Why Is Pluto No Longer A Planet?

In 2006, Astronomers around the world convened at the General Assembly of the International Astronomical Union, which was held in Prague, to decide the definition of a planet. The decision that was made during that assembly killed Pluto’s status as a planet. Today we’re going to explore why they did it.

A Short History Of Pluto

Pluto was discovered in 1930 by Clyde Tombaugh at the Lowell Observatory in Flagstaff, AZ. Astronomers, at the time, had long predicted that there would be a ninth planet. They called this undiscovered planet “Planet X”. Tombaugh was given the time-consuming task of comparing photographic plates which were used to compare the same parts of the sky at different times. Any moving object – like a comet, asteroid or planet – would seem to jump from one photograph to the next.

After a year of this exhaustive work, Tombaugh finally discovered an object in the right orbit, and announced that he had discovered the fabled “Planet X”. Since they had discovered it, his team was allowed to name it. They settled on Pluto, a name suggested by a young school girl in Oxford, England. We now had 9 planets in our solar system.

Enter the 10th Planet?

Fast forward to 2005. An astronomer by the name of Mike Bell made a discovery that shook up the world of astronomy.

 He discovered an object orbiting the Sun that is a bit further out than Pluto, but was larger and had 25% more mass. This new object was named ‘Eris’.

But what should it be classified as? Is it a comet, a Kuiper belt object or a planet?Astronomers heavily debated the classification for the next few months and it was decided that at the general assembly, to be held the following year, a consensus would be voted upon and a decision reached on what exactly a planet is.

The Decision

To define or classify what a planet really is, astronomers had to consider various bits of information that the general public may not know, or take for granted. For example, there are 6 moons in our solar system that are larger than Pluto — should they be considered planets? Pluto is also more than 50% ice. This means that if you were to replace Earth with Pluto, Pluto would form a long tail of gas and ice. Objects that normally exhibit this feature are called comets. Pluto also has a moon that is roughly 50% its size.

All of these quirks and inconsistencies strained Pluto’s status as a planet. A vote was then held on what the criteria should be for a celestial object to be considered a planet. The criteria were decided as follows:

• It needs to be in orbit around the Sun – check
• It needs to have enough gravity to have pulled itself into a spherical shape – check. So far so good…
• It needs to have “cleared the neighborhood” within its orbit – Uh oh. Pluto breaks this rule.

What Does “Cleared The Neighborhood” Mean?

During a planets formation, they become the dominant gravitational object within their plane of orbit. As they interact with smaller bodies, they either slingshot them away thanks to gravity, or consume them through collision.

Pluto is only .07 times the mass of the other objects in its orbital plane. By comparison, the Earth has 1.7 million times the mass of the other objects in its orbit. Any object that meets the first two criteria but not the third is considered a dwarf planet.

What Is A Dwarf Planet?

A dwarf planet is defined as a celestial body that orbits the Sun and is large enough to have a spherical shape as a result of its own gravity — but has not cleared its neighboring region of planetesimals and is not a satellite (or moon).

We currently know of five confirmed dwarf planets in our solar system. They are Pluto, Ceres, Haumea, Makemake, and Eris. It is believed that at least another 40 objects in the solar system are dwarf planets, and up to 200 dwarf planets may be found when the Kuiper belt is explored.

While Pluto is no longer officially considered a planet, it is still an alluring target for study. NASA agrees. In 2006, they launched the New Horizons probe, its mission is to study Pluto then continue on and hopefully study objects in the Kuiper belt. It scheduled to reach Pluto in 2015 and will capture the very first close-up images of the dwarf planet’s surface.

Astronomers and space enthusiasts will marvel at the beauty and remoteness of Pluto, and the painful memories of its demotion will diminish. We’ll be able to appreciate Pluto for what it is, and not worry about its classification.