Explore! Jupiter's Family Secrets

Dunking the Planets


In this 30-minute demonstration, children compare the relative sizes and masses of scale models of the planets as represented by fruits and other foods. The children dunk the "planets" in water to highlight the fact that even a large, massive planet — such as Saturn — can have low density. They discuss how a planet's density is related to whether it is mainly made up of rock or gas.

This activity establishes the relationship between density and composition. It should be conducted before Heavyweight Champion: Jupiter! in order for the children to better distinguish the concepts of size, weight, and mass. These concepts involve more advanced science than previous activities in Jupiter's Family Secrets, and they explore more deeply the science of the Juno mission and the rich information it will return to us. Facilitators who choose to undertake this activity should have a firm grasp of the scientific basis so that misconceptions are not introduced to the children.

This series is appropriate for children ages 10 to 13.

What's the Point?

  • Models can help us compare characteristics of planets.
  • Planets have measurable properties, such as size, mass, and composition.
  • An object's density cannot be determined by its size alone (larger objects are not necessarily more or less dense), but also depends on its mass.
  • A planet's density is related to its composition. The four inner terrestrial planets are dense compared to the four outer planets. The inner planets are made up mainly of dense, solid rock. The outer planets are composed primarily of gas, so their overall density is lower. However, they are larger in size and much more massive!
  • Saturn is massive and second in size to Jupiter, but its density is so low that it would float in water!


For each group of 10 children:

  • One of each of the following fresh fruits and other foods (listed from largest to smallest):
    • (5 1/2"-wide) large mango or potato
    • (4 1/2"-wide) large unpeeled orange, coconut, or cantaloupe
    • (2"-wide) plum
    • (2"-wide) kiwi or lime (not a lemon)
    • (1/2"-wide) small grape
    • (1/2"-wide) large blueberry
    • (1/4"-wide) pea or navy bean
    • (1/5"-long) uncooked orzo pasta
  • 1 (18" wide x 8" deep or larger) bowl, tub, or small wading pool
  • A sink or other access to water
  • Optional: 1 golf ball or ball bearing
  • Optional: 1 ping-pong ball or a marble that is similar in size to the ball bearing

For each child:

For the facilitator:


  • Review the complete background information.
  • Because of the differences between individual fruits and changes in their composition over time, you will need to test your materials before the activity. Substitute as needed, using the Dunking the Planets: Selecting Appropriate Foods to select appropriate fruits and other foods to represent the planets, based on their size and density. Only the 4 1/2"–wide large orange, coconut, or cantaloupe should float; the others should sink.


1. Present the fruits and other foods to the children and explain that you will use them to model the physical properties of the planets. Discuss one important property of the planets represented by this model: size.

  • What's a model?

We use models to help us represent objects and systems so that we can study and understand them more easily. By "a scale model" in this case, we mean a model that has smaller parts but parts that are relatively the same size and distance to each other as the real planets, dwarf planets, asteroid belt, and Sun.

  • What are the names of the planets in the solar system?
    Starting with those closest to the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
  • Which planets are the biggest? Which are the smallest?

As the children answer, give them the fruit or other object that represents each planet and ask them to stand at the front of the room.

2. Optional: Have the children investigate the difference between size and mass by comparing a golf ball and a similarly sized ping-pong ball. Invite them to predict which is more massive before allowing them to hold them.

  • How do the sizes of the two balls compare? They're about the same size.
  • Which is more massive? The golf ball.
  • Is that what they predicted, based on looking at their sizes? Why would they be different? Answers may vary. Encourage discussions about how the golf ball has more mass packed into the same volume, i.e. it is more dense, and help the children discover the concept of density as it relates to the planets as the activity progresses.

Facilitator's Note: Children may not know the difference between mass and weight, so it is important to explore the concept of mass as you introduce this activity. Mass is the amount of matter an object contains and is an intrinsic property of that object — its mass does not change depending on its location, temperature, or any other function of its environment. Weight depends in part on the gravitational pull experienced by the object; an object's weight does depend on its location. An object may weigh less on the Moon, but it still has the same mass as it does on the Earth.


3. Discuss which of the models of planets has the most mass and which has the least.

  • What is mass? Mass is the amount of "stuff" or material that an object has. This can include solid, liquid, and gas.
  • How can we estimate which fruits are the most massive? The least massive? The children may offer that the most massive objects are "heavier," and the least massive objects are "lighter."

It is true that the more something weighs, the more mass it has. Clarify that mass and weight are different, however. An object's mass does not change simply by changing its environment, but its weight can change. For example, a child's mass does not change when she enters a pool. Yet, the water's buoyancy helps her feel "lighter" while swimming and she is able to jump higher or lunge further as a result. Similarly, an astronaut's mass is the same whether he is on Earth or the Moon. His weight on the Moon is only a fraction of his weight on Earth.

4. Invite the children to group the different models of the planets based on their estimated masses. The groups may include " heavy" and "light," or a range of estimated masses. Invite them to record their estimates in their journals.

  • Which of the fruits and food has the most mass? One of the largest objects, such as "Jupiter" or "Saturn" should be heaviest.
  • Is Earth or Jupiter more massive? Jupiter.
  • Is the largest fruit necessarily the most massive? No, size and mass are two different things.
  • What are some examples of objects that have similar sizes (volumes), where one has a lot of "stuff" packed into it compared to the other? The children may have a variety of ideas, such as ball bearings (compared to marbles), bricks (compared to wood blocks), a candy bar (compared to cotton candy), etc.
  • How do we describe objects that have a great deal of "stuff" (mass) for their size (volume)? Dense.

5. Invite the children to describe the different planets' compositions.

  • What materials make up the planets? The inner planets Mercury, Venus, Earth, and Mars are made of rocky material; the outer planets are composed of "gases."

6. In their journals, have the children record their notes and make predictions: Which of the fruit "planets" will float and which will sink if placed in water? Have them consider, in general, which is more dense: rock or gas? Which planets are more dense:  the rocky, inner planets or the gaseous outer giants?

  • Have any of them heard that one of the planets would float, if it were placed in a large enough bathtub?  Saturn would float.
  • Why do they think this would happen?

7. Begin the density experiment. First, invite the children to state their predictions about what will happen when "Saturn" is placed in the water.

  • Is this object light or heavy? Why? What is it made of? It is heavy because it has a lot of mass.
  • Why might it float? Why might it sink? It will feel "heavy" and so the children may guess that it will sink. Some may realize that this object has trapped air in it, which will help it to float.

8. Invite the child holding "Saturn" to place it in the bowl filled with water, and then continue with the other planets.

  • What happened to "Saturn"? It floated.
  • What might happen to the other planets if we put them into the water? They will sink. Let the children place each of the other "planets" in the water, one at a time.
  • Why do the smaller, less massive "planets" like Mercury sink when Saturn floats? The other "planets" are all denser than "Saturn."

9. Compare the properties of the fruits and food to the planets with the children, as a model.

  • Is Saturn big? Does it have a lot of mass? Yes and yes.
  • How is the orange like Saturn? It's bigger and has more mass than most of the other objects, yet it floated.
  • Why would Saturn float if it were placed in water? It is made of a lot of gases that are lighter than water.

Facilitator's Note: The hydrogen and helium that make up much of Saturn are compressed to a density more like that of a liquid or even a solid for much of the planet. However, the planet's overall density is still lighter than water. That does not detract from the enormous amount of mass that the planet has; it is the second most massive planet in our solar system.



Help the children compare the models of the planets to the characteristics of the planets.

  • In this activity, which planet was the least dense? Saturn.
  • Were the giant planets all less dense than water? No, only Saturn was. Why not? The giant planets are made of mostly gas, but they are made of a LOT of gas! The gases are squished, or compressed, to a greater density, and even act like liquids or even solids.
  • The giant planets are made mostly of gas. What are Mercury, Venus, Earth, and Mars made of? Rock.
  • In general, what can we say about what a planet is made of from its density? The giant planets are less dense overall because they are made mainly of gases, and the inner rocky planets are more dense because they are made mainly of rock.
  • If we weighed our models, which planet would weigh the most (and has the most mass)?  Jupiter.
  • The least? Mercury.
  • Which planets were the biggest? Jupiter, followed by Saturn. Smallest? Mercury.

Ask the children to write, in their own words, the relationship between mass, size, and density in their journals.

Reiterate that a planet's density is determined by a combination of its size and mass, and that we can't measure an object's density just by weighing it. Jupiter has the most mass and is very big, but is denser than Saturn. The Juno mission to Jupiter will help scientists map the densities of the planet's deepest layers in order to better understand their composition and structure.

If possible, build on the children's knowledge by offering them a future Jupiter's Family Secrets activity. Invite the children to return for the next activity to discover what effect Jupiter's immense mass would have on them in Heavyweight Champion: Jupiter!

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