Discover How We Came to Know the Cosmos

Chapter 24. The planet Neptune

18th December 2017 by Dr Helen Klus

24.1 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.

Figure 24.1
Image credit

Neptune, image taken by Voyager 2.

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 over 160 years for Neptune to orbit the Sun, 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]

Neptune Fact Sheet[1]

Designation = Ice giant planet
Mass = 1.0×1026 kg (17 × the mass of Earth)
Radius = 24,764 km (3.9 × the radius of Earth)
Density = 1638 kg/m3 (29.7% density of Earth)
Length of Day = 16.1 hours
Length of year = 59,800 Earth-days (164 Earth-years)
Days per year = 89,143 days on Neptune per year on Neptune
Distance from the Sun = 4.5×109 km (30.0 AU)
Orbital Velocity = 5.4 km/s
Orbital Eccentricity = 0.011
Obliquity (tilt) = 28.3°
Mean Temperature = -200 °C
Moons = 14 (including former Oort Cloud object, Triton)
Ring System = Yes

24.2 Neptune’s moons

Neptune has a faint ring system and 14 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’s spherical.[8]

24.2.1 Triton

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.[9] 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 (discussed in Chapter 26), 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.[9] 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.[9]

24.3 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.

Photograph of Triton.

Figure 24.2
Image credit

Triton, image taken by Voyager 2.

Photograph showing clouds on Neptune.

Figure 24.3
Image credit

Clouds of Neptune, image taken by Voyager 2.

24.4 References

  1. NASA, Neptune: By the Numbers, NASA Solar System Exploration.

  2. Galle, J. G., Monthly Notices of the Royal Astronomical Society 1846, 7, 153.

  3. Lassell, W., Monthly Notices of the Royal Astronomical Society 1846, 7, 157.

  4. NASA, Planetary Fact Sheet, Planetary Science - NASA.

  5. NASA, Neptune: In Depth, NASA Solar System Exploration.

  6. NASA, All About Neptune, Space Place - NASA.

  7. NASA, Neptune’s Interior, NASA Solar System Exploration.

  8. NASA, Neptune: Moons, NASA Solar System Exploration.

  9. NASA, Triton: In Depth, NASA Solar System Exploration.

  10. NASA, Triton: Overview, NASA Solar System Exploration.

  11. Cruikshank, D. P. in The Outer Planets and their Moons, (Eds.: Encrenaz, T., Kallenbach, R., Owen, T., Sotin, C.), Springer, 2005.

  12. NASA, Triton, Voyager - The Interstellar Mission - NASA.

  13. NASA, Neptune, Voyager - The Interstellar Mission - NASA.

  14. NASA, The Voyager Planetary Mission, Voyager - The Interstellar Mission - NASA.

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How We Came to Know the Cosmos: Space & Time

I Pre 20th Century theories

1. Constellations

2. Latitude and Longitude

3. Models of the Universe

4. Force, Momentum, and Energy

5. Newton’s theory of Gravity

6. The Age of the Universe

II 20th Century discoveries

7. Einstein’s theory of Special Relativity

8. Einstein’s theory of General Relativity

9. The Origin of the Universe

10. Galaxies

11. Stars

12. Red Giants and White Dwarfs

13. Supergiants, Supernova, and Neutron Stars

14. Black Holes

III Missions to planets

15. The planet Mercury

16. The planet Venus

17. The planet Earth

18. The Earth’s Moon

19. The planet Mars

20. The Asteroid Belt

21. The planet Jupiter

22. The planet Saturn

23. The planet Uranus

24. The planet Neptune

IV Beyond the planets

25. Comets

26. The Kuiper Belt and the Oort Cloud

27. The Pioneer and Voyager Missions

28. Discovering Exoplanets

29. The Search for Alien Life in the Universe

30. Where are all the Aliens?

V List of symbols

31. List of symbols

32. Image Copyright