It’s been some number of years now since the “demotion” of Pluto from a planet to a dwarf planet, according to the new (as of 2006) definition of a planet by the International Astronomical Union. I think most people familiar with the matter agree that trying to be precise on the matter is dumb, and that the definition they arrived at is also dumb. Requirement 1 is that the planet is a body in orbit around the Sun, which is a bit too local as there are now thousands of known extrasolar planets. Requirement 3 is just confusing: that the body has “cleared the neighborhood” about its orbit. Like, what? Who cares?
The one part of the definition that holds some sway purely for reasons of symmetry and beauty is that the body be large enough that its gravitational field can form it into a roughly spherical shape (hydrostatic equilibrium). Not that asteroids and comets aren’t interesting themselves, but it’s not unreasonable I think for us to pay more attention to the larger bodies, including the larger moons. I’ve been using the word worlds to loosely talk about such objects. Is it big enough to become round? Then it’s a world and worthy of our attention.
If the planet non-issue wasn’t already dead and buried, I’d expect the bandages to be freshly torn from the wound as the family of our solar system, defined as those worlds we have good images of, will welcome two new members in 2015: Pluto and Ceres.
Both worlds are large enough to be spherical, and we only have fuzzy images of them as of this writing. However, each is getting an up-close-and-personal visit from human robotic emissaries in 2015: New Horizons flying by Pluto while Dawn will actually enter into an orbit about Ceres. Notably, as the spacecraft get closer and closer to their respective target worlds, the images will get better and better: New Horizons will beat Hubble’s resolution by early May and Dawn will beat, well, Hubble’s resolution again by around January 26.
And if that wasn’t cool enough, we should also remember that Pluto really forms a twin-world system with its moon Charon, and we’ll be getting high resolution imagery of it too. I wasn’t old enough to share in the excitement of the Pioneer and Voyager missions making our small part of the universe knowable; I’ve always had access to good quality color images of the outer worlds. Very soon now, young children will just always have been familiar with Pluto and Ceres as well.