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9: Exoplanet Detection - Doppler Method

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    Lab Overview

    Lab Goals

    The goal of this lab is to develop the scientific practices of collecting and interpreting data:

    • Make connections between measured data and planetary orbit positions/velocities
    • Identify patterns relating mass (planet and star), orbital distance, orbital speed, and measured light properties for doppler measurements

    Equipment and Setup

    This lab requires the following equipment on each table.

    • Computers with internet for simulator

    Submission Instructions

    Submit a brief lab report that includes the following sections:

    • Doppler Graphs

    Write your answers to the Discussion Questions using the Claim-Evidence-Reasoning format.

    Background Information

    Gravity and Orbits

    The orbital speed of an object depends on the mass of the star and the distance between the planet and the star. For elliptical orbits, planets move faster when they are closer to the star in such a way that results in the planet moving through equal area wedges in equal amounts of time (Kepler’s 2nd Law).

    Doppler Shift

    When the source of waves moves toward you, the wavelength decreases a bit. If the waves involved are visible light, then the colors of the light change slightly. As wavelength decreases, they shift toward the blue end of the spectrum: astronomers call this a blueshift (since the end of the spectrum is really violet, the term should probably be violetshift, but blue is a more common color). When the source moves away from you and the wavelength gets longer, we call the change in colors a redshift. The greater the motion toward or away from us, the greater the Doppler shift. If the relative motion is entirely along the line of sight, the formula for the Doppler shift of light is

    v = (Δλ/λ) c

    where λ is the wavelength emitted by the source, Δλ is the difference between λ and the wavelength measured by the observer, c is the speed of light, and v is the relative speed of the observer and the source in the line of sight. The variable v is counted as positive if the velocity is away from the observer, and negative if it is towards the observer.

    Lab Instructions

    Simulation Setup

    Open the Radial Velocity simulation from the University of Nebraska Astronomy Education website:

    Part 1 - Doppler Measurements

    Setup - Theoretical

    • Set eccentricity to zero
    • Start the animation
    • Adjust the animation speed as needed
    • Adjust the video of the orbit as needed

    Discussion Questions

    1. What is happening in the orbit at the following velocities:
      1. Max (most positive)
      2. Min (most negative)
      3. Zeros (if any)
    2. How do the maximum and minimum velocities compare?

    Part 2 - Planet and Star Parameters

    Setup

    • Set eccentricity to zero
    • Start the animation
    • Adjust the animation speed as needed
    • Adjust the video of the orbit as needed

    Procedure

    1. Play around with the slider to increase and decrease the planet mass. Identify changes (if any) to the following:
      1. Orbital Period
      2. Max/min velocities
      3. Closeness of data points to theoretical curve
    1. Play around with the slider to increase and decrease the star mass. Identify changes (if any) to the following:
      1. Orbital Period
      2. Max/min velocities
      3. Closeness of data points to theoretical curve
    2. Play around with the slider to increase and decrease the semimajor axis. Identify changes (if any) to the following:
      1. Orbital Period
      2. Max/min velocities
      3. Closeness of data points to theoretical curve

    Discussion Questions

    1. Make a claim about how orbital period and max/min velocities relate to:
      1. Planet Mass
      2. Star Mass
      3. Semimajor Axis
    2. Propose a proportional relation for max/min velocity by putting variables in the numerator or denominator:

    Part 3 - Eccentricity

    Setup

    • Set eccentricity to zero
    • Start the animation
    • Adjust the animation speed as needed
    • Adjust the video of the orbit as needed

    Procedure

    1. Change the eccentricity to values between 0 and 0.8. Identify the following at different eccentricities:
      1. How does the shape of the doppler plot change?
      2. Does the planet spend more/less time at fast/slow speeds?
    2. Set the eccentricity to some value above 0.5 and change the longitude in the system orientation box.
      1. How does your view of the system affect the measured doppler plot?

    Discussion Questions

    1. Identify a pattern relating eccentricity to the shape of the doppler plot. Explain the cause of this change.
    2. Explain how your view point (longitude) of an orbit with high eccentricity affects the doppler plot

    Part 4 - Data Quality

    Setup - Actual

    • Check the show simulated measurements box
    • Adjust different amounts of data and different amounts of noise. As you explore, trying turning on/off the show theoretical curve box. Consider the following as a starting point:
      • High Noise, Low Data
      • High Noise, High Data
      • Low Noise, Low Data
      • High Data, Low Noise
      • Medium Data with High and Low Noise
      • Medium Noise with High and Low Data
    • After exploring those options, continue exploring the relations between these two measurement parameters.
    • Draw four Doppler Plots (with the dots for data) that show differences between them and identify the effects of low, medium, high noise and data.

    Discussion Questions

    Overall Question: How does the amount of data and the noise in data affect the ability to “see” the doppler curve and determine planetary properties?

    Subquestions that might help:

    1. Does it seem better to have more or less data? More or less noise?
    2. How does the amount of data affect the clarity of the pattern?
      1. Is there a minimum number where the data do not clearly represent the theoretical curve?
      2. Does the amount of noise affect this minimum?
    3. How does the level of noise affect the clarity of the pattern?
      1. Is there a maximum amount of noise that loses the pattern?
      2. Does the amount of data affect this maximum?

    This page titled 9: Exoplanet Detection - Doppler Method was last modified on Tue, 06 Oct 2026 03:17:33 GMT and is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Andrew Totah-McCarty.

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