Download Is there life outside of Earth? Activity 2: Moving Stars and Their Planets

 Is there life outside of Earth?
Activity 2: Moving Stars and Their Planets
Overview
In this activity, students are introduced to the wobble-method (officially known as the
radial velocity method) of detecting planets. The activity starts with an introduction to
Newton’s Third Law of Motion. Students use a model to explore the effects of a planet’s
mass on the star’s motion. Students then explore how the wavelength of light that a
star emits is apparently changed as the star moves towards and away from a telescope.
Students use a model to discover the influence of orbiting angle on the ability to detect a
planet with the wobble method. Students then use an integrated model to explore the
effects of planetary mass and orbiting angle concurrently. Finally, students learn about
telescope precision and noise in the data.
Learning Objectives
Students will be able to
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describe how an orbiting planet can cause a star to wobble.
explore the effect of planetary mass on a star’s wobble.
explain how wavelength can describe the motion of a star.
demonstrate how a planet’s angle of orbit determines whether or not the planet
might be found.
explain how planets are found using the wobble method.
make claims about the data and determine their own level of certainty with regard
to their claims.
Lesson Plan
1. Estimated Time
This activity should take approximately 45 minutes.
2. Introduce the Activity
In this activity, your students will explore several models of motion.
Page 1: The effect of mass on a star’s wobble
In this model, students will change the mass of planet using the “planet-diameter” slider
and the “Rocky-planet” switch. A rocky planet is denser than a gaseous planet, so the
mass of the rocky planet will be higher than a gaseous planet of the same size.
The mass of the student-created planet is given in the upper right-hand corner of the
model in multiples of Earth’s mass.
Encourage your students to test different combinations of rocky planet and planet
diameter to understand rocky planets and gaseous planets of the same size can have
different effects on the wobble of the star.
Page 2: A star’s movement can change its apparent wavelength
In this model, students will move the star towards and away from a telescope to see
how the apparent emitted wavelength changes.
It is important that students understand that the overall color of the light emitted by the
star it not dramatically changed as the star moves; the wavelength detected by the
telescope changes because the star is either moving closer (compressing the waves) or
moving farther away (decompressing the waves).
Also be sure to discuss with your students the idea that the graph shows the light hitting
the telescope. This means the graph changes only when the wavelength hitting the
lens changes--not when the star motion is changed.
Page 3: The importance of orbiting angle on being able to detect a star’s wobble
In this model, students change the angle of orbit of the planet by tilting the plane (by
clicking and holding the mouse button down in the simulation window, then dragging the
cursor). The telescopes that are used to detect a star’s wobble detect movement
towards and away from the telescope.
Your students’ eyes are now the telescope, so to detect a star’s wobble, they will need
to detect movement of the star into and out of the screen. If the planet does not orbit
the star in a way that makes the star move into and out of the screen (towards and
away from the detection instrument), the planet will not be detected. Students should
try out many different angles of orbit to determine what angles of orbit lead to planet
detection with the wobble method.
This model introduces another slider: the “graph-time-window” slider. This control
allows students to increase or decrease the amount of data they see in the velocity
graph window. Be sure to have students explore and get comfortable with this slider as
it comes up when exploring future models in this investigation.
Page 4: Combinations of mass and angle that lead to planet detection with the wobble
method
In this model, students will make different combinations of mass and angle that result in
the same velocity graph.
Additionally, students can now explore with a list of preset planets in the “PresetPlanets” pull-down menu.
They can also create their own custom planets. To do this, turn on the “custom-planet”
switch when the model is running. Then set the “Rocky-planet” switch to choose a rocky
planet or a gaseous planet and set the planet’s diameter with the “planet-diameter”
slider. To set the planet’s velocity, move the arrow (vector) of the planet. To change
the location of the planet drag the planet around. To start the simulation, turn off the
“custom-planet” switch.
It is easy to get an elliptical orbit this way; it is also easy to select a starting velocity that
doesn’t result in an orbit at all. If a circular orbit is desired, click on the “make circular
orbit” button.
The “Distance-to-star” button zooms the view in and out. It does not change the
distance of the planet from its star, just your viewpoint.
Hopefully, students will explore lots of variations, but be sure to get students to focus on
solving the problem. With the entire class, discuss the different solutions students
found and/or what difficulties students had with solving the problem. Students are
challenged to match the graph.
It is important to tell them that “close enough” is fine; the curriculum focuses on
getting students to see how different variables affect how the data can be interpreted.
Page 5: Limitations of Noise
This page introduces students to the idea of noise and that the graphed data they have
been viewing is too “perfect” to match real data. One way students interpret graphs with
lots of noise is that there are lots of changes going on. In reality the noise is an artifact
of either the environment through which the light passed (gases and other materials in
space), as well as physical limitations of the sensors on the instrument.
Most advances in science are a result of some advance in engineering. New more
sensitive light sensors, or complex telescopic arrays launched into space, make
possible less and less noisy signals.
Page 6: Impact of Noise on Angle of Orbit
In this model, students will experience the data in a way more similar to how scientists
get data about a star’s wobble. Students will set up the model so that the planet is
detectible with a perfect telescope, and then they will decrease the precision using the
“telescope-precision” slider until they can no longer detect the planet.
There is a new control in this model; students can now magnify the y-axis with the
slider. This will allow them to see data that is very small or very large. It is important
that students look at the values on the y-axis to determine the magnitude of the velocity
change. Discuss with your students the importance of scale--similar-looking graphs
with different y-axis scales are not equivalent.
On this page students are asked to focus on how it can be difficult to spot a small signal
inside noisy data, and to explore this through changing the angle of orbit for a planet.
Regardless of the mass of the planet, when the tilt of the orbit is close to 90 degrees
there will be little movement of the star toward and away from the observer. This
produces a weak signal that can get lost in noise.
It is important to talk about experimenter’s bias at this point. Since they know that they
have a planet in the model, some students may “detect” a planet in the noisy data that is
undetectable. It is important to discuss that the noise in the data may obscure planets
that are there or make scientists think that there may be planets present where there
are not. Remind your students that scientists repeat their experiments many times and
ask their colleagues to independently analyze the data so that the experimenter’s bias is
minimized. It is also important to focus on scientists’ certainty about their conclusions
and what scientists can do to increase their certainty.
Page 7: Impact of Noise via Planet Discovery via Star Motion
This page is very similar to the previous one, except this time students are asked to
explore how low mass planets, even when on the ideal orbital plane, may become
undetectable if the data is noisy enough.
This page ends with a challenge for students to determine if a particularly noisy graph
indicates the presence of a planet.
3. Discuss the Activity
Possible discussion questions:
Using light to find planets
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How do scientists use light from distant stars to measure movement?
What do scientists know about the motion of a star from the size of its
wavelength? How is a star moving if its wavelength is shifted towards the
red end of the spectrum (longer wavelengths)? How is a star moving if its
wavelength is shifted towards the blue end of the spectrum (shorter
wavelengths?
“Wobbling” stars
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What is meant by the term “star wobble?” Is Earth’s star (the Sun)
wobbling?
The graphs represent the velocity of the star, not the velocity of the planet.
Why do scientists focus on a star’s motion (and not the planet’s motion)?
How does the model help to explain star wobble?
How does a planet’s angle of orbit affect the ability of scientists to detect
it?
What would the velocity graph look like if there were multiple planets
orbiting the star?
Finding planets with indirect evidence
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The graphs represent the velocity of the star, not the velocity of the planet.
Why do scientists focus on a star’s motion?
How can scientists be sure that they have found a new planet?
How do technological innovations influence the process of science?
4. Answers to Questions
Section 1: Finding Planets Using Star Motion
Page 1: Gravity and Orbits
Q. The motion of a star caused by an orbiting planet is referred to as a “wobble.” What
does this “wobble” motion look like?
A. The “wobble” motion looks like the star moving in a circle around the center of the
graph. The planet is pulling on the star and the star is pulling on the planet. The planet
tugs the star outward and the star tugs inward on the planet.
Q. What happens to the motion of the star when the planet has a very low mass?
A. (A) The star moves around, but not enough for use to see it in the model.
Q. Explain your choice in the previous question.
A. Students should support their choice with evidence from the model. For example,
when the planet’s mass was very low, the star appeared to stop moving, but I know that
it did not actually stop moving because every object has an effect on every other object.
It must be too small for me to be able to see it in this model.
Q. Based on your observations, what is the relationship between movement of the star
and the mass of the planet?
A. When the planet is more massive, the star moves more. When the planet is less
massive, the star moves less. This is because a more massive planet exerts a stronger
gravitational force.
Page 2: The Doppler Effect
Q. Place a snapshot of the model below that shows both short and long wavelengths in
the same picture. Be sure to add text notes to the image that indicate which ones are
short and which are long.
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Q. Imagine you are observing the star through the telescope pictured above.
What happens to the wavelength of the star’s light as it moves away from you?
A. (B) It gets longer.
Q. Imagine you are observing the star through the telescope pictured above.
What happens to the wavelength of the star’s light as it moves towards you?
A. (A) It gets shorter.
Q. You observe light from a star that seems to be shifted toward the red end of the
spectrum.
What does that say about the motion of the star? Explain your reasoning.
A. That means that the star is moving away from the telescope. The wavelengths will
be spread out further if the star is moving away, making its light appear to be red.
Page 3: The Importance of Angle
Q. What orbital angle prevents you from seeing the motion of a star that does have an
orbiting planet?
A. (C) 90 degrees
Q. Explain your choice in the previous question.
A. When the telescope can’t see the wobble, it can’t record any movement. When the
planet’s orbiting angle is at 90 degrees relative to the telescope, the telescope can’t see
the star’s wobble at all. It’s at a right angle to the planet rather than a shallower (more
detectable) angle.
Q. A scientist looks for planets by studying the light from a newly discovered star. The
GRAPH of the star's motion (based on shifting wavelengths) indicates no motion of the
star toward or away from Earth. The scientist decides that the star doesn't have any
planets.
Do you agree with the scientist?
A. Student answers will vary, but most should choose “no.”
Q. Explain your choice in the previous question.
A. The scientist may be right or may be wrong. It depends on the angle that any planets
orbit the star and what their masses are. If the planets are very small or orbit at an
angle close to 90°, then they will not be detectable. Or there may not be any planets at
all. What the scientist should say is that no planets were detected with those methods.
Page 4: The Importance of Both Mass and Angle
Q. Place a snapshot here of the first solution to the challenge above.
A. Student answers will vary. The graph should look similar to the challenge graph.
Q. Place a snapshot here of your solution to the challenge above.
A. Student answers will vary. The graph should look similar to the challenge graph.
Q. How do different combinations of planetary mass and orbiting angle create the same
velocity graph? Explain.
A. Both mass and orbiting angle affect how well a planet can be detected with the
wobble method. If the planet is very massive, then it isn’t necessary to have an optimal
orbiting angle to be detected, but if the planet is not as massive, then it needs to orbit its
star at an optimal viewing angle (from Earth) if it is going to be detected at all.
Q. To the right is a velocity graph that was recorded by pointing a telescope at a nearby
star. What are the approximate mass and orbital angle of the planet orbiting the star?
A. Student answers will vary. Answers should include a combination of mass and
orbital angle.
Q. Are you certain about your description of the mass and orbital angle of the planet
that made the graph above?
A. Student answers will vary.
Q. Explain what influenced your certainty rating in the last question.
A. Student answers will vary. Answers should include reference to the possibility of
many combinations that result in the same graph.
Page 5: Limitations of Noise
Q. Does this graph show a planet orbiting a star? What is your prediction?
A. There is a planet there, but it is hard to see, so any one could be correct. This
question has a “self check” on it, so students can see the “real” answer and compare to
their prediction.
Page 6: Impact of Noise on Angle of Orbit
Q. Most of the planets that have been discovered have an orbit that is tilted closer to
zero degrees than to 90 degrees. Why?
A. When the planet is orbiting closer to 0 degrees the effect of its pull is more
prominent, so it is more detectable. If the orbital plane is closer to 90 degrees, then the
signal will be weak and could be lost in the noise.
Q. Why might noise cause a scientist to be unsure about having discovered a planet?
A. Because noise caused the data on the graph to jump up and down, hiding what could
be a faint signal in the data.
Page 7: Impact of Noise on Planet Discovery via Star Motion
Q. Most of the planets that have been found so far have been very massive. Why?
A. Since the telescopes are not very precise, they are unable to detect small planets.
The bigger planets cause bigger shifts in their stars, resulting in changes in the velocity
graphs that can be seen even with all of the noise.
Q. Based on the data to the right and considering both mass and angle, could there be
a planet orbiting this star?
A. Student answers will vary, but the graph does seem to indicate a wobble.
Q. Explain your answer in the previous question.
A. Student answers will vary. Explanations should include an analysis of any perceived
wobble.
Q. Are you certain of your response?
A. Student answers will vary.
Q. Explain what influenced your certainty rating in the last question.
A. Student answers will vary. Answers should include reference to the level of noise in
the graph as well as instrument precision.