Are moons always smaller than planets? Moons are always smaller than the planet that they orbit (move around). A smaller body always orbits around a larger body rather than the other way around because the larger body has more gravity. However, not all of the moons are smaller than all of the planets. There are seven moons in our solar system, including our own moon, that are larger than Pluto. Jupiter's moon Ganymede, the largest moon in the Solar System, and Saturn's moon Titan are both larger than Mercury and Pluto. Earth's moon, Jupiter's moons Callisto, Io, and Europa, and Neptune's moon Triton are all larger than Pluto, but smaller than Mercury.