On this day, August 24th, in the year 2006, America’s favorite planet Pluto was demoted from full planet to dwarf planet. It was the year I graduated high school and I remember it well because even though I, a space nerd with a job at the Houston science museum, had deemed Pluto her favorite planet (it must be so lonely all the way out there!), I was surprised to learn that tons of people had also picked Pluto to be their favorite. The demotion brought up a lot of feelings! So much so that even today, 19 years later, people are still upset about Pluto no longer being a full-fledged planet. That’s why today is known as Pluto Demoted Day.
Why was Pluto Reclassified?
Technically speaking, Pluto was only reclassified, not demoted (even though that’s the name of the day!). The reasons behind this are – and don’t throw tomatoes here – scientifically sound and rather succinct: the International Astronomical Union, opens a new window (IAU) reclassified Pluto because other objects might enter its orbit.
What Makes a Planet?
I’m going to assume you might already know the rules to qualify as a planet, being that you clicked on an article about Pluto, so therefore love space, but in case you don’t, here are the three criteria the IAU uses to determine what qualifies as a planet:
- It is in orbit around the Sun.
- It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
- It has “cleared the neighborhood” around its orbit.
This is important because, though Pluto orbits around the sun and has sufficient mass to assume hydrostatic equilibrium (obviously), its path is not clear of other objects. Mainly, the Kuiper belt, opens a new window.
The Kuiper Belt, in case you don't know, is the donut shaped region beyond Neptune that is filled with icy bodies (like comets) and is thought to be the remnants of the early solar system before the one we know now!
What is a Dwarf Planet?
Honestly exactly what Pluto is – an object that orbits the sun and has enough mass to maintain a nearly round shape (that’s what point number two in what makes a planet means), but does not have its own orbital path. Truly, that's the real key reason to being a dwarf planet - an object that could be a planet, but doesn't have orbital dominance in it's path. Fun fact, the term "dwarf planet" was coined in 2006 because of Pluto! There were other terms thrown around for what Pluto could be classified as instead, like perhaps planetoid or minor planet, but dwarf planet originally came around to categorize smaller planets, as Pluto is smaller than many of the other eight planets, so when Pluto was reclassified, that's the one that stuck.
Are There Other Dwarf Planets?
Yes, there are! Here is a picture of some of the biggest ones, courtesy of NASA:
In Conclusion
Pluto is still a planet, albeit a dwarf planet. The only real change for us Earthlings is instead of saying My Very Eccentric Mother Just Served Us Nine Pizzas to remember the planets and their order, now we have to say My Very Eccentric Mother Just Served Us Nachos, which really only affects you if you absolutely detest nachos but love pizza.
And, of course, if you have to pick another favorite planet of the non-dwarf variety. Mine is Jupiter now, which is objectively better because of the Great Red Dot. Neptune and Uranus are a close second because of the diamond rain.
Read more about the Pluto Controversy:
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