Make a Model of the Solar System

Background Information for Teachers

This section contains a quick review for teachers of the science and concepts covered in this lesson.

The solar system is the system of objects that orbit the Sun directly (e.g. the planets) or indirectly (e.g. Earth's moon). A celestial body is considered a planet in the solar system if it orbits the Sun, if it is heavy enough for gravity to squeeze it into a spherical shape, and if it has "cleared the neighborhood" around its orbit. The latter means that there are no objects comparable in size in the vicinity of its orbit, other than the planet's moons. Eight objects in the solar system qualify as planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Dwarf planets, like Pluto, fulfill the first two criteria, but not the last. Planets are not self-luminous—they do not emit light like the stars—but they can be seen in the sky because they reflect light emitted by other celestial objects.

Planets are approximately spherical, and the equatorial radius is often used as a measure of their size. The orbital distance is the average distance from the planet to the Sun as it circles the Sun. The distance from the Sun to a planet is not constant since the orbits are elliptical, but a circular orbit with the orbital distance as radius is a good approximation. The orbital distance is often expressed in astronomical units (AU). One AU equals roughly the distance from the Sun to Earth, or 149,597,870,691 ± 30 meters. Table 1 lists the radii and the orbital distances of the eight planets of the solar system.

Listed in increasing orbital distance from the Sun, we first encounter Mercury, the smallest of the eight planets. Mercury is only slightly larger than Earth's moon. Next is Venus, a planet with a radius of 6,052 km, only slightly smaller than Earth. Then comes Earth, the planet with the highest average density (5.5 g/cm³), followed by Mars. Mars' radius is about half of Earth's radius. It is a dusty, cold planet, but might have inhabited some form of life long ago. These first four planets are called the inner planets because they orbit closest to the Sun; they are also known as the rocky planets because their surfaces are rocky, showing mountains, valleys, and other formations. The rocky planets are all smaller than the gas planets and they are made of denser material. The next four—Jupiter, Saturn, Uranus and Neptune—orbit farther away from the Sun; they are gaseous planets. Jupiter is the largest, with a radius 11 times larger than Earth's radius; followed by Saturn, whose radius is about 9.5 times larger than Earth's radius. Saturn is the planet with the lowest density (0.7 g/cm³), a density so low that it would float if placed in water! Uranus and Neptune are similar in size, with a radius of 4.0 and 3.9 times the radius of Earth, respectively.

To make a scale model, your students should scale all the distances (radii and orbital distances) by the same factor, called the scale factor. For example, to create a scale model of the eight planets, scaled so the radius of Earth is 1 cm, you use a scale factor of 1 cm/6,378 km because 6,378 km (the actual radius of Earth) multiplied by 1 cm/6,378 km (the scale factor) yields 1 cm (the radius of the Earth model). You can find the radius of each of the other seven model planets by multiplying each planet's actual radius by the scale factor.

Table 2 lists the radii for this model and Figure 2 shows the results. Your students will make similar model planets. Because the radius of the Sun is about 109 times the radius of Earth, the Sun would have a radius of a little over 1 m in this model.

The planets' radii are small compared to their distance from the Sun, and it takes a little more work to calculate how far from the Sun these model planets need to be placed to get an accurate representation of the solar system. Orbital distance is often expressed in AU, so the first step is often to calculate what distance in the model corresponds to 1 AU in the real world. As 1 AU is approximately 150 million km, or 1.5 × 10⁸ km, and 1 km converts to (1/6,378) cm, 1 AU will become 235,183 cm or about 235 m in your model. The scale factor 1 m/6,378 km thus becomes 235 m/1 AU. This is the same factor, only expressed in different units. Next, your students should multiply the orbital distance (in AU) of each planet by the scale factor (235 m/1 AU in this case) to find how far from the Sun the planets need to be placed in the model. Table 3 lists the results for a model where Earth is represented by a sphere with a radius of 1 cm.

You will likely not have space to lay the model planets out in the classroom, nor on the school grounds. Students can, however, use a map of their surroundings to visualize the scaled-down planet distances. They can use a printed map or an electronic application, like Google Maps. If you prefer a model where the solar system fits in the classroom, try the activity Model the Distances between Planets in our Solar System. Unfortunately, when scaling the solar system that much, the planets become too small to be visible.

Additional Background Links

• Solar System Exploration, NASA
• Watch This Guy Build a Massive Solar System in the Desert | Short Film Showcase (video), National Geographic
• Neptune Approach, NASA
• Voyager fact sheet, Jet Propulsion Laboratory

Prep Work (10 minutes)

Familiarize yourself with the scale model presented in the Background Information or view the two videos listed below. These resources will give you information to fall back on when evaluating the students' models while they are making them.

Engage (10 minutes)

Explore (50 minutes)

Split the class in small groups, preferably four students per group. Smaller groups are fine, too. Provide each group with a calculator, play dough, four balloons, a metric ruler, tape, cardstock, and scissors.

It is very hard to get an idea of how large the solar system and the planets are because of the vastness of it all. Scientists make scale models of objects that are too big or too small to gain a better understanding of the scale and to manipulate them easily. Today, you will make a scale model of the solar system with your group.

Imagine shrinking the solar system so much that Earth becomes a small sphere that you can hold in your hand. If we shrink our entire solar system by that much, all the planets will shrink by the same amount, and become something you can hold. The distances between the planets will shrink, too.

In your group, discuss what you think this shrunken model will look like.

Ask students to make a sketch of a scale model of the solar system. This task is included on the Student Worksheet.

Have each student choose two planets. The student will be responsible for doing the calculations for these two planets. Students in the same group cannot choose the same planet so that in groups of four, all planets will be covered. For groups with fewer students, the students who are done early can work on the remaining planets.

In your group, use the information provided in the table listing the radii of the planets of the solar system to choose a scale factor. You need to choose the scale factor so you can model all the planets with the play dough and the balloons you are provided.

(The table is included in the PowerPoint presentation and the Student Worksheet.)

You can give the students the following formulas and example:

If you want to create a model where Mercury is represented by a sphere of 1 m radius, you need to scale 2,440 km down to 1 meter. The scale factor is 1 m/2,440 km. A planet of 1 m radius is too big for your model, however, so choose a factor that will allow you to model all the planets with the play dough and the balloons you are provided.

If needed, discuss with the class what a scale factor is and give some examples. If necessary, ask the students to make or draw a scale model of an object of their choice as homework.

A scale factor is the factor by which you scale everything up or down and still obtain a model with the same proportions. For example, a scale model of a building and the actual building have the same proportions, the ratio of their height to their width is identical.

Ask students if they can name a few toys that are scale models of real objects; dolls, toy cars, toy tea sets, etc. are good examples. Ask them if they can approximate the scale factor between the real object and the model. Note these scale factors need to be smaller than 1, as the model objects are smaller than the actual objects.

If needed, you can work out an example that speaks to the students' imaginations. Choose an object that students are all familiar with and that has at least two dimensions. Examples are a car, the school, a house, an animal, a soccer field, etc. and scale it down using the formula:

An example is listed in Table 5.

In your group, discuss how you can calculate the size of each model planet using the data provided in the table and the chosen scale factor. Each student should calculate the radii of the model planets they are responsible for and share this information with the group.

In your group, discuss how you will make the model planets. Which ones will be made from play dough and which ones from balloons? Each student should then make the model planets they are responsible for and label them.

In your group, look at the model planets and the radii of the actual planets. Does the model look to scale? How can you know?

An example of what the model might look like is shown in Figure 3 and Figure 4.

The following question is ideal for groups who are done early. You can give groups who struggle to get the work done an idea of the size of the Sun in their model and move on to the next question.

The radius of the Sun is 6.96 10⁵ km. In your group, calculate the radius that the Sun would have in your model. What object could be a good model for the Sun in your shrunken model?

Tell the students that, so far, they have modeled the eight planets and maybe the Sun. Now they have to find out where to place them in their model.

Explain the term orbital distance and the astronomical unit (AU). The planets orbit (or circle) the Sun. These paths are not exactly circular, but elliptical, and the orbital distance is the average distance from the planet to the Sun. Because these distances are so expansive, they are often expressed in astronomical units (AU). One AU equals roughly the distance from the Sun to Earth, or about 150 million km.

Let students write these definitions on their Student Worksheet and let groups calculate the distance 1 AU converts to in their model.

In your group, discuss how you can calculate the distance from the Sun to the planets in the shrunken model, given the data listed in Table 6. Each student should calculate the distance from the Sun to the planets they are responsible for and share this information with the group.

(The table is included in the PowerPoint presentation and the Student Worksheet.)

In your groups, discuss where you would need to place the Sun and each of the planets so the model is to scale. Use an online or a printed map to find landmarks or places that are approximately at the correct distance and well-known to most students in the class.

If time allows, let students prepare a short presentation to show and explain their scale model to the class.

Reflect (10 minutes)

Assess

You can use this quiz to assess student learning after the activity:

• Online quiz, assignable in any LMS
• Quiz (pdf) and answer key (PDF)

The Student Worksheet, the optional group presentation, and the drawing of the solar system students make at the end of the lesson can also be used to evaluate student understanding of the lesson.

Lesson Plan Variations

• In this lesson, the students chose a scale factor. You can also provide the scale factor, or ask them to shrink the solar system until the radius of Earth in their model is, for example, 1 cm, or until the distance from Earth to the Sun is 10 cm. This will ensure all groups have a model that is identical in size.
• If time is short, have the students make a scale model of the planets and show the National Geographic's video listed in the Additional Background section to show students the vastness of the solar system.
• Assign the activities How Big Are the Planets in Our Solar System? and Model the Distances between Planets in our Solar System as homework. These activities remove all the mathematical difficulties of scaling, but still allow students to get a better idea of the vastness and emptiness of the solar system. Let a few students present their models to the class.
• Add other objects to your model, like Earth's moon, the outer boundary of the solar system, or the current location of the Voyager 2 spacecraft.