Written by: Davan Mulligan
Pluto was once known as the ninth and smallest planet in our solar system. It is smaller than the moon, with a width of 2380km. Pluto is so far away from the sun that it takes 248 Earth years to orbit it. In February of 1930, Clyde Tombaugh discovered Pluto, naming it a planet. In August of 2006, the International Astronomical Union (IAU) demoted Pluto from that status. This is largely due to the fact that in 2005, Mike Brown and his team discovered Eris, a body in the Kuiper belt (a ring of icy objects that circles our solar system) that had 27% more mass than Pluto. After the discovery of similar bodies in the Kuiper belt, it became apparent that a clear definition of a planet needed to be explored.
The IAU held a meeting to debate the status of Pluto and what features are required for a celestial body to be classified as a planet. Essentially, there were two logical options: keep Pluto a planet, and name Eris a planet as well, or demote Pluto. The first option would have also meant that Ceres, a small body in the Kuiper belt, and Charon, a moon of Pluto, would need to be classified as planets as well. Choosing the first option would have brought our solar system’s planet count to twelve. The IAU selected the second option.
This ruling created three requirements, as outlined by the IAU, to classify a celestial body as a planet. The body must: (1) have an orbit around the sun; (2) possess enough mass to assume hydrostatic equilibrium (have a near-spherical shape); and (3) have its gravity clear its surroundings from other celestial bodies apart from its own satellite(s). Pluto meets only two of these requirements. As it is too small, Pluto does not have strong enough gravity to clear its surroundings from other celestial bodies. Pluto shares its “orbital neighbourhood” with objects in the Kuiper belt.
To be a classified as a “dwarf planet”, a celestial body only needs to meet the first two of the above-mentioned requirements. Thus, Ceres, Eris, and Pluto are all classified as dwarf planets. Charon is not classified as a dwarf planet, however, because it orbits Pluto. Charon is almost half the size of Pluto, and Pluto-Charon is sometimes referred to as a double planetary system; they almost orbit around each other. However, there is no official double-planet system classification for dwarf planets. Hence, Charon is classified as a moon of Pluto.