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Why Pluto as a Planet Doesn’t Make Sense…Scientifically

Monday, 12 September 2011

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Image 1 - Pluto from the Hubble Space Telescope

There is an emotionally driven debate about Pluto’s demotion (in August 2006) from ‘planet’ to ‘dwarf planet’. Since 1930, people have come to know our solar system with the nine planets and some are offended when a group of scientists (namely the International Astronomical Union) tells us that Pluto no longer meets the criteria as a planet.

The emotion of the situation is understandable. We were taught that Pluto is the last planet to be discovered in our solar system and then decades later astronomers decide that there are only eight planets and Pluto isn’t one of them. The nerve of those darn scientists!

The issue goes to the heart of science. When greater knowledge and understanding mandates scientific change, do we accept the change or do we stick to a decision made scores of years ago by people who had an incomplete understanding of our solar system?

Image 2 - One of many diagrams created to describe the Scientific Method

To answer this may require a quick review about the scientific process. Many parents were taught in school about the ‘Scientific Method.” That is the process of creating a hypothesis, experimenting, adjusting the hypothesis and experimenting more. Now that I’ve explained that, I should tell you that the Scientific Method doesn’t apply to Astronomy.

Yale professor Charles Bailyn¹ explains that the Scientific Method (at least the version parents were taught in school) doesn’t work in Astronomy, nor in many scientific fields because you can’t experiment on the subject matter.² Instead of the hypothesis/experiment model, scientists use an equally effective process of, 1) observation, 2) interpretation of the observations and, 3) classification the subjects being observed into similar groups based on common characteristics. This is followed by more observation, interpretation and reclassification. The observation/interpretation/classification method of science is similar to the Scientific Method in that it uses new knowledge and understanding to update and refine the model, without using experimental data.

This process is why Pluto has been demoted from planet status. Pluto was discovered on February 18, 1930, by Clyde Tombaugh. A year earlier Tombaugh had been given the sole mission to find the ninth planet (known as Planet X.) At that time scientists had a limited knowledge of the detailed structure of our solar system and they couldn’t accurately determine size of an object through a telescope, especially one so far away from Earth. When Tombaugh found an object orbiting the Sun, it was declared to be the undiscovered planet that he was seeking.

Image 3 - How Pluto Compares to Other Objects in the Solar System

However, today we know that there are billions of objects orbiting our Sun. Most are incredibly small and don’t merit a unique identity, but the larger objects fall neatly into distinct groups. The observed groups are as follows:

  1. The Sun is a group of one. There are no other stars in our solar system.
  2. The four rocky, inner planets or terrestrial planets (Mercury, Venus, Earth, and Mars.)
  3. The Asteroid Belt between Mars and Jupiter. These objects range from dust specks to large rocks a few hundred kilometers across.
  4. The four gaseous planets (Jupiter, Saturn, Uranus, and Neptune) make up the gas giants or Jovian planets.
  5. Over 70,000 small, icy objects, including Pluto, extending from Neptune’s orbit and beyond that make up the Kuiper (pronounced ki-per) Belt.

Note that groups 2 through 4 go around the Sun on the same orbital plane and have a nearly circular orbit. Objects in the Kuiper Belt, like Pluto, orbit the Sun in elongated (or elliptical) paths and the travel well off the normal orbital plane of the rest of the solar system. The largest of these objects are much smaller than all the planets and even smaller than many of the moons in our solar system.

As astronomers began to realize that Pluto was undersized and had the characteristics of a group of thousands of small, icy objects in eccentric orbits in the Kuiper Belt, it became obvious that Pluto was misclassified. Pluto failed all the characteristics of the inner four planets, and the four Jovian planets. It didn’t make sense to call it a planet. That became even more evident when a bigger object (Eris) was discovered even farther away than Pluto, which also fell into the Kuiper Belt classification. Thus, the scientists, armed with a greater understanding of our solar system, rescinded the error of 1930 and demoted Pluto.

Some people still see the demotion as unfair; however, there are some positive outcomes for Pluto in the new classification. First, rather than being the lowly, ninth planet; Pluto is the first dwarf planet to be discovered and the first object to be discovered in the Kuiper Belt. Second, Pluto is one of the giants of the dwarf planets, rather than the runt of the regular planets.

The demotion of Pluto is a good lesson for all of us to remember. The scientific process is not a popularity contest, nor is science a stagnant, unchanging collection of knowledge. We know less now than we will tomorrow. That means today’s interpretations of our observations will not necessarily be the same as tomorrow’s interpretations. As for Pluto, it is now classified in a group that has similar characteristics and that makes understanding our solar system easier for those who will take the current knowledge and advance it for our children, and for our children’s children. That is what discovery is all about.

References and Notes:

(I am using the APA style citation for a Professional Website)

¹Yale University. (2011). Charles Bailyn. Retrieved September 11, 2011, from

²Academic Earth (date unknown). Our Solar System and the Pluto Problem. Retrieved September 11, 2011, from

Image Credits:

Image 1 – Courtesy of Hubble Space Telescope, NASA, ESA, and M. Buie (Southwest Research Institute)

Image 2 – Image thanks to

Image 3 – Image thanks to

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