In this activity students use a lamp and polystyrene balls to model how astronomers detect exoplanets using the transit method.

Apparatus and materials

• Lamp (one with an opal globe light bulb is ideal)
• Polystyrene balls of assorted sizes
• Bamboo barbecue skewers (with a length of approximately 30 cm)
• Webcam
• Light Grapher software

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

Health & safety and technical notes

Ask students to be careful when building models as skewers may be sharp. Warn students not to stare directly into the lamp. This activity uses a piece of software called Light Grapher which detects input from a webcam to graphically display the brightness of a model star.

The practical activity

1. Students should set up a lamp to represent their star and attach a ball to a stick or skewer to represent their exoplanet.
2. They should then move their ball on skewer/stick across the front of their lamp and produce a light-curve. You will need to explain how to use the Light Grapher software.
3. Once students have produced a single light-curve, they should predict how the shape of the light curve will change for a bigger and faster exoplanet. Encourage them to think about which variables they need to keep constant (e.g. radius of orbit) in order to test their predictions.

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.

Students use iron and sand to model the composition of the Earth and estimate what fraction of the Earth is occupied by its iron core.

Apparatus and materials

(per group of 2 to 4 students)

• Balance
• Measuring cylinder
• Steal ball bearing or steel block approx. 2 or 3 cm across
• Sand 

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

Health & safety and technical notes

If using ball bearings, remind students that if any fall on the floor they must be picked up promptly so that so no-one slips on them. Give each group a dish to keep them in. A little bit of tissue paper on the balance will stop them rolling off.

The practical activity

1. Introduce the activity by showing a steel ball (to represent the Earth’s core) and some Plasticine. Discuss their different densities. Explain how to calculate density and introduce units. 
2. Wrap a layer of Plasticine around the ball to represent the mantle and crust. What can be said about the average density?
3. You could measure mass and volume of the ball + Plasticine by immersing the ball in water in a measuring cylinder on a balance and then add increasing amounts of Plasticine. However, sand is a better material to represent rock as its density is closer to that of the rock found on the Earth’s surface.
4. They should find that the average density decreases from that of steel as more sand is added.
5. After the activity you may want to discuss the composition of the Earth. Explain that although the crust is of a similar density to sand, the rock in the mantle has a higher density

Students use diffraction gratings to observe the spectra from different sources and deduce how we can work out which chemicals are present in an exoplanet’s atmosphere.

Preperation and safety

Refer to CLEAPSS Laboratory Handbook 9.10.2 for Bunsen burner precautions. Warn students not to stare directly into the lamp.

Equipment

Each group of students will need:

• Access to a filmant lamp and sodium lamp
• Access to other light sources (eg a fluorescent lamp, LED torch, gas discharge tubes)
• Bunsen flame
• Diffraction grating or spectroscope
• Sodium chloride – a few grains 
• Marker Pen 

Procedure

Ask students to:

1. Look at a filament lamp through the diffraction grating or spectroscope.
2. Repeat with the other light sources.
3. Look at the light coming from the Bunsen flame. Drop a few crystals of salt into the flame so that it turns orange.
4. Watch you perform a demonstration of filament lamp shone onto a Bunsen flame and observe the shadow as sodium chloride is added to the flame.
5. Watch you perform the same demonstration but with a sodium lamp in place of the filament lamp.

Discussion prompts

• Are all the colours of the spectrum present??
• Are any colours brighter than the rest?
• For the demonstration, what causes the shadow?

Teaching notes

It is important that students can observe a number of light sources. You may wish to place several around the room and allow students to move around from one to another, recording their observations as they go. Alternatively, you could set up each source in turn at the front of the room so that all students can see the same source and spectrum at the same time.

Some students may find it difficult to observe a spectrum. If you have provided handheld spectroscopes show them how they can change the width of the slit to let more or less light in. If they are using unmounted diffraction gratings they should hold the diffraction grating close to one eye and look directly at the source. Then, by looking to one side, they should see a spectrum. It may help to use card or paper to cover most of the grating, leaving a small slit uncovered.

The demonstration introduces the idea of absorption of light. Students will be familiar with the idea of how shadows are formed, but they may not think of this as the absorption of light. They may never have thought about whether a gas can absorb light.

Learning outcome

Explain how a spectrum can tell us about the elements present in an planet’s atmosphere.

Students use a ball and stick to model the motion of a planet around a star and deduce how seasons may be different to those on Earth.

Preparation and safety

Ask students to be careful when building planets as skewers may be sharp. Warn students not to stare directly into the lamp

Equipment

Each pair of students will need:

• Lamp
• Polystyrene ball
• Bamboo barbecue skewer (length of 30 cm approx.)
• Marker Pen 

Procedure

Ask students to:

1. Carefully push a skewer through the ball to make an planet
2. Mark the N and S poles where the skewer passes through the ball.
3. Draw a line round the ball to represent the planet’s equator.
4. Spin the planet on its axis and discuss with a partner why this gives night and day.
5. Tilt the axis of the planet and move it slowly round the star. Discuss when the planet will experience summer in the northern hemisphere and when it will experience winter.
6. Model an exoplanet with a highly elliptical orbit.
7. Model an exoplanet that has no tilt and orbits with the same face to its star at all times (a tidally locked planet)

Discussion prompts

• How long does an exoplanet day last compared to its year?
• Would there be seasons?
• Could life exist on such a planet?

Teaching notes

If students are unfamiliar with the idea of an exoplanet introduce it. Exoplanets are planets that orbit stars other than our Sun. Astronomers have discovered several thousand and would like to know if any might be home to life.

A planet with a highly elliptical orbit will have seasonal variations, but they would not be like those on Earth. The whole planet will experience the same season at the same time: summer when it is closest to its star and winter when it is furthest away. In the course of a year, its orbit may take it in and out of the ‘habitable zone’ where conditions for life are thought to be most favourable. Life might evolve to hibernate for part of the year, or to aestivate when the temperature is too high.

For a tidally locked planet there are no seasonal variations. Its day is as long as its year. One side of the planet will always be in daylight and hot, the other in permanent darkness and cold. There will be a twilight zone between these two regions which might be a suitable place for life. Alternatively, life might exist beneath the surface.

Learning outcome

Students use their understanding of how seasons arise on Earth to predict how they may be different on other planets.

This poster provides an introduction to the habitable zone around a star and types of exoplanets astronomers have discovered.