Planetary Compositions

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Astronomy, Earth Science: Planet Formation
Grade Range: Middle School, High School
Format: Hands-on, Stage

This demonstration gives a thoughtful look at why the planets are classified as either rocky planets or gas giants. Have some paper towels on hand in case the water is spilled.



  • Small Rocks and Minerals
  • Small Beads or Marbles, Various Types (Plastic, Wood, Ceramic, Glass, etc.)
  • Packing Peanuts
  • Ice Cubes
  • Cotton Balls
  • Small Craft Pom-Poms
  • Water
  • Table Fan
  • 500mL Graduated Cylinder
  • Planet Densities Chart

Safety Precautions

Please read the General Safety Precautions section of the Demonstration Safety page before performing this demonstration.


Presentation (Stage)
  1. Set out the graduated cylinder and add 200mL of water to it. Explain to the audience that you want to see how much water can be displaced by different materials.
  2. Drop the denser objects into the cylinder: the rocks and minerals, marbles, ice cubes, and any ceramic or glass beads. Add enough of these to adjust the water level by 100mL. Point out that the water level rose as you added these things in to a visibly different level.
  3. Drop the less dense objects into the cylinder: plastic and wood beads, cotton balls, pom-poms, packing peanuts. Fill the cylinder to the top with the less dense objects, and point out that since these objects aren't nearly as dense, the water level didn't adjust much.
  4. Set the fan on one end of the table. Pour out the water from the cylinder and lay out all the objects in front of the fan in a pile. Turn the fan on high for 10-15 seconds, and note which objects moved/didn't move. Be sure to have the denser objects underneath the less dense objects, otherwise you might not see anything happen!
  5. Ask the audience the following: What are the planets closest to the sun mostly made of? What are the planets farther from the sun mostly made of? Does the density of the materials seem to affect where the materials are in the solar system?
Presentation (Hands-on)
  1. Set out the graduated cylinder and add 200mL of water to it. Set out all the objects to drop into it, and have the planet density chart out as well. Set the fan on the far side of the table, aiming towards the center.
  2. As students come up, have them drop the various objects into the graduated cylinder to see how dense they are. Have them guess which planets would sink if they could be dropped in water, and which ones might float or sink slowly.
  3. Have the students set a variety of the objects in front of the fan. State that the fan will show the solar wind the sun has, and ask what they think will happen to the objects. Turn the fan on low or medium for 10-15 seconds, and note the results. Do the results seem similar to how the planets are arranged?

Why This Works

Planet Formation is a look at how planets can form, and what factors play into where they form and what they are made of. In our solar system, we know that our planets are in two distinct groups: the rocky planets (Mercury, Venus, Earth, Mars) and the gas giants (Jupiter, Saturn, Uranus, Neptune). Our planets are in these two groups because of two main factors: their overall density, and the sun. When the various objects were dropped in the graduated cylinder, we saw the the denser objects sank and the less dense objects float. This is the first key to why the planets are sorted as they are; the more dense materials, such as rocks and minerals, stay closer together because of their higher density, and over millions or billions of years can form rocky planets. the less dense materials, such as the packing peanuts, are filled with a lot of air, and so they float above and away from the other objects. the gases would collect around the denser core of a rocky planet, and if there is enough gas it might form a gas giant or star. The second factor is the sun, or more precisely the solar wind. the Solar Wind is the constant wave of high energy particles coming from our star, and this wind can push the less dense gases and matter away from the sun. This is why our gas giants are the furthest from the sun, and our rocky planets are closer; the rocky, denser planets could withstand the solar wind better, but most of the gas and less dense matter was blown away to the farther reaches of the solar system! This is why Mercury has no atmosphere, since it is so close to our star, and Saturn has enough gas that the planet is less dense than water!

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