Pluto, the planet that happens to share a name with Mickey Mouse’s dog, was demoted on this day exactly sixteen years ago. The fateful decision that changed the course of Pluto’s history was made by the Planet Definition Committee of the International Astronomical Union.

The main catalyst, of course, was our genius. Through our improvement of technology and better and better telescopes and astronomical measuring devices, we found Eris, another dwarf planet, that happened to be even larger than Pluto. This “unfairness” sparked so much debate that the Planet Definition Committee was formed, a committee with three ideas in mind: allowing both Eris and Pluto to be a planet by making the definition of the planet mass-based, allowing just Pluto to be a planet with the amazing reasoning of “just because,” or making neither of them a planet with their new definition. As proven by the title of this article containing “Demotion,” they chose the last option. The scientific definition of a planet that they decided on is as follows:

A planet is a celestial body that

1. is in orbit around the Sun
2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
3. has cleared the neighborhood around its orbit

Number 3 is the place where Pluto failed, as shown by its being only 0.07 times the mass of objects in its orbit, but Pluto perfectly fits the definition of a dwarf planet, which is shown below:

A “dwarf planet” is a celestial body that

1. is in orbit around the sun
2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
3. has not cleared the neighbourhood around its orbit
4. is not a satellite

Although Pluto was demoted, we can rest easy knowing that Pluto has changed the world of astronomy for the better by sparking the debate of what it truly means to be a planet. Happy Pluto Demotion Day!