The most basic explanation is that we have found and are still finding objects in our solar system that are nearly as big, as big, or bigger than Pluto. 

Either we had to make every new object a planet or we had to reclassify Pluto. It makes way more sense to reclassify Pluto, especially when Pluto is smaller than Earth’s own moon. 

One of the last straws was Eris, another trans-Neptunian object (which means an object in our solar system farther away from the sun than Neptune), which was briefly referred to as “the tenth planet” because it initially appeared to be even larger than Pluto. (Now we know that Pluto actually has the greater volume, but that Eris has the greater mass.) 

I think everyone should really appreciate that it was an object named after the Greek goddess of discord that disrupted the “balance” of the “Nine Planets”.

Some astronomers don’t like the term “dwarf planet”, but the International Astronomical Union as of 2008 has officially recognized five: Pluto, Ceres, Eris, Makemake, and Haumea (https://en.wikipedia.org/wiki/Dwarf_planet).  

The study of space is really hard, guys, because there’s just so much space to study. Our own solar system is in many ways still a mystery to us. Pluto was only discovered in 1930. Remember how recently we got our first decent picture of Pluto? And saw Pluto’s “heart”? That was 2015. 

We’re also coming to understand that our solar system isn’t actually “normal”, now that we’re learning more about exoplanets (which are planets outside our solar system: https://en.wikipedia.org/wiki/Exoplanet). Like how we came to understand that Earth’s moon wasn’t “normal” for every planet. (Our moon is actually really weird, guys.) 

(Did you know that there are objects called “rogue planets”? Also known as “nomad planets” and “orphan planets”? These are planets that don’t have a star to orbit around: https://en.wikipedia.org/wiki/Rogue_planet. Very cool!) 

The scientific community has to work off of what they know. Sometimes, something comes along and changes everything that we think we know. So, we have to adjust. We can’t presume that the universe will adhere to any human mythology; we have to study it as it is; and it’s a “the more we learn, we realize the less we really know” situation. Prof is right. Pluto is not a planet. 

Pluto is a dwarf planet. Near the edges of our solar system, Pluto is keeping company with an unknown number of fellow trans-Neptunian objects. An object by any other name remains exactly as it actually is. 

^^^ anyone interested in the process leading up to Pluto’s demotion should read How I Killed Pluto and Why It Had It Coming by Michael Brown, the guy whose team discovered Eris and whose findings contributed most to the IAU’s ruling on the classification of dwarf planets. It’s a fascinating read.