This post describes the article “Fremde Welten — Die Suche nach Exoplaneten mit Analogieexperimenten thematisieren” (Strange Worlds: Teaching Exoplanet Detection with Analogy Experiments), published in Unterricht Physik (Issue 194, 2023) with Alexander Küpper.

Where Students Start

Before students encounter the transit method, most of them have a clear mental model of how exoplanet detection works: you point a large telescope at a nearby star, and if there is a planet, you see it. “You could see them [the exoplanets] with a telescope/binoculars” and “You can see them with an extremely powerful telescope” are typical responses from year 8–9 students before they work through an actual detection unit.

This is not an unreasonable starting intuition. Telescopes see things far away. Planets are things far away. The inference seems to follow.

What it misses is the contrast ratio problem. A star is not just brighter than its planets — it is overwhelmingly, almost incomprehensibly brighter. In visible light, a star like the Sun outshines Jupiter by roughly a billion to one. Against that glare, the planet is functionally invisible. Direct imaging of exoplanets is possible in special circumstances — young planets far from their stars, imaged in infrared — but for the vast majority of exoplanets, it is not a viable detection method.

The unit described in this article takes that misconception as its entry point and builds from there.

The Direct Imaging Experiment

The first experiment in the unit is a hands-on demonstration of why direct imaging is difficult.

The setup: a student points their smartphone camera at a small light source (a switched-on torch). Directly next to the torch, barely a few centimetres away, is a pin with a coloured head — the “exoplanet”. On the phone’s display, the pinhead is invisible. The torch (star) drowns it out completely.

Students can then investigate what would need to change for the pinhead to become visible. The answer they discover: block the torch with a small disc held in front of the camera at the right distance. With the direct glare suppressed, the illuminated pinhead becomes visible in the image.

This is a coronagraph in miniature. The same principle is used in real direct-imaging instruments like SPHERE on the VLT or the coronagraph in the Nancy Grace Roman Space Telescope. Students discover, experimentally, the essential idea: to see an exoplanet directly, you need to suppress the star’s light without blocking the planet’s.

The experiment also motivates a natural follow-on question: under what conditions does direct imaging work at all? Students can vary the pinhead distance from the torch and its size, exploring qualitatively the conditions under which the “exoplanet” becomes detectable even with partial suppression. The answer — large planets, far from their host star — matches the real observational bias: most directly imaged exoplanets are large, young (still warm from formation), and in wide orbits.

The Transit Experiment

Once the limits of direct imaging are established, the unit introduces the transit method as the primary indirect technique. The pedagogical structure is deliberate: students have already understood that you cannot usually see exoplanets directly, which motivates the question of how else you might detect them.

The transit experiment uses a lamp as the star, a ball moved by hand (approximately periodically) around the lamp, and an Android smartphone running phyphox as the light sensor. When the ball crosses in front of the lamp from the sensor’s perspective, the measured illuminance dips. Students see a real light curve — not a simulation, not a graph from a database, but something they produced themselves from a physical measurement.

Two phyphox experiment files are provided for download (via QR code in the article and at astro-lab.app):

Basic experiment: records the raw illuminance data and displays the light curve. The focus is qualitative — what shape does the dip have? What determines the depth? What determines the period? Students can formulate the relationship between dip depth and planet-to-star size ratio as a qualitative rule (the larger the planet relative to the star, the deeper the dip) without necessarily working through the mathematics.

Extended experiment: adds real-time calculations of the transit depth $\Delta F$, the maximum illuminance $I_*$ and transit illuminance $I_\text{transit}$, the transit duration, and the orbital period. For students who are ready for it, this allows a quantitative derivation of the “planet” radius from the light curve — given a known lamp radius and the measured transit depth:

$$\Delta F = \left(\frac{R_p}{R_*}\right)^2$$

The extended experiment also invites critical engagement with the model: the radius derived from the analogy experiment will differ from the actual ball radius, because the distance ratios in the tabletop setup are not to scale. Making that discrepancy explicit — and asking students why it arises — is good science practice.

Limits of the Transit Method

A recurring theme in the unit is that every detection method has limits, and understanding those limits is part of understanding the method.

For the transit method, the fundamental limit is inclination. A transit is only observable if the planet’s orbital plane is aligned (nearly edge-on) relative to our line of sight. Most exoplanetary systems, viewed from Earth, will not be aligned in this way. The transit method is therefore a biased sample: it preferentially detects planets in edge-on orbits, and it misses most planets entirely.

Students can explore this experimentally: tilt the plane of the ball’s orbit away from edge-on and observe what happens to the light curve. The dip disappears. This connects naturally to a broader point about how astronomical surveys work: when we report “X% of stars have detectable planets”, we are reporting a fraction that has been corrected for this and other observational biases.

The article includes a differentiation note: the limits investigation works well as an open inquiry task, with students formulating and testing their own hypotheses about what orbital configurations produce detectable transits.

Exoplanets as a Curriculum Bridge

One point the article makes explicitly is that exoplanets are not just an astronomy topic but a context that connects to multiple items in the German physics curriculum for Sekundarstufe I. The cross-connections include:

  • Optics: the seeing process (why does the star outshine the planet?), shadow formation, refraction in telescopes
  • Mechanics: orbital period, Kepler’s laws at a qualitative level, the habitable zone as a consequence of stellar luminosity and distance
  • Thermodynamics: planetary surface temperature, the greenhouse effect, albedo
  • Pressure: atmospheric pressure, habitability (a connection developed more fully in the Mission to Mars experiment)

The motivating context — could this planet host life? — sustains student engagement across these topics in a way that treating them in isolation does not.

What Comes After

The transit method is a productive entry point, but the search for extraterrestrial life does not end with planet detection. The article closes by noting that the detected exoplanets need to be analysed for habitability — which depends on orbital radius (habitable zone), stellar temperature, planet radius (mass is not available from transit data alone), atmospheric composition, albedo, and greenhouse effect.

Many of these can be connected back to physics experiment contexts, and the astro-lab project has developed smartphone-based analogy experiments for several of them. Detailed information on these is at astro-lab.app.

For the full pedagogical sequence — from the original astro-lab student laboratory, through the COVID pivot to home experiments, to the return to school — see The Lab Goes Home. For the exomoon extension, which takes the transit experiment further into the question of moon-hosted life, see Can a Planet Have a Moon?.

References

Küpper, A., & Spicker, S. J. (2023). Fremde Welten — Die Suche nach Exoplaneten mit Analogieexperimenten thematisieren. Unterricht Physik, 34(194), 4–9.

Küpper, A., Spicker, S. J., & Schadschneider, A. (2022). Analogieexperimente zur Transitmethode für den Physik- und Astronomieunterricht in der Sekundarstufe I. Astronomie+Raumfahrt im Unterricht, 59(188), 46–50.

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

MSB NRW (2019). Kernlehrplan für die Sekundarstufe I — Gymnasium in Nordrhein-Westfalen: Physik. Ministerium für Schule und Bildung NRW.

Changelog

  • 2025-10-03: Updated the DOI for Spicker & Küpper (2024) to the correct 10.1119/5.0125305.