Under a size cutoff of 10,000 kilometers, there are two planets, 18 or 19 moons, 1 or 2 asteroids, and 87 trans-Neptunian objects, most of which do not yet have names. All are shown to scale, keeping in mind that for most of the trans-Neptunian objects, their sizes are only approximately known. Montage by Emily Lakdawalla. Data from NASA / JPL, JHUAPL/SwRI, SSI, and UCLA / MPS / DLR / IDA, processed by Gordan Ugarkovic, Ted Stryk, Bjorn Jonsson, Roman Tkachenko, and Emily Lakdawalla.
Ever since it was kicked out of the family of planets of our solar system, it's been trying to get back in. This time, in an effort spearheaded by Kirby Runyon of Johns Hopkins University, Pluto wants itself and its 100 closest pals to be called planets.
Runyon proposed that a planet is redefined to focus on its own geophysics: "A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has enough gravitation to be round due to hydrostatic equilibrium regardless of its orbital parameters."
In a scientific poster submitted to the upcoming Lunar and Planetary Science Conference, Runyon pointed out that this new definition of planet would emphasize its intrinsic as opposed to extrinsic properties. For school children and lay people, the definition of planets is easy: "Round objects in space that are smaller than stars."
But what about memorizing the names of all those new planets? No need to do that, Runyon said, instead schools should focus on teaching Solar System's zones and why different types of planets formed at their respective distances from the Sun.
Read more about the new effort to reclassify Pluto as a Planet over at Universe Today.