Is Pluto a Planet?

What You Need


  • Calculators
  • Rulers/tape measures
  • Potential objects to represent bodies in the solar system:
    • uninflated balloons (students can inflate to adjust size) 
    • coconut, grapefruit, small nerf ball, small squash (Jupiter/Saturn) 
    • kiwi, lime, clementine, racquetball (Uranus/Neptune) 
    • grape, blueberry, garbonzo bean (Earth/Venus) 
    • pea, small kidney bean (Mars) 
    • orzo pasta (Mercury/Moon) 
    • rice grain, sesame seed (Moon/Pluto/Eris) 
    • poppy seed, sand grain, salt (Ceres, Sedna, other dwarf planets)
Is Pluto a Planet? By NASA, ESA and G. Bacon (STScI) [Public domain], via Wikimedia Commons


To model objects in the solar system, including Pluto, Eris, Haumea, and Makemake and use the information in the Brown video to draw conclusions about whether Pluto should be considered a planet.


On August 24, 2006, the word “planet” was given its first ever scientific definition by the International Astronomical Union. When this definition was made, and the eight dominant bodies in the solar system were declared, a new class of planets was defined: the “dwarf planets.” Pluto, heretofore considered a planet, was redefined as a dwarf planet.

This lesson explores Pluto’s redefinition as a dwarf planet and asks students to consider what a planet is. Students consider the scientific definition of a planet and a dwarf planet and they compare and contrast the characteristics of planets, comets, asteroids, and transneptunion objects.

As part of this exploration, students will watch a video of astronomer Michael Brown, who made discoveries that contributed directly to the change in Pluto’s status, and he states the reasons for the change in the video. The video and activity help demonstrate that scientists modify classification systems as new evidence is gathered and that scientific discoveries build upon knowledge from previous discoveries. This modification of scientific knowledge exemplifies one of the learning goals of the lesson, which is that "Scientific knowledge is subject to modification as new information challenges prevailing theories and a new theory leads to looking at old observations in a new way." (Benchmarks for Science Literacy, p. 7.)

At this grade level, students should add more detail to their picture of the universe, pay increasing attention to matters of scale, and back up their understanding with activities using a variety of astronomical tools. By studying the controversy over the redefinition of Pluto from a planet to a dwarf planet, students will add to their picture of the universe.

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Planning Ahead

As you gather items to represent the objects in the solar system, remember that the largest body (Jupiter) in the activity is about 87 times larger in diameter than the smallest body (Makemake). If a marble represents Makemake, then a large beach ball is an appropriate size to represent Jupiter.

Before you start the lesson, place the items in Planet Boxes. You may want to set the boxes at different locations in the classroom to avoid crowding. Please be aware that the items listed in the Materials are suggestions only. Feel free to select objects that you think will work for your classroom.


In order to engage students in this lesson, you could begin by asking them if they have heard of the controversy surrounding the designation of Pluto as a dwarf planet. Engage your students in a class discussion about this controversy. Ask students for their definition of a planet versus a dwarf planet. Write down the students’ definitions on a large sheet of paper, the black board, or a SmartBoard.

Then, ask students to use their Is Pluto a Planet? student esheet to go through the What Is a Planet? slideshow. This part of the lesson can be done as homework. They should use the information in this slideshow to help them answer the questions on the What Is a Planet? student sheet. You can find answers to the questions on the What Is a Planet? teacher sheet.

Inform students that they will now engage in some activities to help them compare and contrast the characteristics of planets, comets, asteroids, and transneptunion objects.


Before delving into the question of whether or not Pluto is a planet, students should first learn a little something about the history of discovery of this object. Indeed, it is important to emphasize that our understanding of the universe, our solar system, and the objects in our solar system has changed over time. Emphasize that science is a dynamic process of questioning, hypothesizing, discovering, and changing previous ideas based on what is learned.

To learn more about Pluto, for homework students should use their esheet to go to Pluto and the Developing Landscape of Our Solar System, on the International Astronomical Union site. Students should read just the first two sections: The Discovery of Pluto and The Changing Landscape of the Solar System.

Next, students should use their student esheet to watch Conversations with a Scientist: Michael Brown. In this video, Dr. Brown discusses the solar system's bodies and his role in the redefinition of Pluto from planet to dwarf planet.

Based on the brief reading and the video, students should answer the questions on their Is Pluto a Planet? student sheet. You can find answers to the questions on the Is Pluto a Planet? teacher sheet.

After you go over the questions with the students, tell students that they will engage in a solar system modeling activity. Before you launch into the activity, ask for suggestions as to objects that could represent the bodies in the solar system. Write down students’ ideas on a large piece of paper, black board, or SmartBoard.

Hand out the Solar System Scale and Distance student sheet. Using the student sheet, students should work in groups to create/or find the scaled objects for the solar system. They should classify them according to their scale sizes. Students should make a sign for each body. Point out that the sun is about ten times larger than Jupiter, and ask for suggestions of objects that could represent the sun in the classroom model.

Once they have classified the objects, they should then lay them out the way they are arranged in the solar system. This way, students can see for themselves how scientists went about defining what counted as a planet, dwarf planet, etc.

If you have the space (outdoors, a long hallway, a large open area), you can use the models to demonstrate the relative distances from the sun. The body farthest from the sun (Eris) is about 175 times farther than is the closest body (Mercury). An easy way to start would be to have Mercury a small distance (1 foot, 0.1 meters) from a spot and divide the distance for Mercury into each of the other distances to find their relative locations. Depending on the class, you may need to do the calculations for them in advance.

After the activity is completed, return to the discussion of the definition of a planet versus a dwarf planet that you did in the Motivation. Students can discuss how they chose to group the bodies into categories. One possibility for grouping is suggested by Dr. Brown in the video: the four larger planets in a group, the four smaller planets in a group, and the dwarf planets in another group. Ask if the students would like to make changes to their definition of a planet and make any changes agreed upon by the group.

At this point, it is important to point out to students that they are doing the work of scientists. They created a working definition and revised it based on new information, in this case, the Brown video and their own classroom solar system model. Stress that science is a collaborative effort and that new findings build upon the old ideas and may completely change the previous views.


To assess student understanding, ask students to look at the images on the Space Objects student sheet and classify them as either planets or dwarf planets. You can find a chart with the answers on the Space Objects teacher sheet.


These Science NetLinks lessons can be used to help extend the ideas in this lesson:

You can use the case of the Shoemaker-Levy crash on Jupiter to help students study how scientists came to change their minds about impact craters here on Earth.

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Lesson Details

Grades Themes Type Project 2061 Benchmarks National Science Standards