Transit Method

Overview

This is the core experiment of astro-lab.app. Students move a Styrofoam "planet" around a globe lamp (the "star") and observe the dimming of the star's light every time the planet passes in front of the sensor. The phyphox app records a live light curve and automatically calculates the transit depth, transit duration, and the length of one orbital year.

The activity mirrors the transit method used by the Kepler and TESS space telescopes to detect real exoplanets.

Grades	7–10
Subject	Physics, Natural Sciences
Method	Student experiment
Duration	Variable
Language	German, English
Sensor	Smartphone light sensor, TI SensorTag, or a solar cell on a supported Owon multimeter

Background

When an exoplanet passes directly between its host star and our telescope, it blocks a small fraction of the star's light. This periodic dimming — the transit signal — reveals the planet's existence and allows us to measure its size (from the transit depth) and orbital period (from the time between transits). From the orbital period, Kepler's third law gives us the orbital distance, and combining that with the stellar temperature, we can assess habitability.

Materials

• Globe lamp (e.g. a garden globe light)
• Styrofoam ball (diameter ~5–10 cm)
• Toothpick
• Smartphone (Android) or TI SensorTag with any smartphone, or a solar cell with a supported Owon multimeter
• Smartphone mount

The materials listed are examples. Any round light source and opaque ball will work.

Setup and Procedure

1. Place the lamp in the centre of a table.
2. Position the sensor so that it points directly at the lamp: the smartphone light sensor, a TI SensorTag (for Apple devices, whose light sensor phyphox cannot access), or a solar cell on the Owon multimeter.
3. Open transitmethode.phyphox in phyphox. For the Owon route, owon_digital_multimeter-debug.phyphox is the helper file for reading out the multimeter.
4. Trigger Configuration tab first: place the sensor at its final distance from the lamp and read the maximum signal. Enter the threshold below which the stopwatch triggers in "Threshold for transit measurement". Keep the brightness threshold at its default.
5. Insert the toothpick into the Styrofoam ball as a handle.
6. Start the measurement in phyphox.
7. Move the ball around the lamp in a circle, keeping the orbital plane level so that the ball passes in front of the sensor regularly.
8. After several orbits, stop the measurement and evaluate the light curve displayed in phyphox.

What phyphox Calculates

The custom phyphox experiment acts as an optical stopwatch. It automatically extracts:

• Transit depth — the percentage drop in light intensity (proportional to the square of the planet-to-star radius ratio)
• Transit duration — how long the planet takes to cross the stellar disc; the mean value is shown after six transits (the zeroth transit is neglected)
• Orbital period — the time between successive transits (one "year" of the planet); the mean value is shown after five years

In the Planet Size tab, enter the star radius in cm; phyphox then estimates the planet radius from the transit depth.

Alternative: Motor Drive

Moving the ball by hand rarely produces a perfectly periodic orbit. If a small motor is available, attach it to the ball so the orbital period is constant. This significantly improves the quality of the light curve and is closer to the conditions under which real data is collected.

phyphox Files

• Phone light sensor, TI SensorTag or solar cell on an Owon multimeter: transitmethode.phyphox
• Helper for the Owon multimeter route: owon_digital_multimeter-debug.phyphox

The files contain English, German and French texts. The phyphox files are maintained in the curious-signals-phyphox repository.

Downloads

Worksheet (PDF) and teacher's guide (PDF) are available at astro-lab.app.

Related Publication

Küpper, A., Spicker, S. J., & Schadschneider, A. (2022). Analogy experiments on the transit method for physics and astronomy education at lower secondary level. Astronomie + Raumfahrt im Unterricht, 59(4), 46–49.