Seasons on exoplanets

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.