Worlds in Comparison Activity Guide | EcoTarium

Worlds in Comparison Activity Guide

  • Type of Activity: Classroom or drop-in station; facilitated or self-guided
  • Set up Time: 5 minutes
  • Time to Do: 20 minutes
  • Audience age: 8 years and older
  • Group size: variable, as individuals or in groups of 2 - 4

What's This Activity About?

This activity allows participants to develop an understanding of the relative sizes (volumes) of the planets in our solar system. Families start with a big ball of playdough and divide it up following the steps on the instructions sheet. When they are done, they see how the planets vary in size. By the time they get to tiny Pluto, they are typically quite amazed.


Each group will need:

Setting Up the Activity

This activity works best if the worksheets with the planet names are placed side-by- side on a table, and are arranged to match the order from the Sun. In front of these sheets place the instruction sheet and the playdough and plastic knife on the breadboard (or equivalent). Be sure there is enough room in front of the table for the group to work together. It is crucial to have the indicated amount of playdough for each group. If there is less than three pounds, the Pluto piece will be too small to see! We recommend three pounds each and urge you to try the activity for yourself before leading it.

Suggestions for Introducing the Activity

For any of these scale model activities, it is useful to start by exploring the notion of models. Referring to playthings, such as dolls or toy cars, can be a useful reference for talking about scale models. For a more thorough exploration of this concept with ideas of how to introduce and discuss it, see the introduction to the Saturn Project.

This activity is designed as a self-guided station activity. Nevertheless, if you choose to do so, it can also be a facilitated activity from the beginning. If you facilitate this activity from the start, begin by asking the participants which planet they think is the largest. Which is the smallest? For whatever planet they say is the largest (it will most likely be Jupiter), ask them the following question: If we could combine all the planets together into a big ball, what fraction of that ball would the largest planet be? Might it be 1/9 or 1/5, for example? End the introduction by telling them they will get a better idea after completing this activity.

Note: If people will be using previously used playdough of various colors, you can reassure participants that mixing colors is fine (after all, many planets are multicolored!).

Doing the Activity

Participants start by reading the instructions handout, but they should get into working with the playdough as quickly as possible. They should follow the instructions as to how to divide up their playdough, placing the parts in the proper planet boxes. Each time the playdough is divided up and parts are combined to make a planet, be sure participants roll the combined parts around in their hands until the planet has a ball shape.


Start by asking the group about some of the discoveries they made regarding the sizes of the planets. Were there any surprises? Ultimately direct the discussion so that they realize the smaller planets (except Pluto) are the inner planets, while the larger planets are the outer planets. You may also want to note that more than 96% of the combined volume of the planets is in Jupiter and Saturn (approximately 60% in Jupiter and 36% in Saturn). Those giant planets really ARE giants.

Complementary Activities

Scale models of the Solar System and planets:

  • Pocket Solar System
  • Earth As a Peppercorn
  • Sizing Up the Moon
  • Saturn Project

Other scale models:

  • Birdseed Galaxy
  • Sizing Up the Stars

Materials Sources


Three pound tubs of Play-Doh, Crayola Dough or other similar modeling clay can be purchased from many craft suppliers. As of this writing, Dick Blick sells 3 lb. tubs of Crayola Dough for $7.49 each ( or (800) 828-4548).
Alternatively you can make your own dough using the recipe below. The only drawback is that this can grow moldy, so if you plan to do this activity often, it is recommended that you invest in the commercial product.

Kool-AidTM Playdough Recipe

This recipe makes three pounds of colorful, scented playdough:

  • 5 cups of flour
  • 1 cup salt
  • 4 packages dry unsweetened Kool-AidTM
  • 4 cups boiling water
  • 6 tablespoons vegetable oil

Mix the dry ingredients together in a bowl.  Mix the liquids together and pour them over the dry ingredients. Stir the mixture until it forms a ball (this may take a while — keep stirring). As the mixture cools, it will become less sticky. After the mixture has cooled to room temperature, take it out of the bowl and knead it until it is smooth. Store in ziplock bags in the refrigerator until ready to use.

Originally developed by Dennis Schatz (Pacific Science Center) for Family ASTRO

Adapted by Anna Hurst

© 2006, Astronomy from the Ground Up
Astronomical Society of the Pacific, 390 Ashton Ave., San Francisco, CA 94112