The Moons of Our Solar System

Defining a Moon

Our solar system is a realm of more than just planets orbiting a central star. It is a complex and crowded neighborhood filled with a staggering variety of celestial bodies. Among the most fascinating of these are the natural satellites, or moons. A moon is simply a planetary body that orbits another, larger body that isn’t a star. While we often picture a moon circling a planet, the definition is broader. The dwarf planet Pluto hosts a family of five moons, and even some asteroids, like Didymos, have their own tiny satellites. As of early 2025, astronomers have confirmed the existence of more than 900 moons in our solar system, with estimates suggesting thousands more await discovery.

These worlds come in many shapes and sizes. Some are large enough to be planets in their own right, possessing atmospheres and complex geology. Others are small, irregular chunks of rock and ice, little more than captured space debris. Their origins are just as varied, generally falling into one of three categories. Many moons, especially the large satellites of the gas giants, likely experienced co-formation, condensing from the same swirling disk of gas and dust that created their parent planet. Another method is through a giant impact, a violent collision between two large objects that flings debris into orbit, which then coalesces to form a moon. This is the leading explanation for our own Moon and the satellite system of Pluto. Finally, some moons are formed through capture, where a wandering body like an asteroid or a Kuiper Belt object strays too close to a planet and is ensnared by its gravity. This process is thought to explain the moons of Mars and Neptune’s largest moon, Triton.

This incredible diversity of worlds provides a rich tapestry of planetary science. Each moon is a unique destination with its own story, offering clues about the formation of its parent planet, the history of the solar system, and even the potential for life to exist in the most unexpected places.

The Inner Solar System: Earth and Mars

The inner solar system, home to the rocky terrestrial planets, hosts only three moons. Yet these three bodies tell two vastly different stories of satellite formation and evolution.

Earth’s Moon: A Familiar Neighbor

Earth is accompanied by a single, large satellite: the Moon. It is the fifth-largest moon in the solar system and is unique for its large size relative to its parent planet. The prevailing theory for its origin is the Giant-Impact Hypothesis. About 4.5 billion years ago, when the solar system was still a chaotic place, a Mars-sized protoplanet, now named Theia, is thought to have collided with the young proto-Earth. The cataclysmic impact was powerful enough to melt and vaporize rock and metal from both bodies, flinging a massive cloud of debris into orbit around the newly reformed Earth. Over time, gravity drew this material together, and it coalesced to form the Moon.

This theory is strongly supported by evidence gathered from the hundreds of kilograms of rock and soil brought back by the Apollo missions. These samples revealed that the Moon and Earth share remarkable chemical and isotopic similarities, pointing to a common origin. Had the Moon been a captured object, its composition would likely be very different. At the same time, lunar rocks are significantly depleted in water and other volatile elements that vaporize easily. This is consistent with the intense heat of a giant impact, which would have allowed these elements to escape into space before the Moon formed. This evidence effectively ruled out older theories, such as the idea that the Moon was captured by Earth’s gravity, that it fissioned off a rapidly spinning Earth, or that it formed alongside Earth as a separate body.

The Moon’s surface is a geological archive, preserving a history that has long been erased on Earth by weather and tectonics. It is divided into two primary types of terrain. The bright, rugged, and heavily cratered regions are the lunar highlands. These are the oldest parts of the Moon’s crust, composed mainly of a light-colored rock called anorthosite. Their dense cratering is a record of the “Late Heavy Bombardment,” a period of intense asteroid and comet impacts that occurred early in the solar system’s history.

In stark contrast are the dark, smooth plains known as maria (Latin for “seas”). These are not bodies of water but vast basins that were created by colossal impacts and later flooded by volcanic basalt lava that welled up from the Moon’s interior. Because these lava flows occurred after the main bombardment period, the maria have far fewer craters and are geologically younger than the highlands. This two-toned appearance tells a clear story: a violent beginning of constant impacts, followed by a period of massive volcanic activity. The entire surface is covered by a layer of fine, powdery dust and rock fragments called regolith, created by billions of years of smaller meteorite impacts pulverizing the surface. Beyond its scientific value, the Moon plays a vital role for our planet, moderating the wobble of Earth’s axis and contributing to a more stable climate that has been favorable for the evolution of life.

The Moons of Mars: Captured Asteroids

Mars has two tiny moons, Phobos and Deimos, discovered in 1877 by American astronomer Asaph Hall. They are named for the sons of the Greek god of war, Ares, whose Roman counterpart is Mars; Phobos means “fear” and Deimos means “dread.” These moons are nothing like our own. They are small, lumpy, and irregularly shaped, resembling potatoes more than spheres. Phobos, the larger of the two, is only about 27 kilometers across at its widest point. Their surfaces are dark, heavily cratered, and appear to be made of carbon-rich rock mixed with ice, much like certain types of asteroids.

This composition, along with their irregular shapes, strongly suggests that Phobos and Deimos are not native to Mars. The leading theory is that they are captured asteroids that formed in the main asteroid belt between Mars and Jupiter and were later snared by Martian gravity. This illustrates that planetary systems are not closed; they can acquire new members from other regions of the solar system, leading to satellite systems with very different characteristics from those formed in place.

The orbits of these moons are as unusual as their appearance. Phobos orbits incredibly close to Mars, just 6,000 kilometers above the surface—no other moon in the solar system orbits its planet so closely. It whips around Mars three times a day, moving faster than the planet rotates. An observer on Mars would see Phobos rise in the west and set in the east. This proximity, however, seals its doom. Tidal forces from Mars are causing Phobos to spiral slowly inward at a rate of about 1.8 meters per century. In approximately 40 to 50 million years, it will either be torn apart by Mars’s gravity, forming a temporary ring, or it will crash into the planet’s surface. Deimos, orbiting much farther out, is experiencing the opposite effect and is slowly drifting away from Mars. The Martian system is a clear demonstration that satellite systems are not static. They are dynamic and constantly evolving, with moons being gained, lost, and reshaped by gravitational forces over cosmic timescales.

The Jovian System: A Miniature Solar System

Jupiter, the king of the planets, is orbited by an equally majestic court of moons. With 95 confirmed satellites and more being found, the Jovian system is so vast and diverse that it is often compared to a miniature solar system. The majority of these are small, irregular bodies, likely captured asteroids and comets. However, the system is dominated by four giants, worlds so large and complex that they have revolutionized our understanding of how moons can behave.

The Galilean Moons: Worlds of Wonder

In 1610, Italian astronomer Galileo Galilei pointed his newly improved telescope at Jupiter and discovered four points of light orbiting the giant planet. These were Io, Europa, Ganymede, and Callisto, now known as the Galilean moons. Their discovery was a watershed moment in science, providing the first concrete evidence that celestial bodies could orbit something other than Earth, which was a major blow to the geocentric model of the universe.

The inner three of these moons—Io, Europa, and Ganymede—are locked in a gravitational dance known as an orbital resonance. For every single orbit that Ganymede completes, Europa completes exactly two, and Io completes exactly four. This regular, repeating alignment creates a gravitational tug-of-war that constantly flexes and squeezes the moons’ interiors. This process, called tidal heating, generates immense internal heat, which acts as the engine for their remarkable geological activity. This system provides a perfect natural laboratory for observing how planetary evolution is driven by proximity to a massive parent body, with the intensity of geological activity decreasing with distance from Jupiter.

• Io: The innermost of the four, Io is the most volcanically active body in the solar system. Its surface is a vibrant, chaotic canvas of yellow, red, orange, and black, painted by sulfur compounds and silicate lava from hundreds of active volcanoes. Jupiter’s immense gravity creates tides in Io’s solid rock surface that can rise and fall by as much as 100 meters, generating tremendous frictional heat that keeps its interior molten.
• Europa: Next out from Jupiter is Europa, a world with a surface of bright water ice that is one of the smoothest in the solar system. This icy shell is crisscrossed by a network of dark lines and ridges, but it has very few impact craters, suggesting the surface is geologically young. Beneath this icy crust, scientists are confident there lies a vast global ocean of liquid saltwater, which may contain more than twice the amount of water in all of Earth’s oceans. With a source of internal heat from tidal flexing and the potential presence of key chemical ingredients, Europa is considered one of the most promising places in the solar system to search for extraterrestrial life.
• Ganymede: The third Galilean moon is Ganymede, the largest moon in the entire solar system—it is even larger than the planet Mercury. Ganymede is a world of contrasts, with a surface divided into two main terrain types: ancient, dark, heavily cratered regions, and younger, lighter-colored areas marked by a complex system of grooves and ridges. This suggests a dynamic geological history involving tectonic activity. Most remarkably, Ganymede is the only moon known to generate its own magnetic field, likely created by a convecting liquid iron core.
• Callisto: The outermost of the Galilean moons, Callisto, stands in stark contrast to its siblings. Its surface is ancient and extremely heavily cratered, appearing to have changed very little since the solar system’s formation. It is a dark, icy body that has experienced far less tidal heating and shows few signs of the geological activity that reshaped the other Galilean moons. Callisto serves as a pristine record of the intense bombardment that characterized the early solar system, a history that has been largely erased on its more active neighbors.

The Saturnian System: Rings and Remarkable Moons

Saturn is not just famous for its spectacular rings; it is also the “Moon King” of our solar system, with 274 confirmed moons in its orbit. This diverse family ranges from the planet-sized Titan to tiny, irregularly shaped moonlets that look like potatoes, ravioli, or sponges. While most are small, icy bodies, two of Saturn’s moons, Titan and Enceladus, have fundamentally changed our understanding of where the conditions for life might exist.

Titan: An Earth-Like World

Saturn’s largest moon, Titan, is a truly exceptional world. It is the second-largest moon in the solar system and is unique in being the only one with a thick, substantial atmosphere. Denser than Earth’s, Titan’s atmosphere is composed mostly of nitrogen, just like our own, with a small amount of methane. This atmosphere is so thick and hazy that it obscures the surface from view in visible light.

Beneath the orange haze lies a landscape that is hauntingly familiar. Titan is the only other place in the solar system known to have stable bodies of liquid on its surface. But at surface temperatures of around -179 degrees Celsius, this liquid is not water. Instead, Titan has a complete, active methane-based hydrological cycle. Methane clouds form, rain liquid methane and ethane, which then flows across the surface in rivers, carving channels and filling vast lakes and seas. The surface itself is a frigid world where water ice is as hard as rock. Vast fields of dunes, similar to those in Earth’s deserts, stretch across the equatorial regions, but the “sand” is composed of dark, solid hydrocarbon grains that have settled out of the atmosphere.

As if this weren’t enough, there is strong evidence that beneath its icy crust, Titan harbors a deep, global ocean of liquid water. This presents the tantalizing possibility of two distinct potential habitats for life on one world: a surface environment with liquid hydrocarbons, which might support “life as we don’t know it,” and a subsurface water ocean that could host “life as we know it.” These features make Titan one of the most compelling targets for future exploration.

Enceladus: The Ocean World

While Titan is a giant, the small moon Enceladus, only 500 kilometers in diameter, is just as scientifically exciting. Enceladus is the most reflective body in the solar system, its surface coated in a brilliant layer of fresh, clean ice. The reason for this pristine surface became clear when the Cassini spacecraft discovered one of the most dramatic sights in the solar system: massive plumes of water vapor and ice particles erupting continuously from long fissures near the moon’s south pole, nicknamed “tiger stripes.”

These geyser-like jets shoot material hundreds of kilometers into space at high speed. Some of this material falls back to the surface as snow, constantly refreshing its bright coating, while the rest escapes to form Saturn’s faint E-ring. Cassini flew directly through these plumes and sampled their composition. It found water vapor, ice particles, salts, silica, and a variety of simple organic molecules. This chemical cocktail is strong evidence that the plumes are erupting from a global subsurface ocean of liquid saltwater. The presence of silica nanoparticles, in particular, points to ongoing hydrothermal activity on the ocean floor, where hot, mineral-rich water could be venting from the moon’s rocky core. On Earth, such hydrothermal vents are oases for life, supporting entire ecosystems without sunlight.

The combination of liquid water, organic compounds, and a source of energy from hydrothermal vents means Enceladus possesses all the key ingredients for life as we know it. While some scientists debate whether the plumes originate directly from the ocean or from “mushy zones” of partially melted ice within the crust, the existence of this accessible ocean world has made Enceladus a top-priority destination in the search for life beyond Earth. Together, Titan and Enceladus show that the traditional “habitable zone” based on proximity to the Sun is too narrow a concept; internal heating can create life-friendly environments in the cold, distant reaches of the solar system.

Other Major Moons of Saturn

Beyond the two superstars, Saturn has a host of other fascinating, medium-sized icy moons large enough to be spherical.

• Mimas: The innermost of the major moons, Mimas is most famous for a single, enormous impact crater, named Herschel, that is one-third the diameter of the moon itself. This feature gives Mimas a striking resemblance to the “Death Star” from science fiction. Once thought to be a simple, inert ball of ice, recent data suggests Mimas may also hide a subsurface ocean, making its lack of geysers compared to Enceladus a major puzzle.
• Iapetus: This moon is a world of stark duality. Its trailing hemisphere is as bright as snow, while its leading hemisphere is as dark as charcoal. This two-toned appearance was a mystery for centuries. The solution appears to be that Iapetus is sweeping up dark, reddish dust from the ring of another, more distant moon, Phoebe. This material coats its forward-facing side, like bugs on a windshield.
• Rhea: The second-largest of Saturn’s moons, Rhea is an ancient, heavily cratered world composed mostly of water ice and rock. Its surface is scarred with bright, wispy streaks that are thought to be ice cliffs created by tectonic fractures. Evidence also suggests Rhea has a very thin atmosphere of oxygen and carbon dioxide and may even possess its own faint ring system.

The Uranian System: The Literary Moons

Uranus is orbited by 28 known moons, which follow a unique naming convention. Instead of drawing from Greek and Roman mythology, they are named for characters in the works of William Shakespeare and Alexander Pope, earning them the nickname “the literary moons.” The five largest of these moons are fascinating worlds composed of a roughly equal mix of water ice and rock. Their geology reveals a history shaped by both violent external impacts and powerful internal forces, showing that even in this cold, distant region, moons can have dynamic pasts.

The Five Major Moons

Discovered between 1787 and 1948, these five moons present a range of geological histories.

• Miranda: The smallest and innermost of the five, Miranda has one of the most bizarre and varied surfaces in the solar system. It is a chaotic patchwork of different terrains jumbled together, including vast canyons up to 12 times deeper than Earth’s Grand Canyon, terraced layers, and regions of old, cratered plains abutting much younger, grooved landscapes. This strange appearance has led scientists to theorize that Miranda may have been shattered by a catastrophic impact in its past and then haphazardly reassembled from the fragments.
• Ariel: This moon has the brightest and possibly the youngest surface among the major Uranian satellites. It has few large impact craters but many smaller ones, suggesting that some process has resurfaced it relatively recently. Its most prominent features are a vast network of intersecting valleys and fault scarps that crisscross its surface, indicating a history of tectonic activity.
• Umbriel: In contrast to Ariel, Umbriel is the darkest of the five major moons. Its surface is ancient and uniformly covered with large impact craters. It shows few signs of the geological activity seen on its neighbors, with one notable and mysterious exception: a bright, ring-like feature on one side of the moon, the nature of which is still unknown.
• Titania: The largest of Uranus’s moons, Titania’s surface tells a story of both external impacts and internal evolution. It is covered with numerous small craters and a few larger impact basins. More significantly, it is cut by a system of enormous canyons and fault scarps, some stretching for over 1,500 kilometers. These features are evidence that the moon’s interior expanded at some point in its history, cracking the rigid outer crust.
• Oberon: The outermost of the five, Oberon appears to be a classic, ancient icy world. Like Jupiter’s moon Callisto, its surface is old, heavily cratered, and shows very little evidence of internal activity. The floors of many of its largest craters are covered with a dark, unidentified material, possibly a mixture of ice and carbonaceous compounds.

The Neptunian System: A Captured Giant

The system of 16 moons orbiting Neptune is unlike those of Jupiter and Saturn. It lacks a family of large, co-formed satellites in regular orbits. Instead, it is utterly dominated by a single, massive moon that appears to be an interloper. The story of Neptune’s moons is likely one of ancient violence and disruption, with its current configuration being the aftermath of a dramatic capture event.

Triton: A World in Retrograde

Triton is Neptune’s largest moon and the seventh-largest in the solar system. Its most defining characteristic is its orbit: it travels around Neptune in a retrograde direction, opposite to the planet’s rotation. This is a telltale sign that Triton did not form alongside Neptune. Large moons that co-form with their planet always orbit in the same direction as the planet’s spin. A retrograde orbit is a clear indication of capture. Scientists believe Triton was once a dwarf planet orbiting the Sun in the Kuiper Belt, much like Pluto, before it was gravitationally captured by Neptune billions of years ago.

The energy released during this violent capture would have been immense, likely disrupting any original satellite system Neptune may have had, ejecting some moons and causing others to collide and shatter. Triton, therefore, is not just a moon but the “smoking gun” of a transformative event that completely reset Neptune’s family of satellites.

Despite its frigid surface temperature of around -235 degrees Celsius, Triton is a geologically active world. When the Voyager 2 spacecraft flew by in 1989, it discovered cryovolcanism on the surface—geysers erupting plumes of nitrogen gas and dark dust several kilometers into Triton’s extremely thin nitrogen atmosphere. Its surface is relatively young and sparsely cratered, featuring complex terrains like its famous “cantaloupe terrain,” which resembles the skin of a melon. This indicates that geological processes have reshaped and resurfaced the moon over time.

Neptune’s Other Satellites

Neptune’s other moons are much smaller and less prominent.

• Nereid: Discovered in 1949, Nereid is notable for having one of the most eccentric (elongated) orbits of any moon in the solar system. Its distance from Neptune varies by a factor of seven, suggesting its orbit was also severely disrupted, likely during Triton’s capture.
• Proteus: Neptune’s second-largest moon was not discovered until the Voyager 2 flyby. It is a dark, irregular, and heavily cratered body, shaped more like a lumpy polyhedron than a sphere.

Moons of the Dwarf Planets

The existence of moons is not limited to the eight major planets. Far out in the Kuiper Belt, the icy realm beyond Neptune, many dwarf planets also host their own satellite systems. These distant moons are not just celestial companions; they are invaluable scientific tools. By observing a moon’s orbit, astronomers can apply Kepler’s laws to calculate the mass and density of its parent body, transforming a distant point of light into a world with measurable physical properties.

The Pluto System: A Family Born of Collision

Pluto, the most famous dwarf planet, is orbited by a family of five moons: Charon, Styx, Nix, Kerberos, and Hydra. This complex system is believed to have formed from a giant impact between Pluto and another large Kuiper Belt object early in the solar system’s history, a process similar to the formation of Earth’s Moon.

Pluto’s largest moon, Charon, is truly its partner. It is about half the size of Pluto, making it the largest moon relative to its parent body in the solar system. The two are tidally locked and orbit a common center of mass that lies in the space between them, leading many to refer to Pluto-Charon as a binary planet system.

The four smaller moons orbit this binary pair. Recent observations have revealed that Nix and Hydra have particularly unusual surfaces. They are composed of a strange mix of materials, including abundant water ice, ammonia, and a reddish organic material similar to that found on Charon. This unique chemical blend, seen nowhere else, provides further clues about the composition of the debris disk from which these moons formed.

Satellites of Other Distant Worlds

Pluto is not the only dwarf planet with a moon. Several others in the Kuiper Belt are known to have satellites, demonstrating that these systems are common in the outer solar system.

• Eris: This distant dwarf planet, whose discovery prompted the debate that led to the reclassification of Pluto, has a single small moon named Dysnomia. The discovery of Dysnomia was scientifically vital. It allowed astronomers to calculate the mass of Eris, confirming that it was indeed more massive than Pluto and solidifying its status as a significant object in the outer solar system.
• Haumea: This oddly elongated, rapidly spinning dwarf planet has two moons, Hiʻiaka and Namaka. They are named for the daughters of the Hawaiian goddess of fertility. Like the other dwarf planet moons, their orbits provided the first accurate measurements of Haumea’s mass.

Summary

The hundreds of moons orbiting the planets and dwarf planets of our solar system represent a breathtaking diversity of worlds. They are far more than just passive companions. They are geologically active planets, potential abodes for life, and pristine archives of ancient history. Their origins are varied, from the gentle co-formation alongside their parent planets to the violent chaos of giant impacts and gravitational capture.

In the outer solar system, moons like Europa and Enceladus have shattered the old paradigm of a sun-dependent habitable zone, revealing that the internal heat generated by tidal forces can create liquid water oceans far from our star’s warmth. Titan presents an even more alien possibility, with its frigid, Earth-like landscape of liquid methane offering a potential stage for life based on a completely different chemistry.

Even the smallest moons serve a purpose, acting as scientific tools that allow us to weigh distant worlds and understand their composition. From our own familiar Moon, which preserves the story of the solar system’s violent youth, to the captured interloper Triton, which tells a tale of planetary disruption, each moon adds a unique and indispensable chapter to the grand narrative of our solar system’s formation and evolution. They are a testament to the fact that our cosmic neighborhood is filled with a universe of worlds, each waiting to be explored.