What planet's largest moons did Galileo discover?

Galileo discovered the largest moons of Jupiter.

Using an improved refracting telescope, Galileo began observing Jupiter’s moons in January 1610. On 7 January he saw objects near Jupiter that first appeared to be fixed stars. Their positions changed from night to night, revealing that they were orbiting the planet rather than remaining stationary in the background.

The four moons are Io, Europa, Ganymede, and Callisto. Together they are called the Galilean moons. Galileo initially named them the Medicean Stars in honor of Cosimo II de’ Medici and his brothers; the names used today were proposed by Simon Marius, who independently observed the moons around the same time.

The discovery challenged the idea that every celestial body revolved around Earth. It showed that a planet could have its own orbiting bodies, providing important evidence against the traditional geocentric model and supporting the growing case for Copernican astronomy.