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Model the Distances between Planets in our Solar System

Summary

Active Time
20-30 minutes
Total Project Time
20-30 minutes
Key Concepts
The solar system, planets, scale model
Credits
Sabine De Brabandere, PhD, Science Buddies
Make a Model to Explore the Distance from the Planets to the Sun

Introduction

Did you know that the Voyager 2 spacecraft took 12 years to travel from Earth to Neptune, the furthest planet in the solar system? This sounds like a really long time! Maybe not that long if you consider what distance the spacecraft had to travel to get from Earth to Neptune. In this activity, you will make a model of the planets in the solar system and specifically model their distances to scale. Will it explain why the Voyager 2 took so long? Try it and see!

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Materials

  • Yarn, if possible, in up to 8 different colors
  • Measuring tape
  • Thumbtack
  • Two pieces of cardboard, about 6 cm by 6 cm each
  • Cardstock
  • Scissors
  • Marker or pencil
  • Hole punch
    The materials needed to create a scale model of the orbital distances of the eight planets in the Solar System.

Prep Work

  1. Cut out eight labels from the cardstock, 2.5 cm × 6 cm works well.
  2. Punch a hole in the side of each card.
  3. Write the name of a planet on each card: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, so you have one name tag per planet.

    Illustration of eight name tags, one for each planet
  4. Place the pieces of cardboard on top of each other. Push the thumbtack through the center of both pieces of cardboard. The shaft of the thumbtack will represent the Sun.

Instructions

  1. You will make a model of the solar system. Imagine you shrink the solar system so much that the distance from Earth to the Sun becomes 10 cm. When you shrink the solar system this much, all the planets shrink in size, so they become too small to see. You will add labels so you can remember which planet goes where. In the following steps, you will find out how far away from the Sun each of the other seven planets will be in your shrunken model.
  2. For planet Earth:
    1. Tie the end of the yarn to the card labeled "Earth".
    2. On the string of yarn, mark a point 10 cm from the label—the distance from Earth to the Sun in your model.
    3. Cut the string about 3 cm further down so you have a little extra to tie a knot.
    4. Tie the string to the thumbtack—the Sun of your model, so that the distance between the Earth label and the thumbtack is 10 cm.

    A 10 cm string connects a pushpin with the label 'earth'
  3. The Earth circles around the Sun in an orbit that is almost circular.
    Think about:
    Can you make your Earth rotate around the Sun in your model?
  4. Predict what you expect your shrunken solar system to look like. Place your labels on these places. This is your prediction. If you can, take a picture, so you can compare your prediction with your real model at the end of the activity.
  5. The table below lists the eight planets with their orbital distance. The orbital distance is the average distance from the planet to the Sun as they circle the Sun. It is often expressed in Astronomical Units (AU). One AU equals roughly the distance from the Sun to Earth. The last column in the table lists the distances to which the orbital distances get shrunken in your model.
     Illustration showing the orbital distance as the radius of the planet's orbit around the Sun.

    Planet Orbital Distance* (AU)Model Distance (cm)
    Mercury 0.4 4
    Venus 0.77
    Earth 110
    Mars 1 1/215
    Jupiter 550
    Saturn 9 1/295
    Uranus 19190
    Neptune 30300
    *Distances larger than 1 AU are rounded to the nearest half, distances under 1 AU are rounded to the nearest decimal. Consult the Solar Planet Data Sheet for more accurate data.
  6. Add the remaining planets to your model one at a time. For each planet, look up how far the planet's label needs to be from the thumbtack in your model; it is listed in the "Model Distance" column. Then, repeat step 2 marking a point at the distance you just looked up. For example, for the planet Mars, mark a point 15 cm from the label as you want Mars to be 15 cm from the Sun in your model.
  7. Count the planets in your shrunken solar system model.
    Think about:
    Do you have all 8?
  8. If one is missing, you can go back and add it.
  9. The Sun is huge, but still small when compared to the size of the solar system. In your shrunken model, the Sun would have a radius of less than a millimeter—about the size of the thumbtack's shaft.
  10. Look at your model. You might have placed all the planets in a line, or maybe you placed them spread out in all directions, but still resting on the floor or table.
    Think about:
    How would it be in space, would the planets be on a line, spread out in a plane or spread out in all three dimensions? Why would it be that way?
  11. Spread the planets out on the floor or table, but keep the string modeling the distance to the Sun taut. Imagine tiny dots as planets where you have the labels, and you will have a quite accurate snapshot of what a shrunken solar system would look like.

    The labels for the planets Mercury, Venus, Earth, Mars and Jupiter connected with string to a central thumbtack. The planets are spread out around the pushpin.
  12. If you have a picture of your prediction, look back at it.
    Think about:
    In what way is your prediction different from the model you have now? And in what way is it similar?

What Happened?

You probably noticed how the four planets closest to the Sun (Mercury, Venus, Earth, and Mars) are all much closer to the Sun (at 4, 7, 10, and 15 cm from the Sun in your model) compared to the other four planets (Jupiter, Saturn, Uranus, and Neptune). These last four are much more spread out (at 50, 95, 190, and 300 cm from the Sun in your model).

Earth circles around the Sun in about 365 Earth days. Other planets take many more or many less days to make a complete circle around the Sun. The extremes are about 60,190 Earth days for Neptune and about 88 Earth days for Mercury. They are also all moving at different rotational speeds. This explains why the planets are always changing positions relative to each other. In one moment in time, Earth and Mars might be positioned on the same side of the Sun, while at another moment in time, they might be at opposite sides of the Sun. The planets are not nicely placed on a line from the Sun to the furthest planet Neptune as often represented in images, but spread out in an approximate plane, just like you spread out the planets in your model in the last step of the activity.

Digging Deeper

Planets are celestial bodies that orbit (or circle around) a star. In our planetary system, this star is the Sun. 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.

The Solar System is the system of objects that orbit the Sun directly or indirectly. Planets in the solar system are defined as celestial bodies that orbit the Sun, that are heavy enough for gravity to squeeze them into a spherical shape, and that have "cleared the neighborhood" around their orbit. The later means that there are no objects comparable in size in the vicinity of their orbit other than the planet's moons. Eight objects in the solar system qualify as planets, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Dwarf planets like Pluto fulfill the first two criteria but not the last.

 Representation of the solar system where the first and last four planets are lined up. The distance from the Sun to the planets seems to increase with regular steps in this representation.
Figure 1. Commonly seen representation of the solar system. Notice how the distances of the planets to the Sun are not represented to scale, and how in space, the planets are not aligned but rather spread out. This representation does correctly show how all planets orbit in approximately the same plane.

The solar system has the Sun in its center and eight planets orbiting the Sun. Listed in increasing distance from the Sun, we first encounter Mercury. Next is Venus. Then comes Earth, followed by Mars. These first four planets are also called the inner planets as they orbit closest to the Sun. Earth is the most hospitable to life. Mars is the only other planet that, as far as we know now, may have supported life in the far past. The next four planets—Jupiter, Saturn, Uranus and Neptune—are often called the outer planets as they orbit at a much larger distance from the Sun.

As the distances from the Sun to the planets are huge, they are often expressed in Astronomical Units (AU). One AU equals roughly the distance from the Sun to Earth, or about 150 million km (93 million miles). This distance is so large that it takes light form the Sun a little over 8 minutes to cover this distance and reach Earth, but it is still small compared to the distance to Neptune. It takes light over 4 hours to reach Neptune. Considering that light takes a straight path and has a speed of 186,000 miles per second, the fastest thing we know off, you might start to understand why a spacecraft might take 12 years to fly from Earth to Neptune. Spacecrafts do not necessarily take the shortest path, and although they are fast in human terms, they are slow compared to the speed of light.

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For Further Exploration

  • Let your planets circle around the Sun at different speeds and see what happens to their relative positions.
  • Calculate how small the planets would need to be to represent them accurately in your shrunken model. You can find the necessary information on the Solar Planet Data Sheet.
  • Make a second model that accurately represents the different sizes of the planets in the solar system. One way to do this is explained in the How Big Are the Planets in Our Solar System? activity.
  • Measure the distances between planets, example between Earth and Neptune. In what position on their orbit do the planets need to be for the distance to be the shortest possible, and in what position is it the longest possible?

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