This post describes the work behind “Exoplanet Hunting in the Classroom: An Easy-to-Implement Experiment Based on Video-Aided Light Curve Analysis with Smartphones”, published in The Physics Teacher in 2024 (co-authored with Alexander Küpper). It also draws on the earlier German-language paper on analogy experiments for the transit method, published in Astronomie+Raumfahrt in 2022.
The Pedagogical Problem
The transit method is how the majority of confirmed exoplanets have been found. When a planet passes in front of its host star, it blocks a fraction of the star’s light. A sufficiently precise light sensor pointed at the star will record a characteristic dip: a flat-bottomed decrease in flux during the transit, with a precise shape determined by the ratio of the planet’s radius to the star’s radius, the duration of the transit, and the geometry of the orbit.
This is conceptually accessible. The physics is essentially shadow casting — a topic covered in primary school — applied to an astronomically interesting situation. Students understand it quickly and find it genuinely exciting.
The problem is the implementation. How do you actually demonstrate this in a classroom?
Standard approaches divide into three categories, each with limitations:
Simulations and database exercises: Students work with real data from Kepler or TESS, or use a software simulation. These are conceptually valid but remote from physical experience. There is no sensor, no measurement, no uncertainty to grapple with.
Prefabricated kits: Products like PocketLab or Pasco offer purpose-built transit experiment setups. They work, but they are expensive, closed-source, and require manufacturer-specific software. A school that buys a Pasco sensor is locked into the Pasco ecosystem.
DIY benchtop setups: Various published designs use phototransistors, Arduinos, or similar components with a benchtop light source. These are flexible and cheap but require component procurement, assembly, and some technical confidence from the teacher. The barrier to entry is real.
What was missing was an approach that was inexpensive, open-source, required no specialist equipment procurement, and worked at the level of a student experiment rather than a teacher demonstration.
The Smartphone Solution
Modern Android smartphones include an ambient light sensor that is directly accessible via phyphox, the free measurement app developed at RWTH Aachen. Set up the experiment correctly, and the phone records a real-time light curve.
The basic setup requires three things:
- A light source (a standard desk lamp, ideally with a constant-brightness LED bulb to avoid flicker)
- An opaque sphere to act as the “planet” (a tennis ball, a ping-pong ball, anything with a defined circular silhouette)
- A smartphone running phyphox, positioned beneath the lamp at a fixed distance and oriented so the light sensor faces upward
When the sphere is moved across the light path at a controlled height and speed, the light sensor records a transit: a smooth dip in measured illuminance with the flat-bottomed shape characteristic of a planetary transit across a uniformly bright disk.
This is the core experiment. It works. The transit signal is clear enough to measure even with the modest precision of a phone’s ambient light sensor, provided the background illumination is controlled (dark room or at least consistent ambient light).
The iPhoneRoblem and Its Solution
Apple devices do not expose their ambient light sensor through any public software API. An iPhone running phyphox cannot access the sensor that is physically present in the device.
The workaround we recommend: an external Bluetooth light sensor connected to phyphox. Options include the TI SensorTag CC2650, Bluetooth multimeters such as the OWON B35T, or an Arduino Nano 33 BLE Sense. The Arduino option is particularly well-suited to educational contexts: it is open-source, it is inexpensive, and its absence of a built-in operating system makes it more reliable as a pure sensor.
The external sensor approach also has a benefit beyond iPhone compatibility: it produces more consistent data across different devices, since you are measuring at a fixed external point rather than through whatever optical pathway the phone manufacturer chose. For experiments where comparison across student groups matters, this is not trivial.
Video-Aided Light Curve Analysis
The standard approach to a transit experiment is: measure the dip, calculate the planet-to-star radius ratio from the relative depth, done. This works and is pedagogically valid.
The paper introduces a complementary approach: simultaneously recording a video of the “planet” passing in front of the “lamp”, and using the video frames to cross-reference the light curve data.
Why? Because the light curve from a real transit experiment does not look exactly like the idealised textbook version. There is noise. There is baseline drift. The “ingress” and “egress” phases — where the planet is partially in front of the star — are often unclear at smartphone sensor resolution. Students frequently have difficulty connecting the shape of the curve to the physical geometry that produced it.
Video-aided analysis addresses this directly. Frame-by-frame, students can see exactly where the planet was at each moment in the light curve. The ingress becomes visible: when the sphere first touches the lamp’s light cone, the sensor begins to register the dip. The mid-transit flat bottom corresponds to full occultation of a central portion of the lamp. The egress mirrors the ingress. The correspondence between geometry and photometry — which is the conceptual core of the transit method — becomes explicit.
In a teaching context, this turns the error and noise in the light curve from an obstacle into an educational resource. Students can identify specific features of the curve and ask: what was happening in the physical experiment at that moment? The uncertainty is no longer an embarrassment. It is a diagnostic.
Scaffolding Levels
The paper distinguishes three implementation modes, corresponding to different levels of student independence:
Demonstration experiment: Teacher sets up and runs the apparatus. Students observe and discuss. Appropriate as an introduction to the concept before students engage with it independently.
Guided student experiment: Students follow a structured procedure, with specified setup, data collection protocol, and analysis worksheet. Appropriate for students who have not designed their own experiments and for lesson contexts where time is limited.
Open inquiry: Students are given the materials and a research question — “How does the depth of the transit dip depend on the size of the planet?” — and design their own procedure. Appropriate for upper secondary students with experience in experimental design, and for lesson contexts that explicitly address scientific method.
The materials for all three modes are described in the paper. The open inquiry mode is the most demanding but also the most research-authentic: students are not following a protocol but building one, confronting the actual decisions that experimental physicists make.
From the Classroom to the Telescope
A transit experiment with a lamp and a phone is, obviously, not the same as the photometry done by TESS or the James Webb Space Telescope. The planet-star radius ratios measurable in the classroom analog are much larger than for most real exoplanets. The signal-to-noise is worse. The lamp is not a star.
But the method is the same. The measurement principle — flux dip proportional to the square of the radius ratio, duration determined by orbital geometry — is the same physics that Kepler used to find thousands of planets. When students calculate the “radius” of their tennis-ball planet from their light curve, they are doing, in miniature, what professional astronomers do with data from space.
This connection to real research is not incidental to the pedagogy. It is central to it. The transit method works as a classroom experiment not because it is a good demonstration of some abstract physics principle but because it is a genuine slice of how contemporary science actually operates. The question the experiment answers — is there something out there? — is the same question the professional community is asking.
The simulation companion to this work — a browser-based model of transit photometry with full limb darkening, exomoon scenarios, and N-body dynamics — is described in this separate post. The simulation is the place to go when you want to explore parameter space; the physical experiment is the place to go when you want to understand what a measurement actually is.
Connection to the astro-lab
The transit experiment described here grew directly out of the astro-lab project at the University of Cologne, where Alexander Küpper and I had been developing smartphone-based analogy experiments for exoplanet detection since the COVID pivot in 2020. The astro-lab@home established the feasibility of the smartphone approach; the A+R 2022 paper on Analogieexperimente für die Transitmethode explored the design space more systematically; the TPT 2024 paper is the version written for an international teacher audience, with the comparative equipment table, the video-aided analysis technique, and the scaffolding levels made explicit.
If you want to extend the experiment to exomoons — detecting the gravitational wobble that a moon induces in a planet’s transit — that work is described in a later post.
For the curriculum article that places the transit experiment in the NRW Sekundarstufe I context — including the Direct Imaging pre-experiment — see Fremde Welten.
References
Spicker, S. J., & Küpper, A. (2024). Exoplanet hunting in the classroom: An easy-to-implement experiment based on video-aided light curve analysis with smartphones. The Physics Teacher, 62(3). https://doi.org/10.1119/5.0125305
Küpper, A., & Spicker, S. J. (2022). Analogieexperimente zur Transitmethode für den Einsatz in Schule und Hochschule. Astronomie+Raumfahrt im Unterricht, 59(5).
Staacks, S., Hütz, S., Heinke, H., & Stampfer, C. (2018). Advanced tools for smartphone-based experiments: phyphox. Physics Education, 53(4), 045009. https://doi.org/10.1088/1361-6552/aac05e
Changelog
- 2025-10-03: Updated the DOI for Spicker & Küpper (2024) to the correct 10.1119/5.0125305.