Alien World Hunters: Decoding Exoplanets via Transit Method & Light Curves

Materials Needed

• 1 Desk lamp or bright flashlight (to act as the host star)
• 3–4 small spheres of varying sizes (e.g., marble, ping-pong ball, large wooden bead, small clay ball attached to a thin skewer or string)
• Smartphone or tablet with a free light sensor app installed (e.g., Phyphox or Physics Toolbox Sensor Suite) OR graph paper and ruler if using manual estimations
• Measuring tape or meter stick
• Calculator
• Colored pencils / markers
• Exoplanet Discovery Worksheet (printable or drawn on paper)

Learning Objectives & Success Criteria

Learning Objectives

By the end of this lesson, the learner will be able to:

1. Explain how astronomers use the Transit Method to discover exoplanets light-years away without directly imaging them.
2. Analyze and Plot a light curve graph showing changes in stellar brightness over time.
3. Calculate the relative size ratio of a hidden planet compared to its star using the Transit Depth formula: $\text{Transit Depth} = \left(\frac{R_{\text{planet}}}{R_{\text{star}}}\right)^2$.
4. Design a hypothetical exoplanet system and predict its transit signature.

Success Criteria

• ✔ I can accurately demonstrate a transit using a light source, object, and light sensor.
• ✔ I can identify the orbital period and planet size from a light curve graph.
• ✔ I can successfully calculate an exoplanet's radius percentage given a brightness dip value.

Lesson Structure

1. Introduction: Hook & Objectives (10 Minutes)

The Hook: "The Invisible Needle in a Cosmic Haystack"

Instructor Talking Point: "Imagine trying to spot a tiny fruit fly crawling across the headlight of a car that’s driving toward you from 10 miles away in the middle of the night. Sounds impossible, right? Yet, that’s exactly what astronomers are doing when they hunt for exoplanets—planets orbiting stars outside our solar system."

Discussion Question: "Since exoplanets are too faint to be seen directly through normal telescopes, how do you think space missions like NASA’s Kepler and TESS discover thousands of them?"

Real-World Context: Over 5,500 exoplanets have been confirmed using indirect detection. Today, Heidi steps into the shoes of an astrophysics researcher using the Transit Method to detect unseen worlds.

2. Body: Content & Practice (40 Minutes)

Phase 1: Direct Instruction & Modeling ("I Do") — 10 Mins

• Concept Intro: When an exoplanet passes directly between its host star and Earth, it blocks a small fraction of the star's light. This event is called a transit.
• Light Curves: Astronomers plot brightness over time on a graph called a Light Curve.
  • Baseline (100% light) = Clear view of star.
  • Dip in curve = Planet is blocking light during transit.
  • Duration of dip = How long the planet takes to cross the star.
  • Frequency of dips = Planet's orbital period (its year length).
• Math Breakdown: The fraction of light blocked (Transit Depth) is proportional to the area ratio of the planet and star:
  Transit Depth ($\Delta F$) = Area of Planet / Area of Star = $(R_{\text{planet}} / R_{\text{star}})^2$
  Example: If a planet causes a 1% (0.01) dip in light brightness, $R_{\text{planet}} / R_{\text{star}} = \sqrt{0.01} = 0.10$. The planet's radius is 10% of the star's radius!

Phase 2: Guided Simulation Lab ("We Do") — 15 Mins

Activity: The Tabletop Exoplanet Detector

1. Set Up the Star System: Place the lamp (host star) at one end of a table. Place the smartphone with the Light Sensor app running 1 meter away, facing the bulb directly.
2. Calibrate Baseline: Record the lux/brightness value when no object is in front of the star (this is your 100% baseline).
3. Simulate Orbit: Slowly pass a small sphere (Object A - e.g., small bead) across the lamp's light beam between the lamp and the sensor. Observe the live graph on the phone screen drop and return to baseline.
4. Compare Planet Sizes: Repeat the transit with a larger sphere (Object B - e.g., ping-pong ball).
   • Data Collection: Record the lowest brightness reading during Object A's transit and Object B's transit.
   • Guided Calculation: Calculate the percentage drop for both objects together: $$\text{Dip \%} = \frac{\text{Baseline Brightness} - \text{Transit Brightness}}{\text{Baseline Brightness}} \times 100\%$$

Phase 3: Independent Practice & Creative Application ("You Do") — 15 Mins

Task: Exoplanet Discovery Mission Profile

Heidi will choose or create an original "Exoplanet Profile" from the options below and complete the mini-lab challenge:

3. Conclusion: Synthesis & Reflection (10 Minutes)

• Recap Challenge: Have Heidi summarize the transit method in under 30 seconds using key terms (Transit, Light Curve, Baseline, Transit Depth).
• Discussion & Real-World Connection:
  Instructor Prompt: "What limits do you think the transit method has? Can we detect EVERY planet using this method?"
  Key Takeaway: The transit method only works if the planet's orbital plane happens to line up edge-on relative to Earth! If the orbit is tilted, the planet never passes in front of the star from our perspective.
• Exit Slip / Self-Reflection: On a scale of 1–5, how confident do you feel reading light curve data? What's one question you still have about how astronomers study exoplanet atmospheres?

Assessment Plan

Differentiation Strategies