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Why Isn't Pluto a Planet Anymore?

Why Isn't Pluto a Planet Anymore?

Astronomy Crosscutting Concepts Earth & Space Science Is Lesson Plan NGSS Phenomena Science and Engineering Practices Three-Dimensional Learning Middle School High School Grades 6-8 Grades 9-12

Sensemaking Checklist

Welcome to NSTA's Daily Do

Teachers and families across the country are facing a new reality of providing opportunities for students to do science through distance and home learning. The Daily Do is one of the ways NSTA is supporting teachers and families with this endeavor. Each weekday, NSTA will share a sensemaking task teachers and families can use to engage their students in authentic, relevant science learning. We encourage families to make time for family science learning (science is a social process!) and are dedicated to helping students and their families find balance between learning science and the day-to-day responsibilities they have to stay healthy and safe.

Interested in learning about other ways NSTA is supporting teachers and families? Visit the NSTA homepage.

What is sensemaking?

Sensemaking is actively trying to figure out how the world works (science) or how to design solutions to problems (engineering). Students do science and engineering through the science and engineering practices. Engaging in these practices necessitates students be part of a learning community to be able to share ideas, evaluate competing ideas, give and receive critique, and reach consensus. Whether this community of learners is made up of classmates or family members, students and adults build and refine science and engineering knowledge together.


Today's Daily Do, Why isn't Pluto a planet anymore?, is inspired by astronomers Jane X. Luu and David C. Jewitt's discovery of the Kuiper Belt for which they were awarded the 2012 Kavli Prize for Astrophysics. NSTA is excited (dare we say, over the moon) and honored to welcome Jane Luu as The Kavli Foundation keynote speaker at the Engage: Fall20 Virtual Conference. Join us for her live presentation, The Story of Pluto and the Kuiper Belt: How Science Progresses, on Saturday, November 14, 2020, at 11:05 am ET. It's not too late register!

The reclassification of Pluto followed a vote on the first formal definition of a planet at the 26th General Assembly of the International Astronomical Union (IAU) on August 24, 2006. Fourteen years after Pluto's demotion from full-fledged planet to dwarf planet, many people are still in mourning. But did you know Pluto isn't the first celestial object to loose it's planet status in our solar system? In today's Daily Do, students frame their thinking with patterns as they engage in science and engineering practices and confront the nature of science to answer the question, Why isn't Pluto a planet anymore?

Map of Solar System 1846

Experience the Phenomenon

Tell students you have a puzzling phenomenon to share! Ask students to create a space in their science notebooks to record observations and questions that come up. Share the A Plan or Map of the Solar System projected for Schools & Academies with students (either project the image or share the link with students so they are able to zoom in on different parts of the map). Indicate to students the map represents the planets known in our solar system in 1846.

Note to Educators: Ceres, Pallas, Juno and Vesta were identified as planets upon their discovery (1801, 1802, 1804 and 1807, respectively). While astronomer William Herschel, famous for his 1871 discovery of Uranus, advocated to reclassify these celestial objects into a new class of rocky objects - asteroids - as early as 1802, they seem to have maintained their status as planets until at least the mid-1840s (sources vary on this point). NASA Science Solar System Exploration cites 1863 as the year Ceres was widely accepted as an asteroid among astronomers.

Provide students time in the Alone Zone (independent thinking time) to make and record observations. Students will likely have a range of knowledge of the solar system. Encourage students more familiar with the solar to notice what's included on the map and not "what's missing" from it. Refrain from answering individual questions; instead direct students to record their questions in their science notebooks.

Next, ask students to turn to a partner and share observations. As you move around the room, listen for students to describe some or all of the following observation:

  • there are 12 planets
  • three planets, Uranus, Saturn and Jupiter, are much larger than the rest
  • Vesta is the smallest planet
  • Vesta might be an asteroid (orbit is labeled asteroid)
  • the three largest planets are furthest from the Sun
  • four planets have moons
  • one planet has rings
  • 98 comets (comet sightings) are recorded
  • one comet path (orbit) is shown
  • Saturn and Uranus have the most moons (6) and Earth has the least (1)

Ask students to share their observations with the class, first calling on one of the student pairs who shared their noticing of 12 (or maybe 11) planets. Create a class record of observations. Students may have many questions about the map; either create a class list or ask students to make sure to record them in their science notebook.

Tell students you have another solar system map you need to share with them - Rand McNally's Modern Space Map published circa 1950. Again, project the map or share the link with students. This time, ask students to work in pairs to make and record observations. Remind them not to worry about what's not on the map, but to notice what's included. Encourage them to continue to record questions as well.

Map of Solar System (c. 1950)

Assign pairs to form groups of four students. Ask them to first share their noticings with each other. As you move around the room, help keep students focused on sharing observations of the Modern Space Map and not (yet) comparing the two maps. You might ask students to use the talking stick protocol described in the Why are plane designs so different? Daily Do.

Next, ask the groups to work collaboratively to identify and record differences between the two maps. Listen specifically for groups to talk about the the absence of Vulcan from the Modern Space Map, the appearance of the planets Neptune and Pluto, Ceres identified as an asteroid (Pallas, Vesta and Juno are not labeled), and the notation "1,500 to 50,000 known" with regards to the number of asteroids . When you bring the class back together to share the differences between A Plan or Map of the Solar System projected for Schools & Academies and Modern Space Map, make sure to call on these groups first. Record these differences and any others the groups have identified.

Ask students to review the questions they recorded in their notebook and add new questions they have about observations of and/or differences between the two solar system maps. Which three questions about the phenomenon of the changes in the solar system maps between 1846 and 1950 are the most puzzling to them? Ask students to share these questions with their group and note the two most common questions and the one question the group votes is the most intriguing or unique. Bring the groups together to share these three group-selected questions with the class. Students will likely ask

  • What happened to planet Vulcan?
  • Why was Ceres changed from a planet (1846) to an asteroid (c. 1950)?
  • What happened to Pallas, Vesta and Juno? (Are they classified as asteroids like Ceres or did they disappear like Vulcan?)
  • Why is Ceres (and Eros) shaped like a (rolling pin, sea shell, etc.)? Are all asteroids shaped that way?
  • Why is Eros labeled on the Modern Space Map and not Pallas, Vesta and Juno? Is Eros bigger than them?
  • How are new planets discovered?
  • Why does the comet's path cut across the paths (orbits) of the planets? (Why isn't it's path parallel to the orbits of the planets?)
  • What are sun spots?

If students don't ask about Ceres (and Pallas, Vesta and Juno), you might direct their attention to A Plan or Map of the Solar System projected for Schools & Academies and Modern Space Map and ask, "Why do you think Neptune and Pluto are not included on the 1846 map?" Students will likely say they weren't discovered yet. Then point out Ceres, Pallas, Vesta and Juno and say, "These planets were already discovered in 1846. Why do you think they are not included on the Modern Space Map? Were they undiscovered?" Students will (most likely) unanimously agree a planet can't be undiscovered, but may share many different ideas about why Ceres, Pallas, Vesta and Juno aren't included as planets on the c. 1950 map. Point out these differences and ask, "We have a lot of ideas about why these celestial objects aren't included as planets on the Modern Space Map. We see Ceres classified as an asteroid (and presumably Pallas, Vesta and Juno). Should we investigate what changed between 1846 and 1950 that caused astronomers to demote Ceres and the other objects from planets to asteroids?"

Why wasn't Ceres a planet anymore?

Tell students you have data to share for the planets identified on each map. Share the Data for Planets Identified on Solar System Maps link with students. This will enable them to make a copy of the Excel file which contains Table 1. Planets Identified on A Plan or Map of the Solar System Projected for Schools and Academies (1846) and Table 2. Planets Identified on Rand McNally's Modern Space Map (c. 1950). Sharing the Excel version of the data tables allows students to easily sort data which may help reveal patterns. (PDF versions of the data tables can be found in the Why isn't Pluto a planet anymore? collection of resources.)

Say to students, "These are the data astronomers had available to them - in addition to the data contained on the maps - to reach consensus on reclassifying Ceres, Pallas, Vesta, and Juno from planets to a new class of rocky celestial objects called asteroids." You might further share that astronomer William Herschel proposed this new class of objects when Pallas was discovered; he was an early advocate for "demoting" Ceres, Pallas, Vesta and Juno from planet status.

You may need to share with middle school students that eccentricity is a measure of how close a planet's orbit is to a perfect circle. The closer the eccentricity is to 0.0, the closer it is to a perfect circle.

Students might ask you the difference between the two classifications; make sure to let them know formal definitions for neither planets nor asteroids existed during this time period. Put the question back to students, "How might you decide how to categorize the celestial objects represented as planets on the 1846 map?" Students will likely say look for patterns (similarities and differences) in the data.

Ask students to work in the Alone Zone to identify patterns in each of the two data sets and between the two data sets. Encourage students to sort the data in a variety of ways to see if any patterns emerge. As you move around the room, you might ask students some or all of the following questions:

  • What are some similarities and differences among the data in Table 1? Table 2? Between Table 1 and Table 2?
  • What is one way you could classify or group these objects identified as planets on Table 1, to create groups of planets and asteroids that are similar to each other?
  • What attributes (characteristics) are you using to classify the planets and asteroids?
  • Follow up question: To which of your groups would a celestial object with the following characteristics belong: 18% the size of Earth, orbital inclination of 0.9 degrees, and 0.210 eccentricity? What other data would you want or need to help classify the object?

Next, move students into small groups of three or four students. Ask students to share patterns they noticed with their group. You might ask the groups some of the same questions you asked students as they worked independently to identify patterns. Encourage students to consider their observations of the two solar systems maps to support or refine the patterns they identified.

Bring the groups back together. Ask students, "How do you think astronomers used these data to support the argument for reclassifying Ceres, Pallas, Vesta and Juno from planets to asteroids?" Support a class consensus discussion using some or all of the following prompts:

  • What is our evidence for reclassifying Ceres (Pallas, Vesta and Juno) from a planet to an asteroid?
  • Both groups seem to be using the term asteroid/planet but in a different way, could someone explain the difference?
  • Would anyone have put this point in a different way?
  • What ideas are we in agreement about?
  • Is there more evidence or clarification needed before we can come to agreement? What is that?

Refer students back to the class list of questions. Are there any other questions we can now answer? Are there any new questions we need to add to our list?

Students may pose some of the following new questions:

  • Why was Pluto considered a planet in the 1950s when Ceres was not?
  • Why did scientists think planet Vulcan was there in the first place?
  • Why do the planets have different tilts (orbital planes)?
  • Why do the planets have different eccentricities?
  • Why is Mercury considered a planet?
Planetary Data 1846
Planetary Data c 1950

Reflecting on the Nature of Science

Say to students, "Arguing from evidence to support reclassifying Ceres (and Pallas, Vesta and Juno) from planet to asteroid, you acted, thought and talked as scientists. Let's take a little time to reflect on this process of science."

Instruct students to record the following three prompts in their science notebook:

  • How did I experience science findings are frequently reinterpreted based on new evidence?
  • How did I experience scientific explanations are subject to revision and improvement in light of new evidence?
  • Based on my experience, how do science and technology drive each other forward?

Ask students to independently reflect on the lesson and then respond to each prompt using words, pictures and/or symbols to record relevant evidence from their experience.

Next, give students an opportunity to share their reflections with a partner. As you move around the room, listen for students to share ideas you feel would help deepen the class' understanding of the nature of science. When you bring the class back together, call on these students to share their ideas first. Before moving forward with the lesson, reiterate science is a dynamic process that never concludes, which is why scientific knowledge is open to revision in light of new evidence.

Note to Educators: The student prompts represent elements of Understandings about the Nature of Science (NGSS Appendix D: The Nature of Science) and Interdependence of Science, Engineering, and Technology (NGSS Appendix J: Science, Technology, Society, and the Environment). Although the prompts represent elements in the 6-8 grade band, high school students may need the opportunity to develop these ideas to build the foundation for the 9-12 grade band elements.

Why isn't Pluto a planet anymore? (And why is Ceres no longer an asteroid?)

Ask students to recall the characteristics they used to classify celestial objects as planets and asteroids using the data available to astronomers in the 1950's. Why didn't Pluto and Ceres fall into the same category? Ask students to turn and share their thinking with a partner and encourage them support their ideas with relevant evidence from that data. You might ask for three or four shares from the class. Students might say:

  • Ceres (and Pallas, Vesta, and Juno) is under 10% of Earth's size and Pluto is over 10%
  • Ceres is shaped like a rolling pin (according to the Modern Space Map) and Pluto is round
  • Ceres is orbiting the Sun with 1,500 to 50,000 other objects like it (according to the Modern Space Map) and Pluto is by itself (according to the Modern Space Map)

Share with students that on August 24, 2006, the 26th General Assembly of the International Astronomical Union (IAU) voted on the first formal definition of a planet. As a result, Pluto was demoted from planet to dwarf planet while Ceres rose in stature from asteroid to dwarf planet. (Do not share the definition of a planet at this time.)

Ask students, "What questions does this raise? Turn and share one new question you have with a partner." Then ask students, "How many of you are wondering why there wasn't a definition of planet before 2006 or something similar?" Many students will likely raise their hands.

Tell students, "Before we learn the formal definition of a planet, should we find out what changed between the 1950's and 2006 that created the need for a definition?"

The Search at the Edge of the Solar System

Inform students you are going to share the Tumble Science podcast, The Search at the Edge of the Solar System. (Tumble Science is billed as a podcast for kids, but this interview with astronomer Jane Luu is appropriate for all ages.)

Read or share the description of the podcast with your students:

Is there a hard edge to the solar system? This question led to a big, breakthrough discovery that changed the way we picture the solar system - and every other solar system in the universe. The Kuiper Belt is a gigantic field of small, icy objects beyond Neptune, “planet scraps” left over from the formation of the planets. For many, many years, no one believed it might exist. Until astronomers Jane Luu and David Jewitt decided to see what was out there. Jane Luu tells the story of how she helped discover the Kuiper Belt.

Before playing the episode, tell students to find their reflection on the process of science and review the prompts. Ask students, "As you listen to Jane Luu, think about how she might respond to these same prompts. Jot your ideas in your science notebook."

Play The Search at the Edge of the Solar System episode for your students (02:05 - end). Depending on the age of your students, you might choose to pause at the following times and check for students' understanding:

  • 05:26 - In the 1980's, what did most people think was found beyond the orbit of Neptune? How does this compare with the data represented on the Modern Space Map c. 1950?
  • 09:10 - What kept Jane Luu and Dave Jewitt from giving up their search for faint objects beyond the orbit of Neptune?
  • 11:40 - Show students Jane Luu and Dave Jewitt's images of the Kuiper Belt object they discovered (below).
  • 15:25 - How did Jane Luu and Dave Jewitt's discovery of the Kuiper Belt change our scientific knowledge of the solar system?

At the end of the podcast episode, provide students an opportunity to share their ideas about how Jane Luu might respond to the prompts about the process of science with a partner or small group. Bring the class back together; consider using the questions above to engage the class in a discussion about how the discovery of the Kuiper Belt revised scientific knowledge about the solar system.

Kuiper Belt Object

The Story of Pluto and the Kuiper Belt: How Science Progresses

Join us for Jane Luu's live presentation, The Story of Pluto and the Kuiper Belt: How Science Progresses, on Saturday, November 14, 2020, at 11:05 am ET (it's not too late register!) and then come back here for the final piece of this lesson.


The Why isn't Pluto a planet anymore? Daily Do was made possible with generous support from The Kavli Foundation, "dedicated to advancing science for the benefit of humanity, promoting public understanding of scientific research, and supporting scientists and their work."

The Kavli Foundation

NSTA Collection of Resources for Today's Daily Do

NSTA has created a Why isn't Pluto a planet anymore? collection of resources to support teachers and families using this task. If you're an NSTA member, you can add this collection to your library by clicking Add to my library near the top o the page.

Check Out Previous Daily Dos from NSTA

The NSTA Daily Do is an open educational resource (OER) and can be used by educators and families providing students distance and home science learning. Access the entire collection of NSTA Daily Dos.

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