL/Public domain.

Neptune is the third most massive planet in the Solar System, after Jupiter and Saturn. It is the fourth largest planet, having a slightly smaller diameter than Uranus. It takes just under 60,000 days for Neptune to orbit the Sun, this is over 160 years, and one day on Neptune is just over 16 hours long[4]. Like Mercury, Venus, Mars, Jupiter, and Saturn, but unlike Uranus, Neptune is named after a Roman god, the god of the sea[5].

Neptune is an ice giant, like Uranus, and its atmosphere is similar to Uranus', composed mostly of hydrogen, helium, and methane[6]. Neptune's core is mostly composed of ice and rock[7].

Planets in the Solar system, sizes are approximately to scale. Jupiter is the largest planet, followed by Saturn.

The planets, sizes approximately to scale. Image credit: Dave Jarvis/CC-SA.

2. Neptune's moons

2.1 Triton

Neptune has a faint ring system and 13 known moons. The largest of these is Triton, which contains over 90% of the mass of all the Neptunian moons, and is the only Neptunian moon that is spherical[8]. Triton is the only moon in the Solar System to have a retrograde orbit. This mean it orbits in the opposite direction to Neptune's rotation[9a]. It's also the only known moon in the Solar System to have a surface made mainly of frozen nitrogen[10].

Triton is thought to have once been a Kuiper Belt object, like Pluto, but was captured by Neptune while the Solar System was still forming[11].

Triton is composed of a core of rock and metal, with an icy mantle, and active volcanoes[9b]. It may also contain water[12]. Triton has a thin nitrogen atmosphere. This is the coldest atmosphere in the Solar System, at about -235 °C[9c].

Photograph of Triton

Triton, image taken by Voyager 2. Image credit: NASA/JPL/USGS/Public domain.

Photograph showing clouds on Neptune

Clouds of Neptune, image taken by Voyager 2. Image credit: NASA/JPL/Public domain.

3. Missions to Neptune

NASA's Voyager 2 probe came within 5000 km of Neptune in 1989, before passing Triton and discovering six new moons[13][14]. There are currently no plans for a new mission to Neptune.

4. References

  1. NASA, 'Neptune: By the Numbers', last accessed 01-06-17.

  2. Galle, J. G., 1846, 'Account of the discovery of Le Verrier's planet Neptune, at Berlin, Sept. 23, 1846', Monthly Notices of the Royal Astronomical Society, 7, pp.153.

  3. Lassell, W., 1846, 'Discovery of supposed ring and satellite of Neptune', Monthly Notices of the Royal Astronomical Society, 7, pp.157.

  4. NASA, 'Planetary Fact Sheet', last accessed 01-06-17.

  5. NASA, 'Neptune: In Depth', last accessed 01-06-17.

  6. NASA, 'All About Neptune', last accessed 01-06-17.

  7. NASA, 'Neptune's Interior', last accessed 01-06-17.

  8. NASA, 'Neptune: Moons', last accessed 01-06-17.

  9. (a, b, c) NASA, 'Triton: In Depth', last accessed 01-06-17.

  10. NASA, 'Triton: Overview', last accessed 01-06-17.

  11. Cruikshank, D. P., Encrenaz, T. (ed), Kallenbach, R. (ed), Owen, T. (ed), and Sotin, C. (ed), 2005, 'Triton, Pluto, Centaurs, and trans-Neptunian bodies' in 'The Outer Planets and their Moons', Springer.

  12. NASA, 'Triton', last accessed 01-06-17.

  13. NASA, 'Neptune', last accessed 01-06-17.

  14. NASA, 'The Voyager Planetary Mission', last accessed 01-06-17.

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The Star Garden is a science news and science education website run by Dr Helen Klus.

How we came to know the cosmos covers the history of physics focusing on space and time, light and matter, and the mind. It explains the simple discoveries we made in prehistoric times, and how we built on them, little by little, until the conclusions of modern theories seem inevitable. This is shown in a timeline of the universe.

The Star Garden covers the basics for KS3, KS4, and KS5 science revision including SATs, GCSE science, and A-level physics.

Space & Time

Pre 20th Century theories

1. History of Constellations

2. History of Latitude

3. History of Longitude

4. Models of the Universe

5. Force and Energy

6. Newton's theory of Gravity

7. Age of the Universe

20th Century discoveries

1. Special Relativity

2. General Relativity

3. Big Bang theory

4. History of Galaxies

5. Life Cycles of Stars

6. Red Giants and White Dwarfs

7. Neutron Stars and Black Holes

Missions to planets

1. The planet Mercury

2. The planet Venus

3. The planet Earth

3.1 The Earth's Moon

4. The planet Mars

4.1 The Asteroid Belt

5. The planet Jupiter

6. The planet Saturn

7. The planet Uranus

8. The planet Neptune

Beyond the planets

1. Kuiper Belt and Oort Cloud

2. Pioneer and Voyager

3. Discoveries of Exoplanets