Which planet did Galileo name as hosting four of the largest moons, including Ganymede and Io?

• A. Uranus
• B. Jupiter
• C. Saturn
• D. Neptune

Answer: (B)

Galileo’s observations revealed that planets can have their own systems of moons. Using a telescope, he saw four moons actually orbitting Jupiter: Io, Europa, Ganymede, and Callisto. Since Ganymede and Io are two of those moons, the planet that hosts these four is Jupiter. This discovery helped show that not everything orbits Earth and contributed to the shift toward a heliocentric view of the solar system. The other planets listed don’t host Io and Ganymede, so Jupiter is the correct host.