4.4: Non-planetary Bodies

Introduction

Scientists use specific criteria to distinguish planets from other objects in our solar system. In this activity, you will examine characteristics such as mass, radius, orbit, neighborhood clearing, and shape to determine what defines a planet.

4.4.1 Comparing Mass and Radius

Model

Mercury, Earth, and Titan compared by size and mass

1. Rank the objects in terms of size (radius) from smallest to biggest.
2. Rank the objects in terms of mass from smallest to biggest.
3. Titan is not a planet, but the other objects are. Do you think radius is an appropriate criterion to distinguish planets and non-planets? Explain.
4. Do you think mass is an appropriate criterion to distinguish planets and non-planets? Explain.

4.4.2 Comparing Orbits

Model

Solar system objects show Mercury and Earth in orbit around the sun and Titan in orbit around Saturn

1. What object do each of these bodies orbit?
2. Based on your responses above, what might be a useful criterion to distinguish between planets and non-planets? Explain.

4.4.3 Clearing the Neighborhood

Model

Mercury and Earth have clear orbits while Pluto is surrounded by debris

1. Which object(s) have a relatively empty neighborhood around their orbit?
2. Which object(s) have a “crowded” neighborhood around their orbit?
3. Pluto is not a planet, but the other objects are. What distinguishes Pluto from the two planets in this model?
4. Based on the distinction between Pluto and planets, do you think the asteroids in the asteroid belt are planets? Explain why or why not.

4.4.4 Shape

Model

Comparison of spherical planets and irregularly shaped asteroids.

1. How do planets differ from asteroids in terms of shape?
2. Using only this criterion, would you sort Pluto into the planet category or the asteroid category? Explain.

4.4.5 Asteroids vs Comets

Model

1. What substances do you think the dots inside the circle are made of?
2. What substances do you think the outside the circle are made of?
3. Asteroids are small objects (failed planets) that are made mostly of rock. Where do you think asteroids
   can be found?
4. Comets are small objects (failed planets) that are made mostly of ice. Where do you think comets can be
   found?