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.
Astronomy, device, and software documentation checked through 2026-07-11.
The Pedagogical Problem
Transit discoveries dominate the current confirmed-planet archive, although the fraction is time-dependent and the method is biased toward planets whose orbits align with our line of sight. When a planet passes in front of its host star, it blocks part of the observed light. A sufficiently precise photometric measurement may record a dip whose shape depends on radius ratio, impact parameter, orbital motion, stellar limb darkening, exposure time, and noise. Flat bottoms occur in some idealised or non-grazing cases; they are not the definition of a transit.
This is conceptually accessible. The physics is essentially shadow casting — a topic covered in primary school — applied to an astronomically interesting situation. In our teaching design, the familiar geometry provided an accessible entry point. This article does not report a controlled learning-outcome or motivation study.
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.
Our 2024 paper addressed a gap in the approaches it surveyed: an inexpensive, open-source student experiment using a compatible phone, while recognising that some devices require an external sensor.
The Smartphone Solution
Many Android smartphones include an ambient light sensor that phyphox can access, but sensor availability, range, sampling, and optical placement vary by device. The phyphox sensor list explicitly warns that not every phone exposes the same sensors.
The basic setup requires three things:
- A light source whose short-term stability has been checked; LED drivers can 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 dip whose shape reflects the apparatus geometry. It is a transit analogy, not a guarantee of a flat-bottomed astronomical light curve.
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 iPhone Problem and a Workaround
The current phyphox FAQ states that its App Store build cannot access the iPhone’s physical ambient-light sensor. Apple now documents ambient-light data through SensorKit for authorised research use, so “no public API” is too broad; it remains unavailable to this ordinary phyphox experiment.
The paper’s workaround is an external light sensor whose data can be brought into the measurement workflow. Compatibility, firmware, calibration, cost, and availability must be checked for the chosen device; the named 2024 hardware examples are not timeless purchasing recommendations.
The external-sensor approach can remove one source of variation by using the same detector model at a fixed point. It produces comparable data only if the sensors, firmware, calibration, geometry, and acquisition settings have been checked.
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 — can be unclear at smartphone sensor resolution. The design problem is to connect the curve to the physical geometry that produced it.
Video-aided analysis addresses this directly. After synchronising the video and sensor clocks, students can compare the sphere’s recorded position with each region of 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 gives students more experimental decisions than the prescribed versions: they build a protocol rather than only following one. Whether that is suitable depends on prior experience, time, and the learning objective.
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.
The qualitative measurement principle is shared: an occulter reduces detected light. For a small opaque planet crossing a uniformly bright, unobscured stellar disk, the ideal central-transit depth is approximately \(\delta \simeq (R_p/R_\star)^2\). Limb darkening, grazing geometry, spots, dilution, and the classroom lamp/sensor geometry break that simple relation. Students are practising light-curve measurement and model comparison, not recovering an astrophysically calibrated planet radius from any arbitrary lamp.
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 with documented limb-darkening, exomoon, and N-body paths — 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), 226–229. 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
NASA Exoplanet Archive. Confirmed planets and discovery methods (live database). https://exoplanetarchive.ipac.caltech.edu/
phyphox. Supported sensors and Frequently Asked Questions. https://phyphox.org/sensors/; https://phyphox.org/faq/
Changelog
- 2025-10-03: Updated the DOI for Spicker & Küpper (2024) to the correct 10.1119/5.0125305.
- 2026-07-11: Bounded the transit-shape and radius-depth claims to their approximation regimes; corrected Android and iPhone sensor availability; removed timeless hardware/reliability claims; and added the article pages and current documentation cutoff.