Classifying Objects: Its Purpose and Role in Science and Education – or, in Planetary Science, Is Pluto a Planet?

This article has been developed for various science-related events: teacher workshops, tables at family science events, and for distribution to educators around the country for Girl Scout-related events.

Why Do We, as Humans, Classify Things?

We group objects to help us understand and describe the world around us. How we, including scientists, group things may depend on the manner in which we are studying them. In Planetary Sciences, are you interested in the formation of the Solar System and how the planets, etc. interact? Do you study the atmospheres of the giant planets, rocky planets, or icy moons? Are you interested in the geological processes (past or present) on these bodies? Are you interested in where life exists below the surfaces of icy dwarf planets or moons?

Here are some quotes from classroom teachers and science educators I asked about the significance of classifying in science.

Carolyn Hollis, teacher:
We make sense of our world by categorizing. Without the ability to sort by common feature, every event would be a totally new experience, every object unlike every other. We use this understanding of similarities to communicate.

Bill Schmitt, science educator:
The Universe and everything in the Universe cannot be comprehended if everything is unique and independent from everything else.  Our ability to compare (same and different), find patterns, and categorize are just some of the thinking processes that help us bring order to the Universe and enable us to apply our understanding more widely.  It also helps us see the Universe differently, make connections, and recognize diversity and similarity in Nature.  

Chris Anderson, science educator:
Scientific debate can’t occur unless we are using the same language.

Art Lebofsky, science educator:
I have always defined science for my students and colleagues as “a search for
patterns.” It is what the brain does…[allows] us to find patterns which allow for
prediction and extrapolation. Without sorting or categorizing, our brain, and we, could not survive.

Classifying Solar System Objects, Officially and Unofficially:
Astronomers have been classifying and categorizing the objects in the sky for thousands of years: planets (originally including the Sun and Moon), stars, meteors, and comets. As astronomers discovered more objects and learned more about the Solar System, astronomers have had to change how we classify these objects. Since the invention of the telescope, astronomers have learned that the planets orbit the Sun (not the Earth), the Moon and planets are worlds not unlike the Earth, some planets have moons (also called satellites) of their own, there are millions of smaller bodies (asteroids and comets) orbiting the Sun, some of these small bodies (including some of the moons) are geologically active, and some may have subsurface oceans, etc.

For this reason, astronomers have created a classification scheme to help them and us understand how the Solar System formed and how it has evolved since it formed. Presently there are two views as to what is classified as planet:

1. There are eight (and only eight) planets as recognized by the International Astronomical Union in 2006.
2. All objects that are large enough to be spherical, due to their gravity, and thus can have geological processes are planets.

Classifying Solar System Objects, the Activity:
Nearly 20 years ago, I developed a presentation and hands-on activity called “Classifying Objects: Its Purpose and Role in Science and Education.” The presentation was created in response to the ongoing discussion of the status of Pluto as a planet, and it was several years before the resolution by the International Astronomical Union (IAU) in 2006 to reclassify (not demote) Pluto from a planet to the newly created class of Solar System objects called dwarf planets. After that, I changed the title of the presentation to “Classifying Solar System Objects” to reflect the ongoing, sometimes heated, discussion about the reclassification of Pluto, the creation of the class of dwarf planets, and the search for (and claims of the discovery of) Planet Nine.

More recently, I added a second part to this activity. It was first presented at several science teacher conferences and, more recently, Brother Guy Consolmagno and I, along with staff from the Vatican Observatory, presented a part of the activity at the Tucson Festival of Books (TFOB) in March of 2022. The involvement of the Vatican Observatory makes the activities relevant to Sacred Space Astronomy.

Here are some pictures from our presentation at the Tucson Festival of Books. As usual, there are no pictures of me since I take the majority of the pictures!

The following two pictures show the setup of our table and the images in the order they are presented. Since there is a lot of glare off the plastic holders. I have also included individual images in the activity discussion.

Classifying Solar System Objects, the Detailed Activity:
The goal of the activity is to get participants to think like scientists, look at all the evidence, and draw their own conclusions as to how they would classify Pluto and other Small Solar System bodies. Hint: there may be more than one “right” answer! Also, how we classify an object may change as we gather more information.

The following comes from our presentation at TFOB and from the activity that I have created for others to duplicate the presentation. I also do this activity in a more formal setting, such as an educator workshop. This includes an introductory PowerPoint and sorting activity as well as the Solar System Classification activity. If you are interested in having a copy of these activities, please contact me at: lebofsky at psi.edu

Presentation Part A, the Introduction to Classification:
The participants are introduced to classifying in our everyday lives, using the four provided images of animals in Figure 3 as an example. We are all very familiar with a house cat (Figure 5) and pet dog (Figure 6). These are probably what we picture in our minds when we think about a cat (feline) or dog (canine). The participants are then asked to group the four images.

However, how many of you have front yards where you often get to see a different kind of feline and canine? Figure 7 is a bobcat and Figure 8 is a coyote. These probably are not what you would normally picture in your mind for the classification of feline and canine.

We have a set of “talking points” to assist us and the other presenters in their discussions with
participants as to how they might group these animals.

• How would you group these animals?
• Canine vs. feline?
• Domestic vs. wild?
• Size/weight? (There may not be enough information from just the pictures)
• Color?
• Shape of ears?
• Fur: curly vs. straight?
• Length of tail? (There may not be enough information for the cat in a box)

Conclusion to Presentation Part A:
The “correct” answer is that there is no one correct answer. Your answer might be biased by
what interests you most, such as “I am a dog person, not a cat person,” or “I do not like animals
with black fur.”

Question: Is the cat in Figure 5 still a cat when it is no longer in the box? What new information
might we have, such as the length of its tail? Would that cause us to reclassify it? This is not
unlike the situation that astronomers have as they attempt to classify Solar System objects.

---

Presentation Part B, Classifying Solar System Objects:
In the previous section, we did an activity where we classified animals that may or may not be familiar to us. Now let’s see how we do as planetary scientists (astronomers, geologists, atmospheric scientists, biologists, etc.) as we attempt to try to classify Solar System objects.

The presenters discuss the images on the left (Figure 2)—the five classes of Solar System objects—Planets, Dwarf Planets, Moons, Asteroids, and Comets. Here are the classifications that the IAU has published and the representative objects that are in the display:

Small Solar System Objects: objects that orbit the Sun but are too small (not massive enough) for their own gravity to pull them into a nearly spherical shape. These include asteroids and comets.

Note: these definitions pertain only to objects in the Solar System and not to the more than 5,000
exoplanets orbiting other stars.

---

Presentation Part 2, the Challenge:
The participants are then shown a set of seven Solar System objects (Figure 2). These are a subset of the 30 that are available for a classroom setting where more time is available. The participants are asked to match these seven objects with the classes that they belong to. Participants must also justify their answers. This is not easy and even planetary scientists will often get the wrong answers. Why? There is sometimes insufficient information: How large is the object? Does it orbit the Sun or a planet? Does it have a coma that is too faint to be seen in the image?

Here are examples of objects similar to the ones that the participants would be trying to classify as Planet, Dwarf Planet, etc. I am using different images so that I do not “give away” the answers. The only hint participants are given is that there is at least one object in each of the classes. In the actual activity, there is information about the Solar System object on the back of each card so the participants can see if they got them right or not (there is only one right answer based on the existing, official classification scheme).

Try to classify the following objects and then look for the answers at the end of the article below the pictures. How many did you get right? What additional information would have helped you in classifying these objects?

Concluding remarks:

Speaking as a planetary scientist and educator, how we classify anything in the Solar System and beyond should take into account all of an object’s characteristics, not just size, shape, or distance from the Sun. Maybe the answer is much broader than just eight objects or anything that is round. In our Solar System, there are eight planets, each with their own unique properties. Also, since not all the moons of the Solar System are the same, maybe they should be put into separate categories. Finally, our planets are not typical of the known exoplanets around other stars. They, too, should have their own categories and then where do our eight (or nine) planets and hundreds of moons fit in?

As scientists, classification helps us when we are trying to study/understand something. We therefore try to come up with ways to describe things so that we can look for patterns and put them into groups. By creating a definition, it is our goal to create a description so that others can understand the groups we have created and the patterns that we are trying to describe. Sometimes, as you have seen in the activity, this is not easy.

Solar System objects can be grouped/classified by their properties, but there may be more than one “correct” answer, depending on your area of interest. These are the questions to think about:

• How are planets, dwarf planets, asteroids, comets, and moons/satellites grouped?
• Do all objects in the same group have all of the same properties? [no]
• Are there properties of objects that overlap different groups? [yes]

When we (activity participants, and planetary scientists) classify Solar System objects, what do we consider? What are the characteristics of the objects? How are objects in the same classes similar or different? Are there obvious or logical groupings? As you can see from these images, not all objects have all of the same characteristics. There are properties that overlap different classes.

Some Historic Examples of Reclassification:

Here are examples of objects that have been reclassified as our knowledge of the Solar System changed, our definitions changed based on certain characteristics, or as we learned more about the objects.

• Sun: planet → star [Earth not the center of the Universe]
• Earth: center of Universe → planet [Earth not the center of the Universe]
• Venus: star(s) → moon → planet [Earth not the center of the Universe]
• Moon: planet → moon/satellite [Earth not the center of the Universe]
• Galilean satellites: planets → moons [orbited Jupiter as Moon orbited Earth]
• Ceres, etc.: planets → minor planets/asteroids
• Ceres: asteroid → dwarf planet [reclassification of Pluto, etc.]
• Pluto: planet → dwarf planet [reclassification of Pluto, etc.]
• Asteroids, distant objects → active asteroids, comets [at first appeared “star-like,” but then
• showed comet-like activity]

Points for Discussion:
As I have said before, classification is not always clear-cut. Not all objects in a class have all the characteristics of that class. As we learn more, how we classify a Solar System object may change. Also, other factors may change how we classify an object. For example, an icy body may get closer to the Sun and start showing cometary activity; an asteroid may get impacted by a smaller asteroid, creating a dust tail or exposing sub-surface ice, becoming a cometary-like object (we call these active asteroids).

• While there are, as I write this, 207 known moon/satellites orbiting six planets, there are 490 known companions (moons) orbiting 470 asteroids and outer Solar System objects including dwarf planets. While the planets Mercury and Venus do not have any moons, the asteroid 130 Electra has three moons and the dwarf planet Pluto has five moons!
• Of the 207 planetary moons, 148 are probably captured comets/asteroids. As an example, the moon Phoebe was probably an outer Solar System body that encountered another object, sending it into the inner Solar System where it would have become a comet. However, it got too close to Saturn and was captured and is now a moon.
• Triton, a moon of Neptune is larger than Pluto. One migrated inward from the Kuiper belt and was captured by Neptune and is now a moon and the other is an outer Solar System object (Kuiper belt object). If it had not encountered Neptune, Triton might have migrated all the way into the asteroid belt and been another Dwarf Planet. Based on its composition, it has been proposed that this is what happened to Ceres.
• Ganymede, a moon of Jupiter, is larger than Mercury. It and at least four other moons have atmospheres that are thicker than the extremely thin atmosphere of Mercury. The atmosphere of Saturn’s moon Titan is thicker than the atmosphere of Earth and has liquid lakes on its surface (methane and ethane). Several of these moons are also more geologically active than Mercury.

Fig. 15: The surface of Jupiter’s moon Io. Io is the most volcanically active object in the Solar System. Image from NASA’s Galileo mission.
Fig: 16: An overexposed image of Saturn’s moon Enceladus showing its geysers. Image from NASA’s Cassini mission.

Conclusions:
How a planetary scientist might classify a Solar System body may depend on what they are studying, what they are most interested in: cometary activity, atmospheres, geologic processes, etc. Finally, it is always good to remember this quote from scientist and author Isaac Asimov:

The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka,’ but ‘that’s funny…’.

By organizing objects such as planets, dwarf planets, moons, etc. into classes with certain characteristics, we are in a better position to discover things that do not fit into our prior knowledge, i.e., what we were expecting to find. This opens up whole new fields of investigation, for example tidal heating on the satellites of the gas giant planets such as volcanoes on Jupiter’s moon Io or geyser-like jets of water on Saturn’s moon Enceladus. In the case of Enceladus, the presence of geysers implies the existence of a liquid water subsurface ocean where life may exist!

---

The Challenge Answers:

---

Explore the Solar System in NASA’s Interactive Web-based Solar System App: