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Solar System Exploration
Our Solar System: Moons
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Illustration showing many moons compared to Earth.
This photo illustration shows selected moons of our solar system at their correct relative sizes to each other and to Earth.

Moons -- also called satellites -- come in many shapes, sizes and types. They are generally solid bodies, and few have atmospheres. Most of the planetary moons probably formed from the discs of gas and dust circulating around planets in the early solar system.

Astronomers have found at least 146 moons orbiting planets in our solar system. Another 27 moons are awaiting official confirmation of their discovery. This number does not include the six moons of the dwarf planets, nor does this tally include the tiny satellites that orbit some asteroids and other celestial objects.

Of the terrestrial (rocky) planets of the inner solar system, neither Mercury nor Venus have any moons at all, Earth has one and Mars has its two small moons. In the outer solar system, the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune have numerous moons. As these planets grew in the early solar system, they were able to capture objects with their large gravitational fields.

Earth's Moon probably formed when a large body about the size of Mars collided with Earth, ejecting a lot of material from our planet into orbit. Debris from the early Earth and the impacting body accumulated to form the Moon approximately 4.5 billion years ago (the age of the oldest collected lunar rocks). Twelve American astronauts landed on the Moon during NASA's Apollo program from 1969 to 1972, studying the Moon and bringing back rock samples.

Usually the term moon brings to mind a spherical object, like Earth's Moon. The two moons of Mars, Phobos and Deimos, are different. While both have nearly circular orbits and travel close to the plane of the planet's equator, they are lumpy and dark. Phobos is slowly drawing closer to Mars and could crash into the planet in 40 or 50 million years. Or the planet's gravity might break Phobos apart, creating a thin ring around Mars.

Jupiter has 50 known moons (plus 17 awaiting official confirmation), including the largest moon in the solar system, Ganymede. Many of Jupiter's outer moons have highly elliptical orbits and orbit backwards (opposite to the spin of the planet). Saturn, Uranus and Neptune also have some irregular moons, which orbit far from their respective planets.

Color image of moon within Saturn's rings.
Pan is responsible for a gap in Saturn's rings.

Saturn has 53 known moons (plus 9 awaiting official confirmation). The chunks of ice and rock in Saturn's rings (and the particles in the rings of the other outer planets) are not considered moons, yet embedded in Saturn's rings are distinct moons or moonlets. These shepherd moons help keep the rings in line. Saturn's moon Titan, the second largest in the solar system, is the only moon with a thick atmosphere.

In the realm of the ice giants, Uranus has 27 known moons. The inner moons appear to be about half water ice and half rock. Miranda is the most unusual; its chopped-up appearance shows the scars of impacts of large rocky bodies.

Neptune has 13 known moons. And Neptune's moon Triton is as big as the dwarf planet Pluto and orbits backwards compared with Neptune's direction of rotation.

Pluto's large moon Charon is about half the size of Pluto. Like Earth's Moon, Charon may have formed from debris resulting from an early collision of an impactor with Pluto. In 2005, scientists using the Hubble Space Telescope to study Pluto found two additional, but very small, moons. The little moons Nix and Hydra are about two to three times as far from Pluto as Charon and roughly 5,000 times fainter than Pluto. Eris, another dwarf planet even more distant than Pluto, has a small moon of its own, named Dysnomia. Haumea, another dwarf planet, has two satellites, Hi'iaka and Namaka.

How the Moons of Our Solar System Get Their Names
Most moons in our solar system are named for mythological characters from a wide variety of cultures. Uranus is the exception. Uranus' moons are named for characters in William Shakespeare's plays and from Alexander Pope's poem "Rape of the Lock." Moons are given provisional designations such as S/2009 S1, the first satellite discovered at Saturn in 2009. The International Astronomical Union approves an official name when the discovery is confirmed.

Significant Dates
Color image of rocks on the surface of Titan.
Huygens' image of Titan surface. The rocks are about the size of pebbles.
  • 1610: Galileo Galilei and Simon Marius independently discover four moons orbiting Jupiter. The moons are known as the Galilean satellites in honor of Galileo's discovery, which also confirms the planets in our solar system orbit the sun.
  • 1877: Asaph Hall discovers Mars' moons Phobos and Deimos.
  • 1969: Astronaut Neil Armstrong is the first of 12 men to walk on the surface of Earth's Moon.
  • 1980: Voyager 1 instruments detect signs of surface features beneath the hazy atmosphere of Saturn's largest moon, Titan.
  • 2005: The European Space Agency's Huygens probe lands on the surface of Titan. It is the first spacecraft to successfully land on a moon beyond Earth's own moon.
  • 2000-present Using improved ground-based telescopes, orbiting observatories such as the Hubble Space Telescope and spacecraft observations, scientists find dozens of new moons in our solar system.
Moons of Our Solar System
1. Earth's Moon

2. Phobos
3. Deimos

4. Io
5. Europa
6. Ganymede
7. Callisto
8. Amalthea
9. Himalia
10. Elara
11. Pasiphae
12. Sinope
13. Lysithea
14. Carme
15. Ananke
16. Leda
17. Thebe
18. Adrastea
19. Metis
20. Callirrhoe
21. Themisto
22. Megaclite
23. Taygete
24. Chaldene
25. Harpalyke
26. Kalyke
27. Iocaste
28. Erinome
29. Isonoe
30. Praxidike
31. Autonoe
32. Thyone
33. Hermippe
34. Aitne
35. Eurydome
36. Euanthe
37. Euporie
38. Orthosie
39. Sponde
40. Kale
41. Pasithee
42. Hegemone
43. Mneme
44. Aoede
45. Thelxinoe
46. Arche
47. Kallichore
48. Helike
49. Carpo
50. Eukelade
51. Cyllene
52. Kore
53. Herse

54. Mimas
55. Enceladus
56. Tethys
57. Dione
58. Rhea
59. Titan
60. Hyperion
61. Iapetus
62. Erriapus
63. Phoebe
64. Janus
65. Epimetheus
66. Helene
67. Telesto
68. Calypso
69. Kiviuq
70. Atlas
71. Prometheus
72. Pandora
73. Pan
74. Ymir
75. Paaliaq
76. Tarvos
77. Ijiraq
78. Suttungr
79. Mundilfari
80. Albiorix
81. Skathi
82. Siarnaq
83. Thrymr
84. Narvi
85. Methone
86. Pallene
87. Polydeuces
88. Daphnis
89. Aegir
90. Bebhionn
91. Bergelmir
92. Bestla
93. Farbauti
94. Fenrir
95. Fornjot
96. Hati
97. Hyrrokkin
98. Kari
99. Loge
100. Skoll
101. Surtur
102. Greip
103. Jarnsaxa
104. Tarqeq
105. Anthe
106. Aegaeon

107. Cordelia
108. Ophelia
109. Bianca
110. Cressida
111. Desdemona
112. Juliet
113. Portia
114. Rosalind
115. Mab
116. Belinda
117. Perdita
118. Puck
119. Cupid
120. Miranda
121. Francisco
122. Ariel
123. Umbriel
124. Titania
125. Oberon
126. Caliban
127. Stephano
128. Trinculo
129. Sycorax
130. Margaret
131. Prospero
132. Setebos
133. Ferdinand

134. Triton
135. Nereid
136. Naiad
137. Thalassa
138. Despina
139. Galatea
140. Larissa
141. Proteus
142. Halimede
143. Psamathe
144. Sao
145. Laomedeia
146. Neso
Provisional Moons
NASA Official: Kristen Erickson
Advisory: Dr. James Green, Director of Planetary Science
Outreach Manager: Alice Wessen
Curator/Editor: Phil Davis
Science Writer: Autumn Burdick
Producer: Greg Baerg
Webmaster: David Martin
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Last Updated: 10 Mar 2015