Natural satellitesEarth and other worlds

Moons of the Solar System

Earth’s Moon is our constant companion. Beyond it are volcanic worlds, hidden oceans, thick atmospheres, backwards orbits and hundreds of small satellites still being counted.

From Earth and Mars to the giant planets · 24 featured field notes about the worlds that orbit worlds

Known planetary moons: More than 400 · the catalogue keeps growing
Earth’s Moon: 3,475 km across · about 384,400 km away
Lunar cycle: 27.3-day orbit · 29.5-day phase cycle
Largest moon: Ganymede · 5,268 km across
Most volcanically active: Io · a world reshaped by tides
Ocean-moon candidates: Europa · Enceladus · Titan · Triton
Thickest atmosphere: Titan · about 1.5 Earth atmospheres
Backwards orbit: Triton circles Neptune opposite its spin

Worlds seen up close

NASA and mission partners

Galileo view of Earth's Moon, showing dark maria and the bright rays of Tycho crater — **Earth’s Moon from Galileo** The Galileo spacecraft imaged our Moon on 7 December 1992 while travelling toward Jupiter. The bright rays near the bottom radiate from Tycho, a young impact crater. Image: NASA/JPL/USGS.

Colour view of Phobos from about 6,800 kilometres, showing its cratered potato-shaped surface — **Phobos above Mars** The larger and closer of Mars’s two moons has a dark, battered surface and orbits faster than Mars rotates. Image: NASA/JPL-Caltech/University of Arizona.

Global true-colour image of Jupiter's volcanic moon Io — **Io in true colour** Sulfur compounds create Io’s yellow, orange and red surface. Tidal flexing from Jupiter drives hundreds of volcanic centres. Image: NASA/JPL/University of Arizona.

Highest-resolution Galileo mosaic of the fractured icy surface of Europa — **Europa’s fractured ice** Long ridges, bands and disrupted blocks record the motion of a brittle ice shell above an ocean. Image: NASA/JPL-Caltech.

Cassini image of the bright, icy, fractured moon Enceladus — **Enceladus from Cassini** Fresh ice makes this small Saturnian moon brilliantly reflective. Jets from its south pole sample a hidden ocean. Image: NASA/JPL/Space Science Institute.

Cassini radar view through Titan's orange haze, revealing surface features — **Titan beneath the haze** Radar sees through Titan’s organic atmosphere to dunes, plains and drainage systems carved by liquid methane. Image: NASA/JPL-Caltech/ASI.

Voyager 2 parting view of Triton's bright southern hemisphere and dark surface streaks — **Triton’s parting portrait** Voyager 2 found nitrogen frost, dark wind streaks and geyser-like plumes on this backwards-orbiting moon. Image: NASA/JPL/USGS.

What makes a moon?

A moon is a natural body orbiting a planet, dwarf planet or another body other than a star. That orbital relationship is the definition; moons are not defined by size, shape or composition. Some are round worlds with atmospheres and oceans, while others are irregular fragments only a few kilometres across.

Moons are different from rings: ring particles orbit individually, while a moon is a discrete body. They are also different from artificial satellites. The Moon is Earth’s only natural satellite, despite thousands of spacecraft circling our planet.

Orbit defines it

The host can be a planet, dwarf planet, asteroid or even another moon. What matters is the orbit, not the moon’s geology.

Tides do work

Changing gravity can flex a moon’s interior. That tidal energy melts ice and powers Io’s volcanoes and Enceladus’s ocean.

Two orbital families

Regular moons follow close, circular, equatorial paths. Irregular moons are distant and tilted, often moving backwards around their host.

Partners, not copies

Earth and the Moon, and Pluto and Charon, raise strong tides in one another. The shared centre of mass lies outside the larger body.

The known moons, by host

Small-moon counts change as survey observations improve, especially around Saturn and Jupiter. The numbers below follow the catalogue used across this site; the named moons are the ones most useful for understanding the Solar System.

Earth 1 known moon

Our Moon is unusually large relative to its planet. It stabilises Earth’s axial tilt, drives ocean tides and records a history of impacts.

Moon

Mars 2 known moons

Phobos and Deimos are small, dark and irregular. Phobos is slowly spiralling inward and will eventually break apart or strike Mars.

Phobos Deimos

Jupiter 95 known moons

The four Galilean moons are worlds in their own right: volcanic Io, ocean-bearing Europa, giant Ganymede and ancient Callisto.

Io Europa Ganymede Callisto

Saturn 293 · Aug 2026

Saturn’s family spans tiny ring shepherds to Titan. Enceladus vents ocean material into space, while Titan has rivers and lakes.

Enceladus Titan Mimas Iapetus

Uranus 28 known moons

Uranus’s larger moons are named for Shakespeare and Pope characters. Miranda’s patchwork cliffs record a particularly violent past.

Titania Oberon Ariel Umbriel Miranda

Neptune 16 known moons

Triton dominates Neptune’s system. Its backwards orbit and nitrogen geysers point to a captured Kuiper Belt world that is still active.

Triton Proteus Nereid

The wider moon family: dwarf planets have natural satellites too. Pluto’s five-moon system includes Charon, whose mass is about one-eighth of Pluto’s; the pair orbit a shared centre. Explore Pluto and Charon.

Compare the notable moons

Search by name or filter by host system. Every named moon below has its own field note with NASA imagery, key facts, video recommendations and source links. “World type” describes what the moon is like, not its formal orbital class.

Moon finder

Six planetary systems and the Pluto system

24 moons

Moon	Host	World type	Diameter	Orbit	Why it stands out
Moon	Earth	Rocky companion	3,475 km	27.32 d	Stabilises Earth’s tilt; its near and far sides differ dramatically.
Phobos	Mars	Irregular rubble world	22.5 km	7.7 h	Orbits below synchronous altitude and is slowly falling toward Mars.
Deimos	Mars	Small irregular moon	12.4 km	30.3 h	Smooth-looking because loose material fills many of its craters.
Io	Jupiter	Volcanic rocky moon	3,643 km	1.77 d	The most volcanically active world known, heated by Jupiter’s tides.
Europa	Jupiter	Ocean ice moon	3,122 km	3.55 d	A fractured ice shell probably covers a global saltwater ocean.
Ganymede	Jupiter	Large icy moon	5,268 km	7.15 d	The largest moon, larger than Mercury, and the only one with its own magnetic field.
Callisto	Jupiter	Ancient icy moon	4,821 km	16.69 d	One of the most heavily cratered surfaces known, outside Jupiter’s strongest radiation.
Enceladus	Saturn	Ocean ice moon	504 km	1.37 d	Samples of its subsurface ocean erupt from polar fractures.
Titan	Saturn	Ocean moon with atmosphere	5,150 km	15.95 d	Methane lakes and rain sit beneath a thick nitrogen atmosphere.
Mimas	Saturn	Small icy moon	396 km	0.94 d	Its giant Herschel crater gives it the nickname “Death Star”.
Tethys	Saturn	Bright icy moon	1,062 km	1.89 d	Ithaca Chasma wraps around much of this reflective ice world.
Dione	Saturn	Fractured icy moon	1,123 km	2.74 d	Bright ice cliffs cross an ancient landscape and hint at a hidden ocean.
Rhea	Saturn	Ancient icy moon	1,528 km	4.52 d	Saturn’s second-largest moon preserves a crowded impact record.
Hyperion	Saturn	Porous irregular moon	270 km	21.28 d	A sponge-shaped moon with chaotic rotation and extremely low density.
Iapetus	Saturn	Two-tone icy moon	1,469 km	79.33 d	One hemisphere is far darker than the other, with a high equatorial ridge.
Miranda	Uranus	Small icy moon	472 km	1.41 d	A broken-looking mix of cliffs, canyons and grooved terrain.
Ariel	Uranus	Bright icy moon	1,158 km	2.52 d	Uranus’s brightest major moon, crossed by deep valleys and ridges.
Umbriel	Uranus	Dark icy moon	1,169 km	4.14 d	An ancient cratered surface with the mysterious bright feature Wunda.
Titania	Uranus	Large icy moon	1,578 km	8.71 d	Uranus’s largest moon, cut by enormous canyons and scarps.
Oberon	Uranus	Ancient icy moon	1,523 km	13.46 d	The outermost major moon of Uranus, with dark deposits inside old craters.
Proteus	Neptune	Large irregular moon	420 km	1.12 d	Nearly round but still irregular, Proteus is one of the darkest large moons.
Nereid	Neptune	Distant irregular moon	340 km	360 d	An extremely eccentric orbit points to capture or a violent dynamical history.
Triton	Neptune	Captured ocean ice moon	2,707 km	−5.88 d	Orbits backwards; nitrogen geysers and frost mark a young surface.
Charon	Pluto	Large icy companion	1,212 km	6.39 d	Large enough that Pluto and Charon orbit a centre in the space between them.

Selected representative moons, not the complete catalogue. Negative orbital period denotes retrograde motion. Data: NASA/JPL satellite orbital elements and NASA moon pages.

How moons form

01 · Co-accretion

Built around a world

Dust and ice in a circumplanetary disk can clump into moons. This explains many regular satellites travelling near their planet’s equatorial plane.

02 · Giant impact

Made from debris

Earth’s Moon probably formed after a Mars-sized body struck the young Earth. Charon may have grown from debris after a similar collision with Pluto.

03 · Capture

Arrived from elsewhere

Passing asteroids or Kuiper Belt objects can be caught by gravity. Triton’s backwards orbit is strong evidence for capture.

Worth knowing

The Moon always shows Earth nearly the same face because its rotation and orbit are tidally locked.
Ganymede is larger than Mercury, yet it orbits Jupiter as a moon.
Io’s volcanoes can throw material hundreds of kilometres above its surface.
Titan is the only other world known to have stable liquid on its surface, though its seas are methane and ethane.
Phobos completes three orbits around Mars in about one Martian day.
Some irregular moons orbit their planets in the opposite direction to the planet’s spin.

Watch the Moon story

Videos are embedded from YouTube and load only when you open a card.

Follow the worlds that orbit worlds

Ocean moons