Why Is Pluto No Longer Considered A Planet?

Pluto has remained mysterious ever since its detection in 1930.

It used to be the tiniest planet known, even smaller than our own moon. Like the terrestrial planets, it’s thick and rocky. The gas giant Jovian planets are its closest neighbours. This leads many researchers to speculate that Pluto was captured by the sun’s gravity from another part of the galaxy. Pluto was formerly thought to have been one of Neptune’s moons, but this theory was later disproved.

Pluto has an unstable orbit. Our solar system’s planets all follow a rather level path around the sun. Yet, Pluto’s orbit is 17 degrees tilted off this plane as it travels around the sun. It also has a very eccentric orbit that cuts over Neptune’s.

Pluto has one moon, Charon, which is roughly the size of Pluto itself. Some scientists have proposed that instead of treating the two as a planet and satellite, they be considered a binary system.

Could Pluto be considered a planet?
These details fueled the ongoing discussion about whether or not to classify Pluto as a planet. The International Astronomical Union (IAU), a group of trained astronomers, voted to de-list Pluto as a planet twice on August 24, 2006. Planet is defined in Resolution 5A, the first of these resolutions. Although the term “planet” is commonly understood, astronomy has never agreed upon a precise meaning of the term.

Planets are defined as follows in Resolution 5A:
In order to be considered a planet, a celestial body must (a) be in orbit around the Sun, (b) be massive enough for its self-gravity to overcome strong body forces, assuming a hydrostatic equilibrium form, and (c) have cleared the neighbourhood surrounding its orbit.
Pluto’s orbit overlaps Neptune’s, therefore while being roughly spherical and orbiting the sun, it does not qualify. Opponents of the resolution point out that none of the planets in our solar system, including Earth, have cleaned up the space immediately surrounding their orbits. The Earth, for instance, has frequent run-ins with asteroids both inside and outside its orbit.

Dwarf planets and other minor solar system entities were also officially recognised as a new class of planetary-like objects by this resolution. For the purposes of this resolution, a dwarf planet should be defined as:

A celestial body that (a) orbits the Sun, (b) is massive enough for self-gravity to overcome strong body forces, assuming a hydrostatic equilibrium (roughly round) form, (c) has not cleared the neighbourhood surrounding its orbit, and (d) is not a satellite.

Resolution 6A, which likewise calls Pluto a dwarf planet, confronts the issue head-on:
Unfortunately, not all astronomers agreed with Resolutions 5A and 6A. Some people think it’s deceptive to use the phrase “dwarf planet” for objects that aren’t planets, since the name itself implies that the object is a planet. As just a small fraction of the world’s astronomers and planetary scientists were eligible to vote, the resolutions’ legitimacy has been questioned by several astronomers.

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