Moon

A moon, or natural satellite is any small body that orbits a planet. In the Solar System there are 173 known moons, which orbit within six planets.

Mercury and Venus are the only planets in our solar system that don't have moons.

When Galileo discovered Jupiter's four largest moons on January 7, 1610, his observations caused a revolution in astronomy: a planet with smaller planets orbiting it did not conform to the principles of Aristotelian cosmology, which held that all heavenly bodies should circle the Earth.

Ganymede, a moon of Jupiter, is the largest moon in the Solar System. Ganymede is larger in diameter than the smallest planet Mercury. It would be classed as a planet if it were orbiting the Sun rather than Jupiter.

Image of Ganymede's taken by the Galileo orbiter 

One of Jupiter's 62 moons, Europa, is thought to have twice as much water as Planet Earth.

American astronomer Edward Emerson Barnard discovered Amalthea on September 9, 1892 using the 36-inch (91 cm) refractor telescope at Lick Observatory. It was the last planetary satellite to be discovered by direct visual observation (as opposed to photographically) and was the first new moon of Jupiter since Galileo Galilei's discovery of the Galilean satellites in 1610.

Amalthea is a small, irregularly shaped moon with a diameter of about 163 miles (262 kms). It is the fifth largest moon of Jupiter, after the four Galilean moons. Amalthea is composed of dark, rocky material and is thought to be a captured asteroid.

The most detailed existing image of Amalthea 

The Science Master of Eton College Henry Madan, in 1877 suggested the name for the two dwarf moons of Mars, Deimos and Phobos. This was referencing the fact that Deimos and Phobos were twin brothers, the children of the god Ares (Mars in Roman mythology).

Titan is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found.

Titan's atmosphere is so thick and gravity so low that a human could fly through it by flapping any sort of wings attached to their arms.

Titan has oceans of liquid natural gas flowing on its surface and its fuel reserves are so large that they could act as our energy source for thousands of years.

In 2023, astronomers reported that they had detected phosphates on Enceladus, a moon of Saturn. This is a significant discovery, as phosphate is a key ingredient for life as we know it. Phosphorus is essential for the formation of DNA and RNA, the molecules that store genetic information. It is also important for cell membranes and other cellular structures. The discovery of phosphates on Enceladus suggests that the moon may have the potential to support life.

Enceladus is a small moon, but it is geologically active. It has a subsurface ocean that is thought to be liquid water. The moon also has geysers that erupt water vapor and ice particles into space. These geysers are thought to be powered by hydrothermal vents, which are hot springs that form on the ocean floor.

Neptune has 14 known moons, which are named for minor water deities in Greek mythology. By far the largest of them is Triton, discovered by William Lassell on October 10, 1846, just 17 days after the discovery of Neptune itself. Over a century passed before the discovery of the second natural satellite, Nereid.

Triton is unique among moons of planetary mass in that its orbit is retrograde to Neptune's rotation and inclined relative to Neptune's equator. This suggests that it did not form in orbit around Neptune but instead is a dwarf planet that was captured from the Kuiper belt.

Voyager 2 photomosaic of Triton

The next-largest irregular satellite in the Solar System, Saturn's moon Phoebe, has only 0.03% of Triton's mass.

The finding of 243 Ida's moon Dactyl in the early 1990s was the proof that some asteroids have moons; indeed, 87 Sylvia has two.