Why is Pluto no longer a planet?
Pluto once used to be the ninth planet of our Solar System. But it’s no longer a planet, at least by official IAU definitions. This is the story of how Pluto was nerfed from the title of being a planet.
Pluto was first discovered in 1930 by US astronomer Clyde Tombaugh. It was found 4 Billion miles away from the Earth, inside Kepler’s belt. Pluto was known for its methane and nitrogen-rich icy glaciers. It’s around one-sixth of Earth’s size. Pluto has 5 moons with one of them, Charon, being almost half as big as Pluto.
Scientists had always debated regarding the status of Pluto since its discovery. But since the International Astronomy Unit had no exact definition for what a planet was supposed to be, Pluto continued to be a planet for a long while.
But in the 2000s, we started discovering new astronomical objects inside Kepler’s belt where Pluto was located. Quaoar (announced in 2002), Sedna (2003) were the first ones and then in 2005 came the discovery of Eris.
Eris changed how we perceived Pluto and it also opened up the possibility of finding more objects like Eris inside Kepler’s belt. To make matters worse Eris was larger than Pluto in size. If we were to accept Pluto, then Eris would have to be made the tenth planet of our solar system.
As the scientific community became divided, in August 2006, IAU General Assembly came together and one of the discussions was regarding a definition of the term planet. It was here a new definition was adopted. On 24th August it became a resolution through voting by the members of the assembly.
The new definition was this:
A planet is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighborhood around its orbit.
Pluto satisfied the conditions a and b. But failed at C as there are numerous other objects floating around in the orbit of Pluto. The argument here was that for an object to be considered as a planet it has to exhibit gravitational dominance in its orbit. As a result, Pluto lost its title as a planet.
That is how Pluto and other objects discovered in Kepler’s belt became the Dwarf planets.
According to IAU, a “Dwarf planet” is a celestial body that,
(a) is in orbit around the sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
(c) has not cleared the neighborhood around its orbit, and
(d) is not a satellite.
Many scientists and astronomers still do not accept the new definition and treat Pluto as a planet. One of their key argument is that voting something to establish some scientific is against the spirit of science. And the fact remains only 10 percent of the 2,700 member scientists participated in the voting.
Many scientists still consider Pluto as a planet by using a geophysical definition for a planet. That is; A celestial Object is a planet if
It has enough mass to be round.
It has insufficient mass to undergo nuclear fusion in its interior.
Today we celebrate August 24 as Pluto Demoted Day to mark the demotion of Pluto. Ultimately whether we call Pluto a planet or not, it is all just semantics. The science behind celestial bodies remains unchanged no matter which name you use to describe Pluto.