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Is Pluto a Planet?

After ranking among our solar system’s planets for nearly 80 years, we finally have an answer to the question, "is Pluto a planet?"

By Jake AronskindPublished 8 years ago 4 min read

Every space-minded child growing up found their niche among the stars once they were able to name all the planets in order from the Sun. For anyone born after 2006, they might find it a little harder as we continue to ask: Is Pluto a planet? Pluto, the true underdog story of the Milkyway, was declassified as a primary planet after it failed to meet the International Astronomical Union's specifications. The world watched on as the baby brother of the planet family was disowned and thrown into a meaningless group of misfit dwarf planets. So while we brush away our tears as we watch everyone’s favorite underdog become stripped of its planetary ties, let’s find the true reasons why we still question whether or not Pluto is a planet.

Planet X

Before we understand why Pluto is no longer considered a planet, we must first understand how it was mistakenly included in the planetary ranks. Pluto was discovered in 1930 by Clyde Tombaugh, an American astronomer observing at the Lowell Observatory in Flagstaff, Arizona. It was temporarily given the name Planet X. Tombaugh and the rest of the Lowell staff finally decided on the name Pluto after an eleven-year-old English schoolgirl suggested it. Contrary to popular belief, Pluto is not named after Mickey Mouse’s trusted accomplice, but rather the Roman God of the Underworld. It wasn’t until 1992, when astronomers discovered several similarly-sized objects orbiting in what would later be called the Kuiper Belt, that people first began questioning the classification of Pluto.

Image via Universe Today

Criteria of a Planet

In 2005, when an object marginally bigger than Pluto, Eris, was discovered in the same orbit as the dwarf planet, the IAU decided to lay out specific criteria for the classification of a planet. This was the beginning of the end for our solar system’s Cinderella story. If there were an object larger than Pluto that was made up of similar elements, how unique was our presumed ninth planet? The IAU decided that for an object to be classified as a planet, it needed to meet three requirements:

It orbits the sun. Check. Pluto orbits the sun at a range of 4-8 billion kilometers from the center. It has an eccentric and inclined orbit, causing it to sometimes come closer to the sun than Neptune. Fortunately, nature solved that problem. Pluto and Neptune have a stable orbital resonance, meaning that the two massive bodies exert gravitational forces on each other throughout their orbits. This interaction keeps the two objects stable so they don’t clash or push the other one out of its orbit.

It has a spherical shape. Check. This criterion is a little more complicated than it seems, however, as there’s a reason behind every planet’s spherical shape. A planet must have sufficient mass to achieve a hydrostatic equilibrium, which is basically saying that it has enough gravity to pull itself into a sphere. Pluto’s self-gravity overcomes its rigid body force, forcing it into its nearly-round shape. So far so good for this underdog story.

It clears “neighborhood” of surrounding objects. Uh oh. While this questionably-worded criterion might raise a few eyebrows, the facts are against Pluto. Due to a planet’s massive size, they either consume surrounding smaller objects or force them into orbit. While the other eight planets are dominant gravitational forces in their neighborhood of the solar system, Pluto is far from it. The dwarf planet is only 0.07 times the mass of the other objects around it. Meanwhile, Earth is nearly two million times greater than the other masses in its orbit. Check mate—Pluto opposers.

Image via BBC


On September 13, 2006, we finally got our answer to Pluto's planetary question. Along with Eris and its moon, Pluto was classified as a dwarf planet, which is defined as any object in space that meets the first two criteria of the IAU but fails to meet the third. Supporters of Pluto’s declassification, namely led by Neil Degrasse Tyson, held their heads high as everyone’s favorite Milkyway underdog story came to an end. Although Tyson and his fellow planet-killers hammered the final nail of Pluto’s coffin, they don’t deny its significance in our solar system.

More Than Just a Dwarf Planet

While we no longer consider Pluto a planet, the dwarf planet has influenced how astronomers study our solar system in countless ways. Until Pluto, astronomers never laid out a specific criteria on the classification of planets. Luckily, we didn’t lose another helpless planet to the treacherous rules of the IAU. The reason behind Pluto’s declassification also shed light on a completely newfound phenomenon in our solar system. After the discovery of Eris, astronomers noticed an icy belt of small bodies orbiting the sun just past Neptune’s orbitthe Kuiper Belt. Consisting of asteroids and other small objects, the Kuiper Belt is similar to the asteroid belt, but it is much larger. The circumstellar disc is nearly 20 times as wide and almost 200 times as massive as the asteroid belt, which is found between the orbits of Mars and Jupiter. The Kuiper Belt stretches far beyond the orbits of our eight planets. It is also home to several other classified dwarf planets, such as Haumea and Makemake. Although Pluto is no longer considered a primary planet, the recently-named dwarf planet continues to provide astronomers with analysis to further study our solar system.

Image via Space

Although Tyson and the rest of the Pluto-opposers might have pushed for the ultimate decision, it was Pluto itself that pulled the trigger as it ended its reign among the other planets. The planet-that-once-was is not larger than its surrounding masses in orbit. Fortunately, with every fairy tale comes a happy ending. Although Pluto may no longer be considered a planet, it continues to influence astronomers’ findings of our solar system. Pluto’s declassification not only led to a concrete list of criteria to define future planets, but it also led to the discovery of the Kuiper Belt. So, finally, is Pluto a planet? For those countless Pluto-lovers out there, you might want to take a seat. The universe has made its ruling: a resounding NO!

spacefact or fiction

About the Creator

Jake Aronskind

Born from the blood of Ash Ketchum and Lyanna Stark. Walks past unfinished jigsaw puzzles and can't help but solve them. Just might be the most interesting man in Essex County.

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