Why isn't Pluto a Planet?

Feb 22, 2018

Dr. Todd Young Visits the KWIT studios once again with his reasoning on scientifically why Pluto is not a planet here on Siouxland Public Media

I give a lot of planetarium shows to kindergartners and at the end of each program, I like to ask them if they have any questions about space and astronomy.  Most questions I receive are usually not questions and more about how they want to be an astronaut, which is fun. But, occasionally, someone asks a really good question.

Recently, one little boy asked : “Why is Pluto no longer a planet?”  To answer this question, let's start with the criteria for a planet:  1) it must orbit the sun; 2) it must be massive enough for gravity to shape itself into a sphere; and 3) it must have cleared the neighborhood around its orbit of all other objects.  Pluto does meet the first two criteria, but it does not meet the third.  Pluto is in fact in a region of the solar system just beyond Neptune called the Kuiper Belt.  The Kuiper Belt is one of two places in the solar system where comets come from (the Oort Cloud being the other).  Comets are large chunks of ice, rock, metal, and gases, (sometimes affectionately called "dirty snowballs) that sometimes get close enough to the sun.  When this happens, they begin to melt and create the long tail that is typically associated with a comet.

So essentially, Pluto is not considered a planet anymore because it is really just a bigger comet among many other comets!  And Pluto is not even the biggest comet in the Kuiper Belt; that title belongs to an object called Eris.


Follow your curiosity to the Fred G. Dale Planetarium at Wayne State College.