There are more than 181 moons of the various planets, dwarf planets and asteroid in the solar system. The planets Mercury and Venus do not have any moons and neither does the dwarf planet Ceres.
A moon is defined to be a celestial body that makes an orbit around a planet, including the eight major planets, dwarf planets, and minor planets. A moon may also be referred to as a natural satellite, although to differentiate it from other astronomical bodies orbiting another body, e.g. a planet orbiting a star, the term moon is used exclusively to make a reference to a planet’s natural satellite.
The first moons to be discovered outside of the Earth’s moon were the Galilean moons of Jupiter, named after astronomer and discoverer Galileo Galilei. The moons Io, Europa, Ganymede, and Callisto are Jupiter’s largest and only the first four to be revealed, as to date, the planet has, at least, 67 moons.
The Earth...1 Moon
The fifth largest moon in the solar system, Earth's moon is the only place beyond Earth where humans have set foot. The brightest and largest object in our night sky, the moon makes Earth a more livable planet by moderating our home planet's wobble on its axis, leading to a relatively stable climate. It also causes tides, creating a rhythm that has guided humans for thousands of years. The moon was likely formed after a Mars-sized body collided with Earth.
Earth's only natural satellite is simply called "the moon" because people didn't know other moons existed until Galileo Galilei discovered four moons orbiting Jupiter in 1610.
The moon is farther away from Earth than most people realize. The moon is an average of 238,855 miles (384,400 kilometers) away. That means 30 Earth-sized planets could fit in between Earth and the moon.
The moon is slowly moving away from Earth, getting about an inch farther away each year.
Mars has two small moons: Phobos and Deimos. Phobos (fear) and Deimos (panic) were named after the horses that pulled the chariot of the Greek war god Ares, the counterpart to the Roman war god Mars. Both Phobos and Deimos were discovered in 1877 by American astronomer Asaph Hall. The moons appear to have surface materials similar to many asteroids in the outer asteroid belt, which leads most scientists to believe that Phobos and Deimos are captured asteroids.
Even from Mars, the moons don't look like moons. The more distant moon, Deimos, appears more like a star in the night sky. When it is full and shining at its brightest, it resembles Venus as seen on Earth. Phobos has the closest orbit to its primary of any moon in the solar system, but still only appears a third as wide as Earth's full moon.
Phobos orbits only 3,700 miles (6,000 km) from the Martian ground. Its surface is marred by debris that may have come from impacts on Mars. It travels around the planet three times a day, zipping across the Martian sky approximately once every four hours. The fast-flying moon appears to travel from west to east.
Deimos orbits much farther away, tending to stay 12,470 miles (20,069 km) from the red planet's surface. The moon takes about 30 hours, a little over a Martian day, to travel around its host.
Jupiter...67 - 69 Moons
Jupiter has 53 named moons. Sixteen more have been discovered but not given official status or names. Combined, scientists now think Jupiter has 69 moons. There are many interesting moons orbiting the planet, but the ones of most scientific interest are the first four moons discovered beyond Earth - the Galilean satellites.
The planet Jupiter's four largest moons are called the Galilean satellites after Italian astronomer Galileo Galilei, who first observed them in 1610. The German astronomer Simon Marius claimed to have seen the moons around the same time, but he did not publish his observations and so Galileo is given the credit for their discovery. These large moons, named Io, Europa, Ganymede, and Callisto, are each distinctive worlds.
Io is the most volcanically active body in the solar system. Io's surface is covered by sulfur in different colorful forms. As Io travels in its slightly elliptical orbit, Jupiter's immense gravity causes "tides" in the solid surface that rise 100 m (300 feet) high on Io, generating enough heat for volcanic activity and to drive off any water. Io's volcanoes are driven by hot silicate magma.
Europa's surface is mostly water ice, and there is evidence that it may be covering an ocean of water or slushy ice beneath. Europa is thought to have twice as much water as does Earth. This moon intrigues astrobiologists because of its potential for having a "habitable zone." Life forms have been found thriving near subterranean volcanoes on Earth and in other extreme locations that may be analogues to what may exist on Europa.
Ganymede is the largest moon in the solar system (larger than the planet Mercury), and is the only moon known to have its own internally generated magnetic field.
Callisto's surface is extremely heavily cratered and ancient -- a visible record of events from the early history of the solar system. However, the very few small craters on Callisto indicate a small degree of current surface activity.
Saturn has at least 62 moons (some of which are awaiting confirmation) orbiting it, some of them inside the ring system which helps shape the rings. They range from very tiny worlds of less than a kilometer in diameter to spherical moons such as Titan. Their shapes range from irregular worlds to ellipsoidal to almost completely rounded. The first of Saturn’s moons to be observed was Titan in 1655 by Christiaan Huygens, another moon was not found until 1671 when Giovanni Domenico Cassini discovered Iapetus. Two of Saturn’s moons (Janus & Epimetheus) swap orbits every 4 years.
Titan makes up 96 percent of the mass orbiting the planet. Scientists think that Saturn's system may have originally housed two such moons, but the second broke up, creating the debris that formed the rings and smaller, inner moons. Another theory suggests that the system originally housed several large moons, similar to Jupiter's Galilean moons, but two fused into Titan. The violent collision could have scattered the debris that would have later drawn together into the smaller moons.
Some of the moons travel inside the gaps of the rings, clearing paths through the debris. Others orbit farther out. Several of the moons interact with one another, with their orbits moving in resonance with each other. Larger moons may trap smaller moons, keeping them nearby. Sixteen of the moons are tidally locked, with one face permanently turned toward Saturn.
While most of the satellites orbiting other planets take their names from Greek mythology, Uranus' moons are unique in being named for Shakespearean characters, along with a couple of the moons being named for characters from the works of Alexander Pope.
Oberon and Titania are the largest Uranian moons, and were first to be discovered -- by William Herschel in 1787. William Lassell, who had been first to see a moon orbiting Neptune, discovered the next two, Ariel and Umbriel. Nearly a century passed before Gerard Kuiper found Miranda in 1948. And that was it until a NASA robot made it to distant Uranus.
The Voyager 2 spacecraft visited the Uranian system in 1986 and tripled the number of known moons. Voyager 2 found an additional 10, just 26-154 km (16-96 miles) in diameter: Juliet, Puck, Cordelia, Ophelia, Bianca, Desdemona, Portia, Rosalind, Cressida and Belinda.
Since then, astronomers using the Hubble Space Telescope and improved ground-based telescopes have raised the total to 27 known moons. Spotting the post-Voyager moons is an impressive feat. They're tiny -- as little as 12-16 km (8-10 miles) across, and blacker than asphalt. And of course, they're about 2.9 billion km (1.8 billion miles) away from the Sun.
Triton (not to be confused with Saturn's moon, Titan), is far and away the largest of Neptune's satellites. Dutch-American astronomer Gerard Kuiper (for whom the Kuiper Belt was named) found Neptune's third-largest moon, Nereid, in 1949. He missed Proteus, the second-largest, because it's too dark and too close to Neptune for telescopes of that era. Proteus is a slightly non-spherical moon, and it is thought to be right at the limit of how massive an object can be before its gravity pulls it into a sphere.
Proteus and five other moons had to wait for Voyager 2 to make themselves known. All six are among the darker objects found in the solar system. Astronomers using improved ground-based telescopes found more satellites in 2002 and 2003, bringing the known total to 13.
Voyager 2 revealed fascinating details about Triton. Part of its surface resembles the rind of a cantaloupe. Ice volcanoes spout what is probably a mixture of liquid nitrogen, methane and dust, which instantly freezes and then snows back down to the surface. One Voyager 2 image shows a frosty plume shooting 8 km (5 miles) into the sky and drifting 140 km (87 miles) downwind.
Neptune's gravity acts as a drag on the counter-orbiting Triton, slowing it down and making it drop closer and closer to the planet. Millions of years from now, Triton will come close enough for gravitational forces to break it apart -- possibly forming a ring around Neptune bright enough for Lassell to have seen with his telescope.
Pluto, the dwarf planet that was once considered the ninth planet, has a growing entourage of satellites. The tiny world has five moons of varying size in orbit around it that tumble and dance in a strange and chaotic pattern.
Pluto has one very large moon that is almost half the planet’s size. Discovered in 1978, it was named Charon after the demon who ferried souls to the underworld in Greek mythology. (Pluto is named for the god of the underworld.) The huge size of Charon (750 miles or 1,200 kilometers in diameter) sometimes leads scientists to refer to Pluto and Charon as a double dwarf planet or binary system. Pluto's diameter is 1,473 miles (2,370 km).
Pluto and Charon are just 12,200 miles (19,640 km) apart, less than the distance by flight between London and Sydney, Australia. Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation — a Pluto day — also takes 6.4 Earth days. This means Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking.
In 2005, as scientists photographed Pluto with NASA's Hubble Space Telescope in preparation for the New Horizons mission — the first spacecraft to visit Pluto and the Kuiper Belt — they discovered two other tiny moons of Pluto, now dubbed Nix and Hydra. These are two to three times farther away from Pluto than Charon.