The planet Jupiter
The planet Jupiter imaged by Voyager 2 in 1979 (photo: NASA/JPL/USGS) – click to enlarge

Jupiter is the largest planet in the Solar System and the fifth planet from the Sun.

Jupiter is around eleven times the diameter of the Earth and about 1,300 times its volume. However, it is just 300 times as massive as the Earth, as it has a much lower average density.


With just a small telescope, or even a good pair of binoculars, it’s possible to make out the coloured bands in Jupiter’s atmosphere, as well as the four Galilean Moons – Io, Europa, Ganymede and Callisto (see below).

To find Jupiter’s current position in the sky from your location, visit our Night Sky Simulator.

Jupiter Stats:

  • Type: Gas Giant
  • Equatorial Diameter: 142,984 km (11.21 Earths)
  • Mass: 1.898×1027 kg (317.8 Earths)
  • Distance from Sun: 4.950 AU to 5.458 AU
  • Sidereal period (day length): 9 hours 55 min 30 sec
  • Orbital period (year length): 11.862 Earth years
  • Number of Moons:  80
  • Symbol: ♃ (HTML code: ♃)


Jupiter is one of the four ‘giant’ planets in our Solar System, along with Saturn, Uranus and Neptune. It is classified as a gas giant and doesn’t have a solid surface. It is thought that Jupiter has a rocky core at its centre, at least 14 times the mass of the Earth, surrounded by a thick layer of metallic hydrogen, an intermediate layer of liquid hydrogen and an outer layer of gas, composed primarily of hydrogen and helium (about 90% and 10% by volume, respectively).


The outer layer of Jupiter’s atmosphere is covered with thick clouds of ammonia crystals, which form light-coloured bands known as ‘zones’ and dark-coloured bands known as ‘belts’. Giant hurricane-like storms form in the turbulence between these layers, the most recognisable of which is Jupiter’s Great Red Spot – a storm whose diameter has been as much as three times the size of the Earth. The Great Red Spot was first observed in the 1660s, when telescopes became powerful enough to show detail on Jupiter, and has been raging ever since.

Ring System

Jupiter has a ring system, discovered by the Voyager missions in 1979, although they are much fainter than the rings of Saturn and made of dust ejected from three of its moons, rather than ice particles.


Jupiter has a particularly strong magnetic field – about 14 times stronger than the Earth’s – which produces a magnetosphere that is larger in size than the Sun. Plasma particles in parts of Jupiter’s magnetosphere are heated to temperatures that are hotter than any other place in our Solar System, hotter even than the centre of the Sun!


NASA’s Pioneer 10 spacecraft was the first mission sent to explore Jupiter. Launched on 3 March 1972, Pioneer 10’s Closest approach to Jupiter occured on 4 December 1973.

This was followed by Pioneer 11 (launched 6 April 1973), which achieved its closest approach on 3 December 1974 before continuing to explore Saturn.

NASA’s Voyager 1 and Voyager 2 spacecraft (launched 5 September 1977 and 20 August 1977 respectively) were the next missions to conduct flybys of Jupiter. Both Voyager spacecraft then carried on to explore Saturn, with Voyager 2 also continuing to Uranus and Neptune. Voyager 1’s closest approach to Jupiter occured on 5 March 1979 and Voyager 2’s closest approach followed on 9 July 1979.

NASA’s Galileo mission (launched 18 October 1989) was the first spacecraft to enter orbit around Jupiter at 00:27 UTC on 8 December. The spacecraft was deorbited on 21 September 2003, impacting Jupiter’s atmosphere at 18:57:18 UTC. Galileo also carried an atmospheric probe which entered Jupiter’s atmosphere at 22:04 UTC on 7 December 1995, operating for 57 minutes.

Both the Ulysses and Cassini–Huygens spacecraft (collaborative missions between NASA and the ESA) made flybys of Jupiter on 8 February 1992 and 30 December 2000, respectively. However, Jupiter was not a primary target for these missions. In the case of Ulysses, Jupiter was used for a gravity assist to place the spacecraft in a high inclination orbit around the Sun, while Cassini–Huygens’s visit was en route to study Saturn.

NASA’s New Horizons probe (lunched 19 January 2006) also carried out a major observation mission of Jupiter, from January to June 2007, before continuing on to visit Pluto.

NASA’s Juno mission (launched 5 August 2011) is the most recent spacecraft to visit Jupiter. Juno entered into polar orbit around Jupiter on 4 July 2016, and it was originally intended that the probe would be deorbited in February 2018 and burn up in Jupiter’s outer atmosphere. This was to avoid any possibility of impact and biological contamination of one of Jupiter’s moons. However, because of a suspected problem in Juno’s main engine, a burn scheduled for 11 December 2016 was cancelled and Juno remained in its original 53-day orbit until its first flyby of the moon Ganymede on 7 June 2021. Subsequent flybys of Europa and then Io will further decrease Juno’s orbital period to 33 days by February 2024, rather than the 14-day orbit originally intended. Juno’s extended mission is now planned to last until September 2025.

Jupiter’s Moons

Jupiter has 80 known moons (as of 2021). The largest of these are Io, Europa, Ganymede and Callisto (in increasing order of distance from Jupiter), which are easily seen with the aid of a small telescope or even binoculars. They are known as the Galilean Moons as they were first discovered by Galileo in 1610.

In order of increasing distance from Jupiter, these known moons are:

  1. Metis
  2. Adrastea
  3. Amalthea
  4. Thebe
  5. Io
  6. Europa
  7. Ganymede
  8. Callisto
  9. Themisto
  10. Leda
  11. Ersa
  12. Pandida
  13. Himalia
  14. Lysithea
  15. Elara
  16. Dia
  17. Carpo
  18. Valetudo
  19. Euporie
  20. Eupheme
  21. S/2003 J 18
  22. S/2010 J 2
  23. Helike
  24. S/2003 J 16
  25. S/2003 J 2
  26. Euanthe
  27. S/2017 J 7
  28. Hermippe
  29. Praxidike
  30. Thyone
  31. Thelxinoe
  32. S/2017 J 3
  33. Ananke
  34. Mneme
  35. S/2016 J 1
  36. Orthosie
  37. Harpalyke
  38. Locaste
  39. S/2017 J 9
  40. S/2003 J 12
  41. S/2003 J 4
  42. Erinome
  43. Aitne
  44. Herse
  45. Taygete
  46. S/2017 J 2
  47. S/2017 J 6
  48. Eukelade
  49. Carme
  50. S/2003 J 19
  51. Isonoe
  52. S/2003 J 10
  53. Autonoe
  54. Philophrosyne
  55. Cyllene
  56. Pasithee
  57. S/2010 J 1
  58. S/2003 J 24
  59. Pasiphae
  60. Sponde
  61. S/2017 J 8
  62. Eurydome
  63. S/2017 J5
  64. Kalyke
  65. Hegemone
  66. Kale
  67. Kallichore
  68. S/2011 J 1
  69. S/2017 J 1
  70. Chaldene
  71. Arche
  72. Eirene
  73. Kore
  74. S/2011 J 2
  75. S/2003 J 9
  76. Megaclite
  77. Aoede
  78. S/2003 J 23
  79. Callirrhoe
  80. Sinope

Leave a Reply

Astronomy, Cosmology, Space and Astrophysics