How Many Moons Are in the Universe?

We recognize the moon seen from Earth, but any space object orbiting a planet is called a moon. Planets other than Earth in the solar system also have moons. The largest moon is Ganymede of Jupiter, which has a diameter of 5,261 kilometers—bigger than Mercury. Among all moons in the solar system, only Saturn’s moon Titan has an atmosphere. Jupiter’s moon Io is the hottest moon and has many volcanoes. Neptune’s moon Triton is the coldest moon with temperatures of minus 235 degrees Celsius.

There are different ideas about Earth's moon. Some astronomers think it is a piece broken off from Earth, while others believe it is a separate planet linked to Earth. There are believed to be over 150 moons in the solar system. Astronomers keep discovering new moons by observation. Jupiter has the most moons—63 have been discovered one by one.