Short answers, pulled from the story.
Jupiter has 115 confirmed moons as of April 2026, when the Minor Planet Center announced fourteen additional moons on the 9th of April 2026. The count continues to grow as new surveys discover fainter and smaller satellites.
Galileo Galilei made the first certain observations of Jupiter's moons in January 1610, spotting the four large Galilean moons with a 20-times magnification telescope. Simon Marius independently discovered them one day later and supplied the names Io, Europa, Ganymede, and Callisto that are still used today.
The four Galilean moons are Io, Europa, Ganymede, and Callisto. Together they contain approximately 99.997 percent of all mass orbiting Jupiter. Ganymede is the largest, ranking ninth in the Solar System and exceeding Mercury in diameter, though not in mass.
Europa, Ganymede, and Callisto are all suspected of having subsurface water oceans based on evidence gathered by the Galileo spacecraft during its mission from 1995 to 2003. The Voyager probes first detected water ice on Europa's surface in 1979.
Io, Europa, and Ganymede are locked in a 1:2:4 orbital resonance: for every orbit Ganymede completes, Europa completes two and Io completes four. Models predict Callisto will be drawn into the resonance in roughly 1.5 billion years, creating a 1:2:4:8 chain.
Irregular moons of Jupiter are named after lovers, sexual partners, or descendants of the Roman god Jupiter or his Greek equivalent Zeus. Names ending in "a" or "o" denote prograde irregular moons, while names ending in "e" denote retrograde ones. This convention was formalized by the International Astronomical Union in 1975.