Exoplanets in the habitable zone

Students investigate how temperature changes with distance from a heat source and relate this to planetary temperatures.

Apparatus and materials

(per group of 2 to 4 students)

• Radiant Heater or 250 W infrared bulb mounted in a holder
• 2 thermometers (one with a shiny bulb, the other with a blackened bulb)
• 2 clamps and stands
• Meter rule
• Graph paper

Each student will require a photocopy of the instructions and worksheet.

Health & safety and technical notes

Old mains powered radiant heaters with bowl-fi re elements are no longer recommended for use in schools. Refer to CLEAPSS Laboratory Handbook 11.9.2 for safety information and alternatives. A 240 W infrared bulb works well.

Beware of burns: tell students to stop as soon as they feel anything. If a lamp is used, warn students not to look directly into the light as it will be very bright.

The practical activity

1. Students use thermometers to measure the temperature at different distances from a radiant heater. They should start at a good distance (around 70 cm) from the heater and move towards it.
2. Students will probably realise that the temperature will rise as they approach the heater.
3. The shiny bulb thermometer should show lower temperatures as it refl ects radiation away. The blackened thermometer will absorb radiation better.
4. After the students have drawn their graphs, discuss their results and explain why temperature decreases with distance from the star/heater.
5. Also ask students how they think the graph would change for a more powerful heater/star.