“Now that I have a reasonable-resolution global color view of Pluto,” writes Emily Lakdawalla, “I can drop it into one of my trademark scale image montages, to show you how it fits in with the rest of the similar-sized worlds in the solar system: the major moons and the biggest asteroids.”
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.
It's just an accident that Pluto wound up next to Iapetus and Triton, which I think are the two best analogs for what we can see on Pluto's surface. Yet Pluto stands out for its uniquely ruddy color. Charon, too, is unique, for its dark pole, but there are similarities to the similar-sized worlds on the left side of the diagram: Ariel and Dione in particular.
These are the not-planets. Their non-planetary status is a handicap because these are the worlds that we need to get Earthlings excited about exploring. Titan's strange hydrology -- Enceladus' geysers -- the subsurface oceans of Europa and Ganymede -- the dynamic surfaces of Triton and Pluto. And beyond all the worlds pictured here, there are hundreds of Kuiper belt objects that I would include on this montage if we had ever visited them up close. But we haven't yet. So much undiscovered country yet to explore -- but they're all worlds that much of the public is not familiar with.
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.