12. Jupiter has 67 moons, Saturn 62, but Neptune a relatively small 13. Which of Neptune's moons is the largest?
From Quiz Neptune -- Furthest Planet from the Sun
Triton is enormous compared to all of the other moons of Neptune. With a radius of 1,530 km, it is about 80% of the size of our Moon (1,737 km). By comparison, the second-largest of Neptune's moons, Proteus, has a radius of jut 210 km. Triton accounts for 99.5% of the total mass of all 13 of Neptune's moons.
Triton is the seventh-largest satellite in the Solar System, although it is almost exactly half the size of the largest, Jupiter's Ganymede, which has a radius of 2,634 km. Triton is the only large moon in the Solar System with a retrograde orbit -- i.e. it orbits in a direction opposite to its planet's rotation.
When measured in 1989, Triton was found to be the coldest known place in the Solar System with estimated temperature of -235 C.
Of the alternatives, Callisto is the second-largest of Jupiter's moons, Titania is the largest moon of Uranus, and Proteus is Neptune's second-largest moon.