Science 101: Why is Pluto no longer a planet?by: William C. Robertson, Ph.D.

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On August 24, 2006, the International Astronomical Union (IAU) decided that Pluto was no longer a planet, but rather a "dwarf planet". Our understanding of the solar system has not undergone any radical changes, our understanding of Pluto did not change, nor did any great theories in astronomy undergo revision. Whatever the reasoning for the reclassification, it has left quite a few scientists upset and many science educators have hailed this as a major change in scientific understanding.

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Reviews (10)
  • on Fri Oct 09, 2015 10:00 AM

This article highlights the redefining of Pluto as a planet. Bill Robertson does a great job of explaining how and why Pluto is now classified as a Dwarf planet. He then presents an explanation for classification and its role in science. This is a great article that gives teachers a short explanation of the changing of Pluto's classification.


  • on Tue Aug 19, 2014 6:52 AM

I really liked the cartoon included I with this article. It really sums up the answer with a cute visual. But Scientifically, speaking of tubing classification truly makes sense. That is the basis of Science.


  • on Thu Jul 07, 2011 12:40 PM

This article explains why scientists changed their view of Pluto and decided to reclassify it as something different than a planet. We are back to the idea that scientists can change their minds when new information becomes available and the fact that classification of different things can be changed when viewed a different way. I would certainly use the article as in an upper level middle school class studying about the solar system or in an astronomy class in high school. This is what science is all about, change when appropriate. I just hope elementary teachers read this so that upper level science teachers have to reteach something that has changed awhile back.

Adah  (San Antonio, TX)
Adah (San Antonio, TX)

  • on Wed Jul 06, 2011 8:00 PM

This article is a resource for teachers to be able to understand how Pluto was reclassified and no longer considered a planet. Within the article is a discussion on classification and how “Classficiations of things is an important part of doing science, even if it isn’t a goal in and of itself.” Robertson does an excellent job of explaining the process of how Pluto was reclassified while providing an understanding of classification and its role as a tool in the process.

Sandra Gady  (Renton, WA)
Sandra Gady (Renton, WA)

  • on Thu Apr 14, 2011 8:18 PM

Many classroom teachers, and even some of us who teach science all day, can use a little help with the background of science principles. This short article does the trick. We all have heard that Pluto has been declassified as a dwarf planet, no longer a planet. But why, we wonder? In a very clear, concise article, we now know that it is a matter of scientists at the International Astronomical Union discovering several dwarf planets the same size as Pluto. Rather than add to our list of planets, they decided to demote Pluto. Bill Robertson, the multi-talented and writer of easily understandable science concepts, goes on to relate this decision to scientific classification and its importance in science thinking. Easy, breezy science.

Allison Cooke
Allison Cooke

  • on Wed Oct 13, 2010 10:39 PM

This brief article does a great job of explaining why Pluto is no longer considered a planet. It goes on to discuss the role of classification in science as well, which could be a good discussion starter with your students.

Kate Geer  (Louisville, CO)
Kate Geer (Louisville, CO)

  • on Fri Sep 09, 2016 10:39 PM

This article explains how Pluto got demoted to "dwarf planet" recently. Pluto was re-classified based on its relation to other similar celestial objects nearby. The article talks about the importance of classification in science in a way that would be understandable to elementary students. I give it 4 stars.

Samantha Brent
Samantha Brent

  • on Mon Apr 25, 2011 2:59 PM

This article is a good, concise explanation the IAU's decision classify Pluto as a dwarf planet. The article also explains how things are classified in science. This is a good article for anyone who needs a refresher in scientific classification.

Maureen Stover  (Seaside, CA)
Maureen Stover (Seaside, CA)

  • on Thu Apr 07, 2011 10:02 AM

Bill Roberson, the author of the NSTA Press book series, Stop Faking It, weighs in on the importance of the reclassification of Pluto to science.

Arlene Jurewicz Leighton
Arlene Jurewicz Leighton

  • on Tue May 05, 2015 6:10 PM

This article provides readers with a great background of what went into the declassification of Pluto as a planet. There is information on how classification in science is handled and there is a detailed look into what goes into determining it. Creation and understanding of scientific theories is briefly discussed in this article as well. This article over is a great way for teachers to acquire an overview of Pluto becoming a dwarf planet.

Elizabeth  (Lansing, MI)
Elizabeth (Lansing, MI)

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