Ganymede is a moon. It is the largest moon in our solar system but, as you can see in the photo, it is nowhere near as big as its mother planet, Jupiter. Jupiter has three moons, the others are Io and Europa.
Ganymede is 7% of the size of Earth, though it is a little larger than Mercury. It takes about a week to orbit Jupiter. It was discovered by Galileo in 1610. Is there any chance of life? Not as we know it; the surface temperature is about -160C!