Moving Stars and Their Planets Answer Key

1. The motion of a star caused by an orbiting planet is called a “wobble.” Why does the star wobble

when it has an orbiting planet? The star wobbles because the planet is “tugging” on it. The gravitational force from the star acts to keep the planet in orbit, and the gravitational force from the planet acts to move the star as the planet orbits. The pull results in the star “wobbling” as the planet orbits.

2. Based on your observations, what is the relationship between the movement of the star and the

mass of the planet? The larger the mass of the planet, the larger the motion it will cause in the star. Similarly, the smaller the mass of the planet, the smaller the motion it will cause in the star. More massive planets are easier to detect because they cause larger star movements.

3. What happens to the wobble motion of the star when the planet has a very low mass?

The star continues to wobble. 4. Explain your answer.

Even though the motion cannot be easily seen, the star does continue to wobble some. The planet still has a gravitational effect on the star, so it will still move the star. The action will be very small because the planet is very small. (Every action has an equal and opposite reaction.)

5. How certain are you about your claim based on your explanation?

Student answers will vary. 6. Explain what influenced your certainty rating.

Student answers will vary. Scientific evidence includes citing Newton's Third Law of Motion or evidence from the model about how the star moved when the planet's mass was large. Students may be uncertain about their answers because the movement of the star is very small when the planet has a small mass. Note: Zooming in on the model, so that the star fills nearly the entire view, does allow you to see that there is still some motion of the star even with the smallest planet choice. See image for level of zoom. Even at this level of zoom, the motion is subtle. You will need to focus on the edges of the star (noted by the circles) to see the pixels change from yellow to black, showing the motion of the star. You can confirm that it is star motion causing the pixels to change by starting and pausing the model. The pixels will only change from yellow to black when the model is running. Note that this is more apparent on a large, high-resolution screen. The motion is very small, and students may think that the star ceases to move when there is a small planet orbiting it.

7. Take a snapshot that shows both short and long wavelengths. Add notes that show which

wavelengths are short and which wavelengths are long. The red shows long wavelengths, and the blue shows short wavelengths.

8. You observe a star through a telescope. What happens to the apparent wavelength of the star's

light as it moves toward you? It gets shorter.

9. Explain your answer.

As the star moves closer, the waves get compressed. This makes it appear as if the wavelength is getting shorter. This is known as a blue shift.

10. Based on the observations, can the scientist be certain that the star is not moving?

no 11. Explain your answer.

Since the light from the star appears to be yellow, and the star is yellow, the star does not appear to be moving relative to the telescope. If the star were moving closer, the light would appear bluer, and if the star were moving away, the light would appear redder. Just because the star's light appears to be yellow doesn't mean that the star isn't moving toward and away from the telescope. However, if the scientist did not wait long enough to see movement, the star could be moving. Maybe the star is moving very slowly, and the scientist did not observe long enough. You can't tell motion based on a single snapshot in time.

12. How certain are you about your claim based on your explanation?

Student answers will vary. 13. Explain what influenced your certainty rating.

Student answers will vary. Scientific evidence includes citing information from the model about how the wavelength (and therefore the color of the light) changes with motion of the star relative to the telescope. Students may be uncertain that the scientist has waited long enough to see the wavelength of the star's light change, since they will be able to see that the light has to reach the telescope before the scientist can observe it. Some students might discount the idea of time since light travels so fast that the scientist should be able to detect changes in wavelength in just a short snapshot.

14. Which orbital angle (tilt) prevents you from seeing the motion of a star that does have an orbiting planet? 90 degrees

15. Explain your answer.

When the tilt is at 90 degrees, no motion is apparent in the velocity graph. This is because the plane of the planet's orbit is at a right angle to the telescope. The telescope cannot detect motion at a right angle; it needs the motion to be in the same plane. A 0 degree tilt is best for detecting a planet, but even at 45 degree tilt, you can still detect a planet around a star.

16. How certain are you about your claim based on your explanation?

Student answers will vary.

17. Explain what influenced your certainty rating.

Student answers will vary. Scientific evidence includes citing evidence from the model and graph. Students may be uncertain that 90 degrees is the only option, since if the planet is very small, it will be hard to detect at a 45-degree tilt.

18. Do you agree with the scientist? Student answers will vary.

19. Explain your answer.

The velocity graph shows that there is no movement of the star relative to the telescope in this time period. This could mean that there is no planet orbiting in the detectable orbiting angle. But it does not mean that there is no planet orbiting the star. There could be many planets orbiting the star in undetectable angles of orbit (close to 90 degrees). If the planets are small and orbiting at a steep angle, the telescope might not detect them. It could mean that the scientist didn't wait long enough to see motion of the planet. There is no indication of how much time is shown in this data.

20. How certain are you about your claim based on your explanation?

Student answers will vary. 21. Explain what influenced your certainty rating.

Student answers will vary. Scientific evidence includes evidence from the snapshot of the model, showing a flat velocity graph, as well as evidence from previous models, showing that planets orbiting at steep angles are less likely to be detected than planets orbiting at shallower tilts. Students may be uncertain that the scientist has waited long enough to see the star's motion, since there are no units of time on the graph.

22. Take a snapshot that shows a very large, very heavy planet that showed the velocity of the star

(relative to the observer) between -0.2 and 0.2 meters/second, similar to the velocity graph made by the medium-sized planet (pictured above the model). Note that students will not be able to match exactly the provided graph (due to graph autoscaling). The students should make a heavy planet that has a velocity graph range between -0.2 and 0.2 m/s. The following snapshots show acceptable matches to the velocity graph.

The important part is that students recognize that a very large, very heavy planet must be orbiting at a steep angle to show such a small motion on the velocity graph. Note the tilts of greater than 84 degrees in the following snapshots.

23. Explain how different combinations of planetary mass and orbital angle can create similar velocity graphs. When the planetary mass is very high, the star motion will be very high, and when the planetary mass is low, the star motion is low. When the orbital angle is very shallow (close to 0 degrees), the star's motion will be most apparent, and when the orbital angle is very steep (closer to 90 degrees), the star's motion will be most difficult to detect. If you combine high mass with high angle, you can get the same graph as a low mass planet orbiting at 0 degrees.

24. Do you agree with the scientist's conclusion? Student answers will vary.

25. Explain your answer.

The velocity graph shows quite a bit of star motion, but that does not mean that it is caused by a very large planet. It could be caused by a very large planet if the planet is orbiting at a steep angle relative to the telescope. Or it could be caused by a smaller planet orbiting at an angle closer to 0 degrees. There is not enough information in the graph to tell which scenario it is.

26. How certain are you about your claim based on your explanation?

Student answers will vary. 27. Explain what influenced your certainty rating.

Student answers will vary. Scientific evidence includes citing evidence from the model about planetary size and orbital angle and their combined effects on the velocity graph. Students may be uncertain that the velocity graph results from a very large planet, since smaller planets orbiting at a tilt closer to 0 degrees would produce the same velocity graph as a large planet at a higher tilt. Because there are multiple interpretations of the data, there are very good reasons to be uncertain and many possible hypotheses about what caused the graph results.

28. Does this graph show a planet orbiting a star?

I'm not sure. If there is a planet, the change in velocity is small.

29. What more information would you want to be sure of your answer?

I would want to have data from a longer period of time. There is a lot of noise in that graph, and it is difficult to tell whether there is a regular pattern of up and down. It looks like there is, but I would need to see more data to see if the pattern continues despite the noise.

30. Most of the planets that have been discovered orbit their stars at a tilt closer to 0 degrees than to

90 degrees. Why is this true? Real-world telescopes have a lot of noise, so it is difficult to detect the smaller motions of stars caused by planets orbiting at steeper angles. When the planet orbits at a shallower angle, the signal is larger, so it is easier to decipher from the noise. ('Noise' comes from the telescope, the atmosphere, or both. Scientific instruments are not perfect, and there is always some degree of error.)

31. Why might a scientist using modern telescopes be unsure about having discovered a planet?

There is a lot of noise in the real-world data. Even though modern telescopes have less “noise” than earlier telescopes, the data are still not perfect signals. This could lead scientists to be unsure about whether a tiny signal is part of the noise or if it is a real signal that a star is moving due to a planet's orbit.

32. Most of the planets that have been found so far have been very massive. Why?

Larger, more massive planets have more of a gravitational pull on their stars than smaller, less massive planets. This leads to more star motion. The large signal is easier to detect in the noisy real-world data than smaller signals. Large planets are more easily discoverable.

33. This velocity graph was recorded by pointing a telescope at a nearby star.

34. Based on this velocity graph, could there be a planet orbiting this star?

yes 35. Explain your answer.

The velocity graph shows a regular up and down signal, indicating that the star is moving toward and away from the telescope. This motion is quite small (between -0.1 and 0.1 m/s), but it is clearly detectable from the noise in the data (the little ups and downs). This indicates that there is something pulling on that star in a regular pattern. The most likely explanation is for a planet to be orbiting the star.