Classification Rules for Planets
Planets Before Pluto
Planets were understood for many millennia. Early astronomers noticed that some points of light in the sky would wander around compared to the fixed background of points of light. That definition survived through the ages, as the word planet is derived from the Greek word for wanderer! The planets were known to be Mercury, Venus, Mars, Jupiter, and Saturn. The Sun and the Moon were classified as separate entities. Before the Greeks, ancient Babylonian astronomers did not realize that Venus, seen as the morning and evening star, was actually the same planet. After the invention of the Telescope, and the adoption of heliocentrism, the planets were known to be Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Planets After Pluto
Pluto was first assumed to be a planet, with a mass ten times that of Earth. As more evidence grew that Pluto was actually less massive than Earth, and with the discovery of the Kuiper Belt, Pluto’s classification was called into question. The final nail in the coffin for the planet Pluto came when Eris, another Kuiper Belt object about the size of Pluto, was discovered. Due to the new information, astronomers from all over the world gathered to define a planet. These were the rules that they set forth to be considered a planet.
Rule 1
Must be in orbit around the Sun
This rule is fairly obvious, a planet must orbit the Sun. This is the least controversial rule for what defines a planet.
Rule 2
Must be nearly round due to sufficient mass
This rule states that a planet must have sufficient mass to be in hydrostatic equilibrium overcoming the rigid body forces through gravity. Astronomers have argued that this rule does not properly define what constitutes as a nearly round shape.
Rule 3
Must clear the neighborhood around its orbit
This rule truly separates Pluto and Eris from being considered a planet. The Kuiper Belt is full of objects that Pluto and Eris have not cleared. However, thanks to asteroids that are located in Jupiter’s orbit and will not be cleared, named the Trojans, according to this definition Jupiter would not be a planet.
Dwarf Planets
These rules set out the classification criteria for planets, but objects like Pluto and Eris still needed defined. At the same meeting the classification of dwarf planets was set as follows.
Rule 1
Must be in orbit around the Sun
Much like the definition of a planet, a dwarf planet must orbit the Sun.
Rule 2
Must be nearly round due to sufficient mass
Dwarf planets must also be round, the same as the second rule for a planet.
Rule 3
Has not cleared the neighborhood around its orbit
The difference between a planet and a dwarf planet. Since Pluto and Eris have not cleared the Kuiper Belt, the objects are considered dwarf planets.
Rule 4
Must not be a satellite
A rule which rejects satellites from becoming dwarf planets. Without this rule, objects such as the Moon would meet all the previous criteria.
Small Solar System Bodies
All other objects which do not fit within these definitions are considered small solar system bodies. These include asteroids and comets.