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How Big Are the Planets in Our Solar System?

17 reviews


Active Time
20-30 minutes
Total Project Time
20-30 minutes
Key Concepts
The solar system, planets, scale model
Sabine De Brabandere, PhD, Science Buddies
Model the Planets of the Solar System


Did you know that there are more planets than stars in our galaxy? All of these planets circle around a star, but only eight of them—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune—circle around the Sun—the star in our solar system. This activity explores the relative size of these eight planets. Will one be much bigger than the other, or are they all about the same size?

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.


  • Play dough - store bought or homemade.
  • Balloons (4)
  • Metric ruler
  • Cardstock
  • Scissors
  • Marker or pencil
  • Glue or tape
  • Optional: Piece of yarn

    Materials needed to make a scale model of the planets in the solar system as described in this STEM activity.

Prep Work

Cut out labels from the cardstock, 2 cm by 6 cm works well.


  1. You will make a model of each of the eight planets in the solar system. You will start with Earth. Imagine you shrink Earth to a small sphere. Then, shrink all the seven other planets by the same amount. The result will be the model you are about to make!
  2. To represent Earth, make a sphere of play dough with a diameter of 2 cm. The diameter of a sphere is twice the distance of a point on the surface to the center of the sphere. If you feel that is easier, you can also measure the circumference of the sphere to make sure your sphere has the right size. Wrap a piece of yarn around the sphere at its widest point. The length of the string is the circumference of the sphere. When your sphere has a diameter of 2 cm, the piece of yarn (or the circumference) will be 6.3 cm.

    Illustration showing a string wrapped around a sphere with a diameter of 2 cm. The string is a little over 6 cm long.
  3. Take a label, write "Earth" on it. Stick a corner of the label in your model of Earth.
    Think about:
    Do you think most planets in the Solar System are larger, smaller or about the same size as planet Earth?

    The corner of a label reading 'earth' stuck in a play dough ball
  4. The table below lists the eight planets with the relative size of their diameter compared to Earth's diameter.

    Planet Relative size of the planet's diameter compared to Earth's diameter* Relative size of the planet's circumference compared to Earth's circumference*
    Mercury 1/2 1/2
    Venus 1 1
    Earth 1 1
    Mars 1/2 1/2
    Jupiter 11 11
    Saturn 9 1/2 9 1/2
    Uranus 4 4
    Neptune 4 4
    *These ratios are rounded to the nearest half. Consult the Solar Planet Data Sheet for more accurate data.

    Think about:
    Looking at the table, can you find how large the diameter of Mercury is compared to Earth's? In your model, Earth has a diameter of 2 cm, what should the diameter of Mercury be in your model?
  5. According to the table, Mercury's diameter is half as long as Earth's diameter (the ratio is 1/2). In reality, Mercury's diameter is a little smaller than half Earth's diameter. The circumference follows the same ratio, so its circumference will also be a little shorter than half as long. Make a play dough sphere for Mercury with a diameter of a little under 1 cm, or a circumference of a little under 3.1 cm.
  6. Take a label, write "Mercury" on it. Stick a corner of it in your model of Mercury.
  7. Find Venus in the table.
    Think about:
    Can you find out how large Venus needs to be in your model?
  8. Venus is similar in size as Earth. Make a sphere similar in size as Earth, and label it Venus.
  9. Next is Mars.
    Think about:
    Looking at the table, can you find out how large Mars needs to be in your model? Is there another planet you already made that is similar in size as Mars?
  10. According to the table, Mars' diameter is half as long as Earth's diameter (the ratio is 1/2). In reality, Mars' diameter is a little over half Earth's diameter. Make a play dough sphere for Mars with a diameter of a little over 1 cm. Label it "Mars."

    Four play dough balls, a small one labeled Mars, two larger ones labeled Earth and Venus respectively and another small one labeled Mercury.
  11. Find Jupiter in the table.
    Think about:
    Can you find out how large Jupiter needs to be in your model?
  12. Jupiter has a diameter that is 11 times longer than Earth's diameter, so in your model, its diameter needs to be 22 cm (circumference 68 cm). You probably do not have enough play dough to make a sphere that big. Instead, blow up a large balloon until it has a diameter of about 22 cm and tie it. Make a label for Jupiter and glue it to your balloon.
    Think about:
    Does anything stand out about Jupiter?
  13. Next is Saturn.
    Think about:
    Looking at the table, can you find out how large Saturn needs to be in your model? Would you use a balloon or play dough to model Saturn?
  14. Blow up a large balloon until it has a diameter of about 19 cm and tie it. Label it "Saturn."
  15. The next planet is Uranus.
    Think about:
    Can you find out how large Uranus needs to be in your model? Would you use a balloon or play dough to model Uranus?
  16. Model Uranus with a balloon blown up to a diameter of 8 cm (circumference 25.1 cm). You can use a smaller size balloon or blow up a large balloon a little bit. Label this balloon "Uranus."
  17. Still one more planet.
    Think about:
    Which planet are we missing? Can you find its size?
  18. Neptune is similar in size to Uranus, so model Neptune like you did Uranus. Label this balloon "Neptune."
  19. Place your model planets in the order they are listed in the table (first Mercury, then Venus, ... ). This is the order they appear in in space, Mercury being closest to the Sun, and Neptune being the furthest away.

    The scale model with four small planets followed by two huge ones, the last two are medium in size.
  20. These are the planets of our solar system, but we did not model the sun.
    Think about:
    Would you know how large you would need to make the Sun in if you wanted to add the Sun to your model?
  21. The Sun is huge, about 109 times larger than Earth. Add it to your solar system model if you find a sphere that is about 2 meters in diameter.

What Happened?

Your model has Mercury first, with a diameter of a little under 1 cm. Then comes Venus and Earth, both with a diameter of 2 cm, followed by Mars (diameter a little over 1 cm). These are the four inner or rocky planets modeled with play dough. The next four planets are much larger, they are the gaseous planets and are modeled by balloons. First is Jupiter, the largest planet. In your model, it has a diameter of 22 cm. Next is Saturn (diameter 19 cm), Uranus and Neptune, both with a diameter of 8 cm.

Digging Deeper

Planets are celestial bodies that orbit (or circle around) a star. In our 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. A celestial body is called 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 later 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.

 Illustration showing the eight planets of the solar system.

The Solar System has the Sun in its center and eight planets orbiting the Sun. Listed in increasing orbital distance from the Sun, we first encounter Mercury, the smallest of the eight. Mercury is only slightly larger than Earth's moon. Next is Venus, a planet with a radius of 6052 km, only slightly smaller than Earth. Then comes Earth, the planet with the highest average density (5.5 g/cm³), and then Mars. Mars' radius is about half Earth's radius. It is a dusty, cold planet, but might have inhabited some form of life long ago. These first four planets are all rocky planets, their surface is a thin layer of rock. The rocky planets are all smaller than the gas planets; they are made of denser material. The next four—Jupiter, Saturn, Uranus and Neptune—are gaseous planets. Jupiter is the largest, its radius is more than 11 times larger than Earth's radius, followed by Saturn, whose radius is about 9.5 times large 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.

The planets diameters are small compared to their distance from the Sun. In a model where the Earth's diameter is 2 cm, the Earth model would be (on average) at 235 m from the Sun. Mercury, the planet closest to the Sun, would be placed at 90 meters from the Sun and your Neptune model would be at about 7.05 km (7050 m) from the Sun! If you like to explore the distances of planets to the sun in more detail, look for the activity listed in the Further Exploration section.

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For Further Exploration

  • Calculate how far you would need to place your model planets from each other and from the Sun to represent the distance from the planets to the Sun correctly. Look up in Google Earth or on a digital map where you would need to place your model planets to represent the distances correctly. Can you find an open space where you can place your inner (or rocky) model planets so the distance and the size of the planets are represented to scale? Why is it hard to add the gaseous planets?
  • Model the distances from planets to the Sun as explained in this Model the Distances between Planets in our Solar System activity.

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