What collective name is given to Io, Europa, Ganymede, and Callisto?

Io, Europa, Ganymede, and Callisto are collectively called the Galilean moons, or Galilean satellites. They are Jupiter’s four largest moons and were the first moons discovered orbiting a planet other than Earth.

Galileo Galilei observed them with an improved telescope in January 1610. Their changing positions beside Jupiter showed that they were not fixed stars but bodies circling another world. This observation challenged the Earth-centered Ptolemaic model and supplied powerful support for a Solar System in which objects could orbit something besides Earth.

The moons are ordered outward from Jupiter as Io, Europa, Ganymede, and Callisto. Io is intensely volcanic, Europa has a smooth icy surface and likely a subsurface ocean, Ganymede is the largest moon in the Solar System, and Callisto is heavily cratered and ice-rich.

They are not “Trojan moons,” a term associated with objects sharing an orbit near a planet’s stable Lagrange points. “Ptolemaic moons” and “Titanian moons” are not standard collective names for Jupiter’s quartet.