The International Astronomical Union’s 2006 definition of a planet demoted Pluto, dropping the consensus number of planets in our solar system from nine to eight.
The word “planet” originally described “wanderers”. According to IAU, “planet” is defined as a celestial body that 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 shape and has cleared the neighborhood around its orbit. The only difference between a planet and a dwarf planet is that the latter haven’t cleared the neighborhood around its orbit.
Icy, rocky Pluto had been the smallest of the nine planets, its diameter under three quarters of the moon and nearly a fifth of Earth. Currently, Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
That portion of IAU’s 2006 formula- which required that a planet and its satellites move alone through their orbit- excluded Pluto. Otherwise, Pluto fits the IAU definition: It orbits the Sun and it is massive enough that the forces of gravity have made it round. If a planet is far away from the Sun, its size needs to be bigger to clear it’s orbit. If Earth revolved around the sun in Uranus’ orbit, it wouldn’t be able to clean out its path and would thus not qualify as a planet.
As of now, Pluto have gained its status as the largest object in the outer solar system. The question that arises now is that if astronomers consider giving it back its former title —a full-fledged planet?
The battle over Pluto’s planethood may be more semantic than anything else. This issue is about something more than one’s childhood nostalgia. What matters in the end isn’t whether Pluto is a planet or not. What matters is that we consider our world carefully. That we think not just about what Pluto is like as a celestial body, but about how we think about celestial bodies and their place in the universe. It’s about deciding what’s important.