Montage by Emily Lakdawalla. The Moon: Gari Arrillaga. Other data: NASA/JPL/JHUAPL/SwRI/UCLA/MPS/IDA. Processing by Ted Stryk, Gordan Ugarkovic, Emily Lakdawalla, and Jason Perry.
The solar system contains dozens of objects that are large enough for self-gravity to make them round, and yet are not considered planets. They include the major moons of the planets, one asteroid, and many worlds in the Kuiper belt. The ones that we have visited with spacecraft are shown here to scale with each other. A couple of items on here are not quite round, illustrating the transition to smaller, lumpier objects.