The short answer depends on when the count was made and what qualifies as a confirmed moon. A useful dated snapshot is the 2023 inventory: it counted 285 known natural satellites orbiting the eight planets. Most were not little versions of Earth’s Moon. They range from small, irregular objects to worlds larger than Mercury, with very different landscapes and histories. Because astronomers keep finding and confirming small moons, a total without a date can quickly become misleading.
A 2023 count, planet by planet
In that 2023 snapshot, Mercury and Venus had zero known moons. Earth had one, Mars had two, Jupiter had 95, Saturn had 146, Uranus had 27 and Neptune had 14. Add those figures and the total is 285. The giant planets account for nearly all of it: Jupiter, Saturn, Uranus and Neptune together had 282 known moons, while the four inner planets had three.
These are inventory figures, not a claim that each planet has a fixed, final number of moons. Counts can change when observations reveal faint objects, when follow-up work establishes an orbit, or when an object’s status is reassessed. Different lists can also reflect different cut-off dates or criteria for counting a moon as confirmed. The total is therefore best understood as “known and counted in this dated inventory,” rather than the number of every moon that exists.
The imbalance has a physical explanation. The giant planets are much more massive than Earth and can hold many objects in orbit; their outer regions also contain populations of small moons. Many are irregular in shape and travel on distant or tilted paths. The large moon totals do not mean that every moon is comparable to the bright, round one visible from Earth.
The largest moons change the scale
The biggest moons are worlds in their own right. Jupiter’s Ganymede is the solar system’s largest moon, with a diameter of about 5,268 kilometres. It is larger in diameter than Mercury, whose diameter is about 4,879 kilometres. That comparison is surprising because Mercury is a planet and Ganymede is a satellite; what matters for size is the body itself, not its label or what it orbits.
Saturn’s Titan is close behind, at about 5,150 kilometres across. It has a substantial atmosphere and lakes and seas of liquid hydrocarbons on its surface. Ganymede and Titan are both larger than Earth’s Moon, which is about 3,475 kilometres in diameter. Earth’s companion is therefore not close to the top of the size ranking, even though it dominates our night sky and has shaped calendars, tides and human exploration.
Size alone does not tell the whole story. Jupiter’s Io is smaller than Earth’s Moon but is exceptionally volcanically active. Europa, also smaller, is an icy world whose surface and interior make it a major subject of scientific interest. Ganymede has its own magnetic field. These contrasts show why “largest moons” is a useful starting point, not a ranking of scientific importance. For a visual sense of how moons and planets fit into the wider system, the site’s interactive solar system map shows current planet positions.
Jupiter and Saturn: crowded systems
Jupiter’s four largest moons—Io, Europa, Ganymede and Callisto—are often grouped as the Galilean moons, after Galileo Galilei’s observations in 1610. They are large enough to have distinct identities: Io’s volcanic surface, Europa’s ice, Ganymede’s scale and Callisto’s heavily cratered terrain offer four very different records of the same planetary system. They are only a handful within the much larger inventory of Jupiter’s known satellites.
Saturn’s count in the 2023 snapshot was even higher, but the numerical lead does not mean that its moons are all large. A large share of the total consists of small satellites, including objects associated with rings and groups of moons. Saturn also has Titan, a prominent world with a thick atmosphere, and Enceladus, a smaller icy moon known for material erupting from its south-polar region. A count treats each confirmed moon as one entry; it does not communicate its size, activity or scientific interest.
Distant moons and Earth’s close companion
Uranus and Neptune add dozens of moons to the planetary inventory, including a few large ones and many smaller satellites. Neptune’s Triton is notable for orbiting in the direction opposite Neptune’s rotation. That unusual path is one reason scientists think Triton may have been captured rather than forming in its present orbit alongside Neptune. Uranus’s major moons are much smaller than Ganymede and Titan, but they include varied icy and rocky bodies.
Earth’s Moon is modest in a planet-by-planet census, yet its relationship with Earth is unusually prominent. It is large compared with its host planet, and close enough to appear as a bright, detailed object to the naked eye. Its changing phases are a consequence of the Sun lighting different portions of the half of the Moon facing Earth. The guide to Moon phases explains that geometry; a moon phase calendar gives exact phase times by month.
What a moon count can—and cannot—tell you
A planet’s moon total is a snapshot of observation and classification as much as a description of its surroundings. Small, distant moons are harder to detect than large ones, and a newly spotted object needs enough observations to establish that it follows a stable orbit. That is why a familiar table of counts should carry a date, and why a newer inventory may differ without the underlying planetary systems having suddenly changed.
For an observing plan, the count is less useful than a local view of the Moon. Its phase, rising time and position depend on date and location; the New York City moon dashboard, for example, provides phase and illumination along with moonrise, moonset, altitude, azimuth, distance and a sky view with local weather. A solar system tally answers how many moons had been counted in a particular inventory. Looking up tells a different story: which one is above your horizon tonight.