Albedo and Reflectivity

Overview

Even when an exoplanet sits inside its star's habitable zone, the temperatures on its surface depend heavily on how much of the star's light the planet reflects. This quantity — albedo — is shaped by the planet's surface and cloud cover. This experiment models the effect in two versions: a simple one in which a shiny Christmas ball orbits a lamp and the reflected light shows up as a small "secondary transit", and a phyphox file version that compares the reflected light of different surfaces at a fixed geometry.

Grades	7–10
Subject	Physics, Natural Sciences
Method	Student experiment
Duration	Variable
Language	German, English
Sensor	Smartphone light sensor (Android) or TI SensorTag

Background

Albedo is the fraction of incident light that a surface reflects, expressed as a number between 0 (perfectly black) and 1 (perfectly white). Earth's average albedo is ~0.30; Venus sits at ~0.75 (Bond albedo) because of its thick cloud layer; the Moon has ~0.12. For exoplanets, albedo affects the equilibrium surface temperature and therefore habitability. It also produces a characteristic variation in reflected light as the planet orbits its star — a signal that next-generation telescopes are beginning to detect.

The same ball-orbiting-lamp geometry from the transit experiment (see Transit Method) is used here, but the "planet" is now highly reflective. When it orbits behind the lamp, its reflected light is blocked and the signal drops slightly: a secondary transit.

Materials

• Globe lamp (e.g. a garden globe light)
• Shiny Christmas tree ball (~6–8 cm diameter)
• Hanging wire or hook for the ball
• Smartphone (Android) or TI SensorTag with any smartphone
• Smartphone mount
• Different surfaces to compare (phyphox file version)

The materials listed are examples. Any spherical reflective object (e.g. a ball of aluminium foil) suspended by a thread will work.

Setup and Procedure

Simple version (as in the CAPjournal article)

1. Place the globe lamp in the centre of a table.
2. Position the smartphone so that the light sensor points toward the lamp and open a light-sensor experiment (illuminance) in phyphox.
3. Replace the "planet" by a reflective ball, e.g. a Christmas ornament, and move it in a circular orbit around the lamp.
4. When the ball passes behind the lamp, its reflected light is blocked and the signal drops slightly (secondary transit).
5. The changes are much smaller than in the transit experiment, so zoom into the graph in phyphox or export the data and examine it in other software.

phyphox file version

1. Keep the geometry fixed: lamp, sensor and surface always in the same positions.
2. Open albedo.phyphox in phyphox and choose the tab "Phone reflectance proxy" (phone light sensor) or "SensorTag reflectance proxy" (TI SensorTag light sensor).
3. Measure the reflected light of the first surface, then of the others, and compare the results.
4. The tab shows the maximum and minimum signal and the reflectance proxy (max − min)/max in %. This is a qualitative classroom analogue, not a calibrated planetary albedo.

Discussion

• What determines the size of the signal change?
• How would the curve look for a dark, matte ball versus a highly polished one?
• Actual phase curves of exoplanets measured by Kepler show similar periodic variations. What can astronomers infer from the curve's shape?
• Moon phases follow the same geometry — how does this connect?

Alternative

Moving the ball by hand rarely produces a perfectly uniform orbit. If a small motor is available, use it to drive the ball at a constant rate. This greatly improves the periodicity of the signal and makes quantitative analysis possible.

phyphox Files

• Phone or TI SensorTag (tabs "Phone reflectance proxy" / "SensorTag reflectance proxy"): albedo.phyphox
• Earlier version for the TI SensorTag only: albedo_sensortag.phyphox

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