In this activity students use a computer model to study the orbits of planets around a sun.

What do the orbits of planets around a star look like?

Our Solar System consists of many types of objects circling around the Sun, held in their orbits by gravity. Name all of the objects you can think of that orbit the Sun. Write down what you know about each one.

The PhET model Orbits is opened in a separate window.

This activity addresses NSES standards for earth and space science and inquiry at grades 5-8 for earth in the solar system
(http://books.nap.edu/readingroom/books/nses/6d.html#es).

This activity will look specifically at planets, which are relatively large objects circling the Sun. You will run a model and be able to change a planet’s mass, velocity and position, so that you can see what effect these factors have on its orbit. Before you start the model, make the following predictions.

• Does the Sun move as the planets go around it, or is it stationary?
• If the Earth were more massive but otherwise had the same position and velocity, would its orbit be the same or different?
• Are all orbits circles?
• Do planets closer to the Sun have greater or smaller velocities than more remote planets?

1. Open the PhET model Orbits. It begins with a single planet going around a sun. Use START, STOP, and RESET to control the model. This picture shows how to adjust the model.
   
2. Click START and observe the model. See how the sun wobbles a bit. Why is that?
3. The sun has an orbit too, but why is it so much smaller than the planet’s orbit?
4. You may have answered that the sun is more massive than the planet. To see if this matters, make the sun’s mass 1000. How does that change the wobble? Why? Now make the sun’s mass 50. How does that change the wobble? Why?
5. Is the planet’s orbit a true circle? Use the tape measure to find out. What are the largest and smallest diameters of the orbit? What are the largest and smallest distances of the planet from the sun?
6. Explore velocity. Decrease the planet’s velocity, using the boxes in the lower left. They can be changed after you click RESET. What happens to the orbit?
7. Increase the planet’s velocity. What happens to the orbit?
8. Change the planet’s X velocity until its orbit is close to a perfect circle. Record the velocity.
9. Change the planet’s X position to 80, keeping the same velocity. What will the orbit look like? Try it. Was your prediction correct?
10. With the planet in this position, change the planet’s X velocity until its orbit is close to a perfect circle, using the boxes in the lower left. Record the new value. Is it greater or less than the velocity for a circular orbit farther away from the sun?
11. Set the planet’s velocity to zero. What happens?

1. Explore mass. Will the planet follow the same orbit regardless of its mass? What do you think? Suppose you made only one change in the model: change the planet’s mass from 1 to 0.1. How would the orbit change?
2. Now test your guess. Use the drop-down menu to select “Sun and planet.” Set the planet’s mass to 1 and run the model. Measure the orbit DIAMETER from left to right, using the tape measure.
3. Reset the model and change the planet’s mass to 0.1. Leave the tape measure in place. Does the orbit change?
4. Reset the model and change the planet’s mass to 0.01. Leave the tape measure in place. Does the orbit change?
5. Add a moon (“Sun, planet, moon” in the drop-down menu) and run the model. Does its orbit look surprising?

1. Based on what you observed in the model, do you think it would be accurate to say that the Sun’s gravity pulls on the Earth and that the Earth’s gravity pulls on the Sun? Or is just one of these statements true? What is your evidence?
2. Summarize what you learned in your experiments about how increasing or decreasing a planet’s velocity affects its orbit.
3. Describe the appearance of a moon’s orbit as it circles a planet, from the point of view of the sun.

Based on what you observed in the model, answer this puzzle:

• The asteroid belt is a band of “rocks” together in an orbit between Earth and Mars. Some are tiny and some are many kilometers across. Even though they are so different in size, they all stay in the same band of orbits. How is this possible? Why don’t the smaller ones fall toward the Sun?

Make the planet as heavy as the sun. What happens?

For other PhET models, go to the Physics Education Technology website.