Starspots and Exoplanets

Overview

Sunspots and their analogues on other stars — starspots — are cooler, darker regions on the stellar surface. When a transiting planet crosses in front of a starspot, it temporarily blocks less light than usual, creating a characteristic brightness bump inside the transit dip. This experiment extends the transit method setup with one small addition: a circle cut from a Post-it note is placed on the globe lamp to simulate a starspot. Students compare light curves with and without the spot and discover how stellar activity can complicate exoplanet detection.

The method follows Silva (2003), who first proposed using transit light curves to detect starspots.

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

Starspots are regions of stronger-than-average magnetic field that suppress convection, making the surface locally cooler (and therefore darker) than the surrounding photosphere. On the Sun, spots are typically ~1500–2000 K cooler in the umbra than the photosphere and can persist for weeks. On active M-dwarf stars, spots can cover a significant fraction of the stellar disc, making them a major noise source in transit photometry.

When a transiting planet passes over a starspot, the total light blocked by the planet temporarily decreases, because the spot was already darker. The result is a small upward bump inside the transit dip — a diagnostic feature used by Kepler and TESS to map stellar surfaces and to separate stellar noise from genuine transit signals.

Safety note: Before the experiment, test whether the globe lamp gets hot after a few minutes. If it does, the Post-it note may be a fire risk. Use an LED globe lamp, which stays cool.

Materials

• Globe lamp (LED recommended — must not get hot)
• Styrofoam ball (~5–10 cm diameter)
• Toothpick
• Post-it note or similar adhesive paper, scissors
• Smartphone (Android) or TI SensorTag with any smartphone, or a solar cell with a supported Owon multimeter
• Smartphone mount

Setup and Procedure

1. Cut a small circle (~2 cm diameter) from the Post-it note.
2. Stick the circle on the surface of the lamp to represent a starspot.
3. Set up the transit experiment (lamp in centre, sensor facing lamp, phyphox open with transitmethode.phyphox — see the Transit Method experiment for full setup instructions, including the trigger configuration).
4. Start the measurement and orbit the Styrofoam ball around the lamp as in the transit experiment.
5. Observe the light curve. Each time the ball passes in front of the sensor it produces a transit dip. When the ball also crosses the Post-it spot, a faint upward bump appears inside the dip.
6. Optionally, film the ball's path from the sensor's point of view with a second smartphone and synchronise the video with the phyphox recording to help students interpret the curve.

Example Light Curve

A clean transit produces a flat-bottomed dip. With the spot present, the dip shows a small upward deviation at the moment the "planet" crosses the "spot". The size of the bump depends on the size of the spot and its contrast with the surrounding photosphere.

Discussion

• How does the transit depth change when the planet crosses the starspot versus the surrounding photosphere?
• If you did not know the spot was there, how might you misinterpret the light curve?
• Real exoplanet surveys (Kepler, TESS) require ground-based follow-up partly to rule out stellar activity. Why is multi-wavelength observation helpful for distinguishing spots from planets?

phyphox Files

• Phone light sensor, TI SensorTag or solar cell on an Owon multimeter: transitmethode.phyphox

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

Reference

Silva, A.V.R. (2003). Method for Spot Detection on Solar-like Stars. The Astrophysical Journal, 585(2), L147–L150. https://doi.org/10.1086/374324