By: Carolina Gonzalez

Pluto's Discovery

Percival Lowell first proposed the existence of an unknown 9th planet when he theorized that wobbles in the orbit of Uranus and Neptune were caused by an unknown planetary body. Lowell and other scientists worked for over a decade to try to prove his theory and on February 18th, 1930 Clyde W. Tombaugh found what was thought to be a new planet, later named Pluto.

The Kuiper Belt

The Kuiper Belt lies just outside Neptune's orbit. It is said that it contains the leftover remnants of the solar systems beginnings that are important to understanding the solar systems birth. Pluto was the first object that was found in the Kuiper Belt which is why scientist thought it was a major planet.

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Pluto Not a Planet?

In 2003, an astronomer thought that he had found a new planet. He named it Eris. Eris is further away and larger then Pluto. Finding Eris caused other astronomers to talk about what makes a planet a planet. The result was that the International Astronomical Union created the Planet Definition Committee. This committee was made up of scientists, astronomers, and educators. The committee made a resolution that stated things like the differences between dwarf and major planets making Pluto a dwarf planet.

Planets vs. Dwarf Planets

Before the Planet Definition Committee there was no official definition of a planet. The official definition is now a celestial body that orbits the sun, is large enough to have its self-gravity pull itself into a round or near spherical shape and has cleared anything in its orbit.

A dwarf planet is smaller and can also orbit in a zone that has other objects in it. A solar system body is an object orbiting the sun that is to small to be a planet or dwarf planet.

If Pluto is no longer a planet, why is Mercury?