The ten biggest natural satellites orbiting the Sun's planets, ranked by mean diameter using NASA/JPL figures.
Quick answer: 1. Ganymede, 2. Titan, 3. Callisto, 4. Io, 5. Moon, 6. Europa, 7. Triton, 8. Titania, 9. Rhea, 10. Oberon.
Our solar system has hundreds of known moons, but only a handful are true giants. The largest, Jupiter's Ganymede, is wider than the planet Mercury, and the top two, Ganymede and Saturn's Titan, both exceed 5,000 kilometers across. This list ranks the ten largest moons by mean diameter. Every figure is derived from NASA's Jet Propulsion Laboratory Planetary Satellite Physical Parameters database, the authoritative source for satellite radii, and cross-checked against NASA's individual moon fact pages.
Jupiter's Ganymede is the largest moon in the solar system and the only one known to generate its own magnetic field. It is bigger than the planet Mercury and, if it orbited the Sun, would be considered a planet.
Saturn's Titan is the second-largest moon and the only one with a thick atmosphere, denser than Earth's. It hosts lakes and seas of liquid methane and ethane on its frigid surface.
Jupiter's Callisto is the third-largest moon and one of the most heavily cratered objects in the solar system. Its ancient, icy surface has changed little in billions of years.
Jupiter's Io is the most volcanically active body in the solar system, with hundreds of erupting volcanoes driven by tidal heating. It is the innermost of the four Galilean moons.
Earth's Moon is the fifth-largest natural satellite and the only one humans have visited. It is unusually large relative to its parent planet and stabilizes Earth's axial tilt.
Jupiter's Europa hides a global saltwater ocean beneath its smooth icy crust, making it a prime target in the search for life. It is the smallest of the four Galilean moons.
Neptune's Triton is the largest moon of the outer ice giants and orbits its planet backwards, a retrograde path suggesting it was a captured Kuiper Belt object. It has active nitrogen geysers.
Uranus's Titania is the largest of that planet's moons, a mix of ice and rock marked by huge canyons and fault valleys. It was discovered by William Herschel.
Saturn's Rhea is the planet's second-largest moon, a heavily cratered ball of ice and rock. It is slightly larger than Uranus's Oberon.
Uranus's Oberon is the outermost of the planet's large moons and, by a narrow margin, the tenth-largest in the solar system. Its dark, cratered surface hints at a mix of ice and rock.
| # | Name | parent_planet | orbital_period_days | discovered | mass_relative_to_moon |
|---|---|---|---|---|---|
| 1 | Ganymede | Jupiter | 7.15 | 1610 (Galileo Galilei) | 2.02 |
| 2 | Titan | Saturn | 15.95 | 1655 (Christiaan Huygens) | 1.83 |
| 3 | Callisto | Jupiter | 16.69 | 1610 (Galileo Galilei) | 1.46 |
| 4 | Io | Jupiter | 1.77 | 1610 (Galileo Galilei) | 1.21 |
| 5 | Moon | Earth | 27.32 | Prehistoric (known since antiquity) | 1 |
| 6 | Europa | Jupiter | 3.55 | 1610 (Galileo Galilei) | 0.65 |
| 7 | Triton | Neptune | 5.88 | 1846 (William Lassell) | 0.29 |
| 8 | Titania | Uranus | 8.71 | 1787 (William Herschel) | 0.048 |
| 9 | Rhea | Saturn | 4.52 | 1672 (Giovanni Cassini) | 0.031 |
| 10 | Oberon | Uranus | 13.46 | 1787 (William Herschel) | 0.041 |
Moons are ranked by mean diameter in kilometers, computed as twice the mean radius published in NASA/JPL's Planetary Satellite Physical Parameters database. Only natural satellites of the solar system's eight planets are included (dwarf-planet moons such as Pluto's Charon are excluded). Where NASA's published rounded fact-sheet value differs slightly from the JPL radius, the difference is within measurement uncertainty.
Ganymede's mean diameter (about 5,268 km) is roughly 120 km larger than Titan's (about 5,150 km) per NASA/JPL figures. Titan looks bigger in some comparisons because its thick atmosphere extends far above its solid surface, but the ranking here uses solid-body diameter.
Yes. Ganymede and Titan are both larger than the planet Mercury (4,879 km in diameter), though they are far less massive because they are made largely of ice.
This list covers moons of the solar system's eight planets. Pluto is classified as a dwarf planet, so its largest moon Charon (about 1,212 km) is excluded even though it is sizable.
Very close. NASA/JPL gives Rhea a mean radius of about 763.8 km and Oberon about 761.4 km, a difference of only a few kilometers in diameter. Rhea is the larger of the two and takes the ninth spot.