On this day in 2006, the International Astronomical Union (IAU) approved a new definition of “planet” that excluded Pluto. The vote at the IAU approved the following definition of a planet:
- is in orbit around the Sun,
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
- has cleared the neighborhood around its orbit.
Pluto fails this last test. In fact, many other objects, including at least one that may be larger (Eris), lie in the same orbit as Pluto around the Sun. However, Pluto does meet the definition of a “dwarf” planet:
- is in orbit around the Sun,
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
- has not cleared the neighborhood around its orbit, and
- is not a satellite.
Hence, Pluto meets this definition whereas Europa, which is larger than Pluto, does not because it fails the last two tests – it has not cleared the neighborhood and it is a satellite.
The IAU vote was not popular. As one editorial argued, the third criteria about clearing the neighborhood makes little sense:
That last criterion states that a planet must be the gravitationally dominant object in the area of space in which it orbits. This rule makes sense for somewhere like, say, Earth, which is far more massive that the Moon and anything else along its orbital path. But out in the Kuiper Belt, where neighbouring bodies are far, far more distant than in the inner Solar System, Earth would not necessarily be able to clear its neighbourhood.
He continues:
This argument predates the flyby of Pluto in July 2015 of NASA’s New Horizons spacecraft, but the images returned by that spacecraft really help make the case: Pluto is an enigmatic world with towering ice mountains, vast glaciers of nitrogen ice, a tenuous atmosphere, a thick, outer icy carapace and a probably liquid water ocean below, all atop a huge rocky interior. By any geological measure – including the fact that there are surface processes acting on Pluto today – Pluto is a planet.
This will get more interesting once we throw in the exoplanets. Stay tuned.
Our Awesome Universe 