Most people alive now spent the vast majority of their lives thinking of Pluto as the 9th planet in our solar system, but all that changed in 2006 when the tiniest planet orbiting our sun was demoted to a “dwarf planet.” Yet, nine years later, people still talk about the issue, and there are even t-shirts and other memorabilia sporting phrases like, “When I was a kid, we had 9 planets.”

Some of this is just people reminiscing and having fun, but there’s also a certain amount of confusion surrounding the issue. Put simply, people don’t understand why the International Astronomical Union (IAU) declared Pluto a dwarf planet in 2006.

The most common misconception is that Pluto is no longer a planet simply because it is so small (which is not true). In reality, Pluto’s status as a planet was relatively secure until 2003 when astronomers discovered another celestial body orbiting beyond the then-planet.

It was named Eris, and it is bigger than Pluto.

For decades, astronomers had thought that the solar system’s planets ended with Pluto, yet suddenly there was an even larger object orbiting even further away. That got astronomers talking about the very definition of the word, “planet,” and the subsequent discoveries of still more celestial bodies beyond Pluto further fueled the debate.

Ultimately, the IAU decided to adopt a new class of object called a “dwarf planet,” which is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a natural satellite (moon).

The new definition of a planet is basically the same, except for part “c,” because — unlike dwarf planets — planets have cleared their orbital neighborhoods. This simply means that, in order to be classified as a planet, an object has to become gravitationally dominant in its orbit, clearing the area of all bodies of comparable size other than its own satellites and objects under its gravitational influence.

Pluto simply hasn’t met this criterion. Not only is one of its moons more than half its size (which is unusual, though not technically a disqualifier), but it also shares its orbit with a variety of other bodies in an area shaped like a flat disc known as the Kuiper Belt, which extends from the orbit of Neptune to approximately 5 billion miles out from the Sun.

It should be noted, however, that there are astronomers who disagree with both the interpretation of and the very definitions themselves. That being said, these are still the most widely accepted definitions of planets and dwarf planets and the only ones approved by the International Astronomical Union.

So, as difficult as it is to say, Pluto is no longer a planet.


More on dwarf planets: NASA – Dwarf Planets


Author: Mary Ringwood


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