EX PLANET EX PLANET PHYSICS EX PLANET EX PLANET · five curriculum linked practical activities for 11-14 year olds ex physics planet ex physics planet ex physics planet physics ex

FIVE CURRICULUM LINKED PRACTICAL ACTIVITIES FOR 11-14 YEAR OLDS

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CONTENTS

Introduction 1

Activity 1: The transit method 2 Making predictions

Activity 2: The habitable zone 6 Temperature-distance graphs

Activity 3: Exoplanet atmospheres 10 Absorbing and transmitting light

Activity 4: Planet density 14 Modelling the Earth

Activity 5: Day and night, seasons 18

Planetary orbits and spins

CREST activity: Exoplanets: A new Earth? 22 Bronze Research Project

Acknowledgements This resource was developed by the Institute of Physics (IOP) and is based on the Exoplanet Physics Project originally developed by the Institute as part of the Stimulating Physics Network funded by the Department for Education.

This resource was written by David Sang, Jemma Duncombe and Taj Bhutta. Illustrations and design are by Lesley Lee and Light-Grapher is used with permission from the NASA Kepler Mission. We would also like to thank Jon Clarke and Brendan Ickringill for their support and advice and the British Science Association for help with developing the CREST activity.

Cover image: An artist’s impression of exoplanets orbiting Kepler-444, a star that is that hosts five Earth-sized planets in very compact orbits. Picture Credit: Tiago Campante/Peter Devine.

OVERVIEW

1

INTRODUCTION

Exoplanets are planets that orbit stars other than our Sun. This resource was developed to help bring this new and exciting area of research into the classroom. It consists of five practical activities matched to the 11-14 curriculum.

Each of the activities is standalone, takes around 40 minutes to complete and can be used either in lessons or as part of a science club. Each offers plenty of opportunity for extension work and includes a taking it further section to allow students to build on what they have learnt through independent research. They can also be used individually, or in combination, as the basis for a CREST Award (an enrichment scheme run by the British Science Association). One “pick-up and run” CREST research project idea is included. Others are available via www.iop.org/exoplanets.

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Apparatus and Materials (per group of 2 to 4 students)

• 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

• Laptop preloaded with Light Grapher software

Each student will require a photocopy of the instructions and worksheet (pages 4 and 5 respectively).

Health & Safety and Technical NotesAsk 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 software and instructions are available to download via www.iop.org/exoplanets. Light Grapher is a Flash file (.swf) and will run in any Flash enabled internet browser.

Learning objectivesAfter completing this activity, students should

• understand that the transit of a planet in front of its star temporarily reduces the star’s measured brightness.

• understand that a light-curve is a graph of “brightness” against time.

• describe and explain how different factors (including size of exoplanet and orbital speed) affect the light-curve observed during a transit.

Introducing the activityIntroduce the idea of an exoplanet and explain why they are difficult to observe. (They are very distant and much smaller than stars, and they are not sources of light.) Explain that a

number of techniques have been developed to observe exoplanets so that we now know of thousands. Explain that they are going to model the transit method in which the brightness of a star is measured as the exoplanet orbits it.

The practical activityStudents should set up a lamp to represent their star and attach a ball to a stick or skewer to represent their exoplanet. 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. 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.

About light-curvesThe brightness is shown as a percentage, with the percentage of brightness on the y-axis and the time on the x-axis. An idealised light curve for a Jupiter-like planet crossing the disc of a Sun-like star is shown in figure 1a.

On their worksheet, students are provided with a light curve and asked to sketch curves for a faster and a bigger planet. These are illustrated in figure 1b. A faster exoplanet moves across the face of the star more quickly and so the dip in intensity lasts for a shorter time. A larger exoplanet obscures more of the star’s surface during a full eclipse and so the dip in intensity is larger.

Students are asked to think of a third variable to test. A likely choice is the distance of exoplanet from star. For their model star system, the proximity of the webcam means that increasing the orbital distance may significantly increase the size of the dip. If they move the planet close to the web-cam they may even observe a total eclipse. In practice the distance between an exoplanet and its host star is negligible compared to the enormous distances from Earth. After the activity you could discuss this limitation of their model when discussing relative scales (see Scale models of star systems below).

Another variable which will affect the light-curve is the orientation of the exoplanet’s orbit around its star. Students

THE TRANSIT METHOD

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

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are likely to model the orbit so that it is “edge on” when viewed from Earth. This produces a dip in intensity that lasts the longest. For other orbital orientations the transit duration will either be reduced or no transit will be observed at all (Figure 1c). This is an additional complication which students may come up with but which you may not wish to introduce if it does not arise in your class.

Scale models of star systemsStudents often hold misconceptions about the relative size of planets, stars and distances between them. You could ask them to look more closely at the vertical axis of light-curve on their instruction sheet. The planet illustrated (Kepler 444f) is similar in size to the Earth and orbits a Sun-sized star 117 light years away. The dip in the brightness is 0.01 %. What does this imply about Earth-sized planets? (They are much smaller than stars).

You could also discuss how large a scale model would have to be to represent the Solar System. Assuming they use a typical 6 cm diameter light bulb for the Sun, the Earth would be the size of a grain of sand about 6 metres away and Jupiter would be the size of a pea at 33 metres. If students were to model observing the transit of Kepler 444f, the exoplanet would be about 50 cm from the light bulb but the observer on Earth (the webcam) would have to be over 48 thousand kilometres away!

Figure 1a: Predicted light curve for a Jupiter-sized planet transiting a Sun-sized star. The corresponding positions of the planet (a-g) are also shown.

Figure 1b: Predicated light curves for a faster and bigger planet. The examples illustrated are for a planet with (i)1.5 times the speed and (ii) 1.5 times the diameter of that shown in figure 1a.

Figure 1c: Light curves for different orbital orientations. Light curve A is for a planet whose orbital plane is exactly edge-on from our point of view. For other orientations the transit duration is either reduced (B) or no transit is detected (C).

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Taking it furtherOnce students have investigated the Transit Method, you could ask them to use the internet to find out about one other way of detecting exoplanets. (There are at least five other techniques used to detect exoplanets although several require an understanding of Physics well beyond Key Stage 3.)

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FINDING EXOPLANETS: THE TRANSIT METHOD

In this activity you will build a model of an exoplanet orbiting a star to investigate how scientists use a transit to detect exoplanets.

What you’ll need:• Lamp

• Polystyrene balls

• Skewers

• Computer with webcam and Light-Grapher software

What you need to do:1. Using the lamp as your star, decide how to

model the transit of a planet as it orbits around the star.

2. Measure the brightness of your star using Light-Grapher software on the computer (your teacher will show you how to do this).

3. Use the Light-Grapher software to capture the light-curve of your star as your planet orbits. Can you produce results similar to the light-curve above?

4. Think about how varying the size and speed of the exoplanet might affect the shape of the graph. On the worksheet sketch light-curves for a faster and bigger exoplanet; also think of one more variable to test.

Exoplanets are too small and far away to see directly, even with the most powerful telescopes. So how can astronomers detect them? When an exoplanet passes in front of its star (an event known as a transit), it blocks some of the star’s light. For a short time, the star’s brightness decreases. So, if astronomers detect that a star’s brightness decreases and then increases again, they can deduce that there is a planet orbiting the star.

The light-curve for an exoplanet called Kepler-444f. Each point on the curve was plotted by taking the average of many measurements.

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INSTRUCTIONS

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• 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 (pages 8 and 9 respectively).

Health & Safety and Technical NotesOld 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.

Learning objectivesAfter completing this activity, students should be able to:

• understand that the temperature of a planet depends on its distance from its star, surface refl ectively and atmosphere

• understand that the habitable zone is the region of space around a star where the average surface temperature of a planet will allow liquid water to exist.

Introducing the activityIntroduce the idea of an exoplanet and explain that we are interested to know whether life might exist on any of the observed exoplanets.

Explain that liquid water is likely to be necessary for life. There are two reasons for this: many substances can dissolve in liquid water, and many of the chemical reactions necessary for life take place most effi ciently in the temperature range around 0°C to 50°C. That’s why our body temperature is maintained close to 37°C.

Discuss the graph on the student instruction sheet; planets close to the Sun are hottest, those furthest away are coldest. Ask them to explain this, given that the surface temperature of the Sun is about 5500°C and the temperature of deep space is -270°C (almost absolute zero).

Students may not appreciate that the temperature of a planet arises from a balance between energy absorbed from the star and energy radiated into space. You may want to discuss a planets energy balance at the end of the activity.

The practical activityStudents 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. Warn them not allow their thermometers to get hotter than 100°C.

Students will probably realise that the temperature will rise as they approach the heater. It is more interesting if you can provide two thermometers per group: one with its bulb blackened using soot or vegetable black, the other with its bulb made shiny using aluminium leaf or foil. If this is not possible ask half the class to work with black thermometers and the other half with shiny thermometers and then pool results at the end.

The shiny bulb thermometer should show lower temperatures as it refl ects radiation away. The blackened thermometer will absorb radiation better.

After the students have drawn their graphs, discuss their results and explain why temperature decreases with distance from the star/heater; the radiation ‘spreads out’ as it travels and so becomes less intense (see fi gure 2a). Also ask students how they think the graph would change for a more powerful heater/star. Other questions to help students link

THE HABITABLE ZONE

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

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their results to planetary temperatures and habitable zones are provided on the worksheet. Answers to these questions are shown in Figure 2b.

About the habitable zoneThe habitable zone is usually defi ned as the region around a star within which an orbiting planet would be able to support liquid water at their surfaces. Colloquially it is also called the Goldilocks zone as it is neither too hot, nor too cold for life to evolve as we know it.

You could explain how astronomers are able to estimate the size of a star’s habitable zone. Refer students to the planetary temperatures on the instruction sheet, both predicted and actual. Explain that the predicted temperatures (the dotted line) were calculated by assuming that the planets absorb all the radiation that falls on them; this is the (theoretical) equivalent of an ideal black thermometer. Ask them if they think treating the planets as black objects is a good model? Students should conclude that for most of the planets in the Solar System this seems to be a reasonable approximation. The differing results they obtained for black and silver thermometers should

help them provide at least one reason why planets may be not be at the predicted temperature; planets that refl ect more light absorb less of the incident solar energy. Another complication is a planet’s atmosphere; particularly if it contains a high concentration of greenhouse gases. For the Earth the (natural) greenhouse effect means it is about 30ºC warmer than predicted. Venus has a much thicker atmosphere and the greenhouse effect is more extreme. Venus is 500ºC warmer than predicted by black-body radiation calculations.

Figure 2a

The intensity of the radiation emitted by a source decreases with distance. For a star (a spherical source) doubling the distance results in a fourfold decrease in intensity.

Figure 2b

Student worksheet answers

1. (i) Temperature decreases/goes down

(ii) Venus

(iii) Earth has a temperature between 0°C and 100°C / it is in the habitable zone

2. (i) 20°C (or whatever room temperature is).

For a star lowest temperature will be -270°C (accept anything below -200°C)

(ii) The shiny thermometer refl ects (more infrared-radiation)

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Taking it furtherOnce students have developed a better understanding of the habitable zones, you could ask them to use the internet to fi nd out about how stars evolve over time. What implications does this have for the Sun’s habitable zone? (As the Sun enters it red giant phase towards the end of its life it will become larger and brighter. The habitable zone will move further out.)

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ESTIMATING TEMPERATURE: THE HABITABLE ZONE

In this activity you will investigate how the temperature varies close to a radiant heat source. (This is your ‘star’.) Find out how the temperature depends on its distance from the star for two thermometers (these are your ‘planets’).

What you’ll need:

• Radiant heater or infrared light bulb

• 2 thermometers, one with a shiny bulb, the other with a blackened bulb.

• 2 clamps and stands

• Metre rule

• Graph paper

• graph paperSafety: Take care when working with a radiant heater. Do not get too close to it as you could be burned. If you are using a bulb do not look at it directly.

What you need to do:1. Mount the shiny thermometer in a clamp.

It should be vertical with its bulb at the same height as the heater/bulb. Repeat with the blackened thermometer.

2. Place each thermometer at a distance of 70 cm from the heater. Wait until the temperature has become steady. Record the distance and temperatures in a table.

3. Move the thermometers 5 cm closer to the heater. Record their temperatures when they are steady. Repeat at 5 cm intervals.

Safety: Take care that your temperature readings do not exceed 100°C (the limit of the thermometers)

4. Use the graph paper to draw a graph of temperature against distance for each thermometer. (Use the same graph axes for both.)

If life is to exist on an exoplanet, it is likely to depend on liquid water. Water is liquid between 0°C and 100°C. If an exoplanet is too close to its star, it will be hotter than 100°C, and its water will boil away. Exoplanets that are colder than 0°C will be icy. For life to flourish, an exoplanet must be at just the right distance from its star, in the star’s ‘habitable zone’.

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Uranus NeptuneNote: The size of the planets is not shown to scale.

Habitable zone

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ACTIVITY 2

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• Access to a variety of light sources (filament lamp, fluorescent lamp, sodium lamp, LED torch)

• Bunsen flame

• Diffraction grating or spectroscope

• Sodium chloride – a few grains

Each student will also require a photocopy of the instructions and worksheet (pages 12 and 13 respectively).

Health & Safety and Technical NotesRefer to CLEAPSS Laboratory Handbook 9.10.2 for Bunsen burner precautions. Warn students not to stare directly into the lamp.

A cheap alternative to using a standard diffraction grating or spectroscope is available via www.mindsetsonline.co.uk (search for ‘CD spectroscope’)


• describe how a spectrum of light can be produced using a spectroscope/diffraction grating.

• understand that a spectrum shows the different wavelengths present in the light from a source.

• understand that a spectrum can tell us about the elements present in the light source.

• describe and explain how an absorption spectrum can tell us about the elements present in an exoplanet’s atmosphere.

Introducing the activityIntroduce the idea that, to find out more about distant stars and exoplanets, astronomers analyse their light. Show how to observe a spectrum by looking through a diffraction grating/spectroscope.

The practical activityIt 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. This will allow you to discuss what they are observing so that you can be sure that all students have seen a number of different spectra.

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.

You may have access to a set of discharge tubes each of which contains a different gas at low pressure. By connecting each in turn to a power source you can show the different colours produced, and their spectra.

To produce a sodium flame, either sprinkle a few grains of salt in the flame or use a metal rod dipped in concentrated salt solution.

EXOPLANET ATMOSPHERES

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.

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About diffraction gratings Traditionally, prisms are used to show spectra. These do not work well for observing different sources, whereas diffraction gratings can be relied on to produce good spectra.

It is not necessary to discuss how diffraction gratings work. Treat them as a useful piece of equipment for splitting light into its component wavelengths or colours.

About the demonstrationThis 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.

You can see a video version of the demonstration at www.iop.org/exoplanets

You need to be able to shine a bright white light at a Bunsen burner so that a shadow is cast on a screen or wall. You will also need a sodium lamp to use in place of the white light.

The student worksheet shows the stages in building up this demonstration (see Figure 3a for expected responses). You can discuss each step as you go along, or you can demonstrate each step and leave students to complete the sheet, following up by asking students to present their ideas.

About the atmospheres of exoplanetsExplain that the sodium flame has a shadow in sodium light because the light from the sodium lamp is absorbed by the sodium atoms in the flame. So the shadow of the flame shows that it contains sodium.

Go on to explain that, when an exoplanet passes in front of its star, some of the starlight is absorbed by the exoplanet’s atmosphere. Astronomers look for changes in the spectrum of the light from a star. If they observe that some wavelengths become dimmer as the exoplanet transits across in front of it, they can deduce which elements and compounds are present in the planet’s atmosphere.

This is similar to the observation that the Sun’s spectrum has certain wavelengths ‘missing’. These appear as dark absorption bands in the Sun’s spectrum and allow us to identify the elements present in the Sun’s atmosphere.

Figure 3a

Student worksheet responses

Demonstration Observation Explanation

White light is shone at a Bunsen flame. There is no shadow of the flame. The flame transmits white light.

Sodium chloride is added to the flame. There is no shadow of the flame. The sodium flame transmits white light.

Light from a sodium lamp is shone at a Bunsen flame.

There is no shadow of the flame. The flame transmits sodium light.

Sodium chloride is added to the flame. A shadow of the flame can be seen on the wall.

The sodium flame absorbs sodium light.

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Taking it furtherStudents can find out about the spectra of stars with exoplanets and how these indicate the substances present in the exoplanets’ atmospheres. They should look for examples of exoplanets with oxygen and water in their atmospheres as these may be home to life similar to that found on Earth.

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There are two ways to split up light to see the spectrum of wavelengths it is made up of: using either a prism or a diffraction grating. In this activity you will use a diffraction grating to see the colours present in light from some different sources.

What you’ll need:

• Light sources (filament lamp, fluorescent lamp, sodium lamp, LED torch, Bunsen flame)

• Diffraction grating or spectroscope

• Sodium chloride (common salt) – a few grains

What you need to do:1. Look at a filament lamp through the diffraction

grating or spectroscope. You will see a spectrum. Record the colours you observe.

2. Repeat with the other lamps. Are all the colours of the spectrum present? Are any colours brighter than the rest? Record your observations.

3. Look at the light coming from the Bunsen flame. Now drop a few crystals of salt into the flame so that it turns orange. (This colour is due to the sodium atoms in the salt.) Observe the spectrum of this light. Which colours can you see? Which is brightest?

Emitting and absorbing lightThe sources you have observed are sources which emit light, just like a star. The spectra are called emission spectra, and these can tell us about the chemical elements present in a star.

Planets are colder than stars. We see them by reflected and transmitted light. Astronomers can find out about planets and their atmospheres by seeing what wavelengths of light they absorb.

Spectrum

If you look at stars in the night sky, you may notice that they have different colours. This is because some stars (the reddish ones) are cooler than others. Blue-white stars are the hottest. Astronomers can also find out the chemical elements present in a star. They do this by separating out the different wavelengths in the star’s light.

UNDERSTANDING STARLIGHT: EXOPLANET ATMOSPHERES

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ACTIVITY 3

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ACT

IVIT

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WO

RKSH

EET

Tak

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.

14


• Balance

• Measuring cylinder

• Steal ball bearing or steel block approx.. 2 or 3 cm across

• Sand


Health & Safety and Technical NotesIf 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.


• measure mass and volume.

• calculate density from mass and volume.

• understand that planets can be classified according to their densities.

Introducing the activityIntroduce the idea of an exoplanet and explain why they are difficult to observe. (They are very distant and much smaller than stars, and they are not sources of light.)

Explain that astronomers can determine the radius and mass of an exoplanet, and hence deduce its density. By comparing an exoplanet’s size and density with that of the Earth and other planets, they hope to find Earth-like planets orbiting other stars.

Explain that the Earth is made of two materials: the dense iron core and the less dense outer rocky region (mantle and crust). Its average density is between the densities of iron and rock. They are going to use a simplified model to estimate what fraction of the Earth is iron (by volume).

The practical activityYou could 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. (For ease of calculation g/cm3 rather than kg/m3 are used throughout this activity).

Wrap a layer of Plasticine around the ball to represent the mantle and crust. What can be said about the average density? (It must be between that of steel – 7.9 g/cm3 and that of Plasticine – 1.9 g/cm3.)

(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.)

A blank table for tabulating results and calculations is provided on the worksheet. Alternatively students can use a Microsoft Excel spreadsheet for processing data. Remind them that before taking readings for the sand-steel mixture they should place the measuring cylinder on the balance and zero it.

They should find that the average density decreases from that of steel as more sand is added. Typical results are shown in figure 4a. The equation for calculating the steel-percentage by volume is provided on the worksheet and a graph of density against percentage provides a straight line from which the percentage that gives a density of 5.5 g/cm3 can be read (see figure 4b). They should get an answer of between 50-60%.

After the activity you may want to discuss the composition of the Earth (figure 4c). Explain that although the crust is of a similar density to sand, the rock in the mantle has a higher density (between 3 and 6 g/cm3). What does this imply about the size of the core? Will it be bigger or smaller than their estimate? (They should conclude that their estimate provides a maximum size for the core; the actual volume will be lower). There is also the additional complication that the iron in the core is denser than the steel they have used in their model.

PLANET DENSITY

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.

ACTIVITY 4

TEACHER NOTES

15

About planetary densitiesFor the Solar System, the masses of planets can be deduced from the orbital speeds of their moons – a moon orbiting a massive planet has to orbit quickly to avoid being pulled in by its strong gravity. Their radii can be measured from photographs taken using telescopes, or by observing them transiting across distant stars.

The chart on the student instructions shows how we can divide them into the higher density rocky planets and the lower density gas giants. (The gases are, of course, frozen.) Astronomers would like to fi nd examples of Earth-like exoplanets. They can then concentrate their efforts on trying to discover whether they may have signs of life such as atmospheres containing oxygen and methane.

It is harder to fi nd the mass and radius of an exoplanet. The radius can be found from the transit light curve – the initial dip takes longer for a bigger exoplanet (see teacher notes for activity 1). The mass can be found from the wobble of the parent star as the exoplanet orbits it – the star moves in a small circle and this can be detected from the Doppler shift in its light.

Figure 4a – Typical results

A AB C D E F

1 Volume of steel (cm3)

Total mass (g)(sand + steel )

Volume of sand (cm3)

Total volume (cm3)

(sand + steel )

Steel percentage by

Volume

Average density (g/cm3)

2 7.6 60.1 0 7.6 100 % 7.9

3 87.6 11 18.6 41 % 4.7

4 110.1 20 28.6 27 % 3.9

5 130.1 28 35.6 21 % 3.7

6 160.1 40 47.6 16 % 3.4

7 182.6 49 56.6 13 % 3.2

0

1

2

3

4

5

6

7

8

9

0% 20% 40% 60% 80% 100%

Figure 4b – Average density against steel percentage. The percentage that gives a density equal to that of the Earth (5.5 g/cm3) can be read from the graph.

Figure 4c – Layers of the Earth, their approximate densities and composition. Density depends on depth as well as composition. For example, the iron core’s density increases from around 10 g/cm3 (at its outer edge) to around 13 g/cm3 (at its centre).

Crust(low density rock)

Mantle(high density rock)

Core(iron)

3

5

10

13

10-70 2900 6400

Den

sity

(g/c

m3 )

Ave

rage

den

sity

(g/c

m3 )

Depth below surface (km)Steel percentage (by volume)

EX PHYSICSPLANET EX

PHYSICSPLANET

PHYSICSPLANETEX

PHYSICSPLANETEX

Taking it furtherStudents can research the densities of some known exoplanets and identify ones that have similar densities to Earth.

16

The biggest planets of the Solar System (Jupiter and Saturn) have the lowest densities. They are gas giants, made of frozen gas or ice. We couldn’t live there. The Earth is more dense. It is a rocky planet with an iron core.

The Earth’s average density is about 5.5 g/cm3. That is in between the density of rock (about 2.5 g/cm3) and the density of iron (7.9 g/cm3).

In this activity you will fi nd out how much of the Earth is rock and how much is iron.

What you’ll need:

• Balance

• Measuring cylinder

• Steel ball bearing or steel block approx. 2 or 3 cm across

• Sand

What you need to do:The steel represents the core of a planet. The sand represents the rocky exterior.

1. Measure or calculate the volume of the steel. (Decide on your own method for this.)

2. Zero the balance. Measure the mass of the steel.

3. Work out the density of steel using

4. Remove the steel from the balance.

5. Follow the instructions on the student worksheet to work out the density of a steel-sand mixture and the percentage of the Earth made of iron.

EX PHYSICSPLANET EX

PHYSICSPLANET

PHYSICSPLANETEX

PHYSICSPLANETEX

To fi nd out what an exoplanet is made of, astronomers look at its size and mass. From this they can calculate its density. This will help them to decide whether it is likely to be a rocky planet like the Earth or a gas giant.

FINDING AN EARTH-LIKE EXOPLANET: PLANET DENSITY

Density of steel = mass of steelvolume of steel

VenusMercuryEarth

Mars

JupiterSaturn

Uranus

Neptune

80000

60000

40000

20000

0

0 1 2 3

Average density (g/cm3)

RockGas or Ice Rock with Iron Core

4 5 6 7

Radi

us (k

m)

Note: Planets are not to scale

ACTIVITY 4

INSTRUCTIONS

ACT

IVIT

Y 4

WO

RKSH

EET

17

EX

PHYS

ICS

PLA

NET

EX

PHYS

ICS

PLA

NET

PHYS

ICS

PLA

NET

EX

PHYS

ICS

PLA

NET

EX

Tak

ing

it

furt

her

Use

the

inte

rnet

to fi

nd o

ut a

bout

the

dens

ities

of s

ome

exop

lane

ts. W

hich

are

like

ly to

be

gas

gian

ts?

Whi

ch m

ight

be

Eart

h-lik

e?

PLA

NET

DEN

SITY

: M

OD

ELLI

NG

TH

E EA

RTH

AA

BC

DE

F

1V

olu

me o

f st

eel

(cm

3)

Tota

l m

ass

(g

)(s

an

d +

st

eel )

Volu

me o

f sa

nd

(c

m3)

Tota

l vo

lum

e

(cm

3)

(sa

nd

+ st

eel )

Steel

perc

en

tag

e b

y V

olu

me

Ave

rag

e

den

sity

(g

/cm

3)

20

100

%

3 4 5 6 7

Reco

rd th

e vo

lum

e of

the

stee

l in

cell

A2

of

the

tabl

e

Plac

e th

e m

easu

ring

cylin

der

on th

e ba

lanc

e an

d ze

ro it.

Plac

e th

e st

eel o

n th

e ba

lanc

ean

d re

cord

the

ma

ss in

cel

l B2

of th

e ta

ble

Add

som

e sa

nd a

nd r

ecor

d th

e to

tal m

ass

and

volu

me

of

sand

in c

ells

B3

and

C3

of th

e ta

ble

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dual

ly in

crea

se th

e am

ount

of s

and,

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cord

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the

tota

l mas

s in

col

umn

B an

d vo

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e of

san

d in

col

umn

C o

f the

tabl

e

For

each

of y

our

valu

es c

alcu

late

the

tota

l vo

lum

e (s

teel

+ s

and)

. Rec

ord

your

ans

wer

s in

co

lum

n D

Plot

a g

rap

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f ave

rage

den

sity

aga

inst

ste

el

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enta

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se th

e gr

aph

to w

ork

out t

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enta

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f the

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th o

ccup

ied

by th

e co

re

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each

of y

our

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es c

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late

the

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enta

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f the

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l vol

ume

that

is s

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ing

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rd y

our

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ers

in c

olum

n E

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each

of y

our

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es c

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late

the

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ity u

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rd y

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er in

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umn

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l % =

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olum

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e th

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ring

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g g

g

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g g

g

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g g

g

0.00

ACT

IVIT

Y 4

WO

RKSH

EET

18


• 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)

• Marker Pen


Health & Safety and Technical NotesTell students not to stare directly into the lamp.


• explain how day and night relate to planetary rotation.

• explain how seasons relate to the tilt of a planet’s axis.

• describe how day and night and seasons may be different on different planets.

• discuss how life may adapt to differing conditions of light and temperature on exoplanets.

Introducing the activityThis activity extends a conventional treatment of day and night and seasons by asking students to apply their understanding to how exoplanets may differ from Earth. Introduce the idea of an exoplanet if this is unfamiliar to students. Explain that several thousand have now been observed and that astronomers seek to compare them to the familiar planets of the solar system. In particular, they would like to know if any might be home to life, and if any might even have advanced life forms comparable to humans.

Explain that astronomers can determine the radius of an exoplanet’s orbit around its star (by timing its transit) and also determine whether its orbit is circular or an elongated ellipse.

The practical activityExplain that a lamp represents a star and a polystyrene ball represents an exoplanet in orbit around it. Briefly remind students of why we experience day and night and seasons.

Students have to push a skewer through a ball to represent the exoplanet’s axis. (You might want to do this for them in advance.)

They should mark the poles and the equator as reference points.

Working in pairs or small groups, students demonstrate night and day and seasons to each other. Encourage them to describe what an observer on the exoplanet would notice in terms of movement of the star in the sky, light intensity and temperature.

They should then go on to model the two types of exoplanet described on their worksheet and discuss them in the same terms as above. They should consider the possibilities for life in these alien worlds. (‘Life’ could mean human-like creatures, or organisms like bacteria which are more capable of living in a range of habitats.)

They could present their findings either in the form of an illustrated written report, or as a presentation to the class.

About exoplanetary orbitsOn their worksheets student are asked to consider seasons and day and night on two exoplanets.

1: An exoplanet that orbits with the same face to its star at all times.

This type of planet is similar to the way in which we always see the same face of the Moon, and the exoplanet is described as ‘tidally locked’ to its star. Such planets rotate slowly. The time it takes to complete a rotation about its axis is equal to the time it takes to complete an orbit. Its day is as long as its year. Whether the planet experiences any seasonal variations or day-night cycles depends on the tilt of the planets axis. You may choose to limit the discussion to the simplest case of no axial tilt (see figure 5a).

Tidally locked planets are usually close to their stars and so the star will look big in the sky compared to how we see the Sun. The side of the exoplanet facing the star will always be in daylight and will always be hot. The back of the

DAY AND NIGHT, SEASONS

Students model the motion of a planet around a star and investigate how day and night and seasons may be different on other planets.

ACTIVITY 5

TEACHER NOTES

19

exoplanet, facing away from the star, will be in permanent darkness and hence 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. For planets with an axial tilt life may only be able to survive if it migrates back and forth between cooler and hotter regions throughout its year-long day.

2: A planet with an eccentric orbit.

Planets move in elliptical orbits, with the star at one focus. You could introduce this concept using two pins and a string to generate an ellipse (see www.iop.org /exoplanets).

Most of the planets in the Solar system have a low orbital eccentricity and move in an almost circular path. The Earth’s distance from the Sun varies by only about 1% during the course of a year. This contributes in only a small way to seasonal variations. Our seasons come about because of the tilt of the Earth’s axis.

An exoplanet with a more eccentric orbit will experience seasons differently: summer when it is closest to its star,

winter when it is furthest away (see figure 5b). Note that the whole exoplanet will experience the same season at any time, however, if it is tilted the northern and southern hemisphere temperatures at any given latitude will still vary.

An eccentric orbit may take the exoplanet in and out of the ‘habitable zone’ (where conditions for life are thought to be most favourable) in the course of a year. Life might evolve to hibernate for part of the year, or to aestivate when the temperature is too high. Organisms would require energy stores to keep them going through these times.

Figure 5a – An exoplanet that orbits with the same face to its star at all times.

Figure 5b – A planet with an eccentric orbit

(i) No Axial tilt The simplest case for a tidally locked planet is one with no axial tilt. For such a planet the star will always appear at the same point in the sky and the point on the planetary surface closest to the star will be hottest with the star directly overhead. No part of the planetary surface (e.g. P) will experience day-night or seasonal cycles.

(ii) With Axial tilt For an exoplanet with a tilted axis the star would move vertically in the sky as the planet orbited (but not across the sky) and there will be some temperature variation throughout its year-long ‘day.’ Whether there will be a day-night cycle will depend on latitude. For positions on the planetary surface such as Q the star will never set and nightfall will never occur. For positions such as R night will fall for some part of the cycle.

P

Q

R

P

Q

R

P

Q

R

P

Q

R

Whole planet experiences summer

Whole planet experiences winter

Taking it furtherStudents can research the range of conditions where life is found on Earth. In particular, they could find out about extremophiles, organisms which live in extreme conditions of darkness, temperature, pressure and chemical environment. They could consider whether this makes it more likely that life exists elsewhere in the universe and what signs we might look for in the search for life on exoplanets.

EX PHYSICSPLANET EX

PHYSICSPLANET

PHYSICSPLANETEX

PHYSICSPLANETEX

20

Here on Earth, we experience seasons. This is because the axis of the Earth is tilted. In the summer, our part of the Earth is tilted towards the Sun and the weather is warm. In the winter, we are tilted away from the Sun and the weather is cold.

If we lived on an exoplanet, would it have seasons like the Earth? In this activity you will fi nd out about two types of exoplanet which are very different from the Earth.

What you’ll need:

• Lamp

• Polystyrene balls

• Marker pen

• Bamboo skewers

What you need to do:The lamp represents a star. A polystyrene ball represents an exoplanet. A skewer through its centre represents the axis on which it spins.

1. On your ‘exoplanet’, mark the N and S poles where the skewer passes through the ball. Draw a line round the ball to represent the exoplanet’s equator.

2. You should know why we experience night and day. Make your exoplanet spin on its axis and discuss with your partner why this gives night and day.

3. You should know why we experience seasons. Tilt the axis of your exoplanet and move it slowly round the star. Make sure that the axis is always tilted in the same direction (for example, towards the window). Discuss when the planet will experience summer in the northern hemisphere and when it will experience winter.

An exoplanet is a planet orbiting a star other than the Sun. Astronomers have discovered several thousand exoplanets orbiting stars in our galaxy, the Milky Way.

LIVING ON AN EXOPLANET: DAY AND NIGHT, SEASONS

The Earth when it is winter and daytime in the UK.Sunlight

N

S

N

S

EX PHYSICSPLANET EX

PHYSICSPLANET

PHYSICSPLANETEX

PHYSICSPLANETEX

ACTIVITY 5

INSTRUCTIONS

ACT

IVIT

Y 5

WO

RKSH

EET

21

EX

PHYS

ICS

PLA

NET

EX

PHYS

ICS

PLA

NET

PHYS

ICS

PLA

NET

EX

PHYS

ICS

PLA

NET

EX

DA

Y A

ND

NIG

HT,

SEA

SON

S: P

LAN

ETA

RY O

RBIT

S A

ND

SPI

NS

1.

Ast

rono

mer

s ha

ve d

isco

vere

d th

at s

ome

exop

lane

ts o

rbit

thei

r st

ar s

o th

at th

e sa

me

side

alw

ays

face

s th

e st

ar. M

ove

your

exo

plan

et r

ound

its

star

in th

is w

ay. D

iscu

ss w

heth

er th

is p

lane

t will

exp

erie

nce

day

and

nigh

t. W

ill it

exp

erie

nce

seas

ons?

2.

Ast

rono

mer

s ha

ve d

isco

vere

d th

at s

ome

exop

lane

ts h

ave

orbi

ts th

at a

re n

ot c

ircul

ar. T

hey

orbi

t the

ir st

ars

in e

long

ated

elli

pses

. For

par

t of t

he

year

they

are

clo

se to

thei

r st

ar, b

ut th

en th

eir

orbi

t tak

es th

em m

uch

fart

her

away

. Mov

e yo

ur e

xopl

anet

in a

n or

bit l

ike

this

. Dis

cuss

wha

t the

se

ason

s w

ill b

e lik

e on

suc

h an

exo

plan

et. H

ow w

ill it

s se

ason

s be

diff

eren

t fro

m w

hat w

e ex

perie

nce

here

on

Eart

h?

ACT

IVIT

Y 5

WO

RKSH

EET

Tak

ing

it

furt

her

Imag

ine

that

hum

ans

wer

e se

nt to

live

on

an e

xopl

anet

. Con

ditio

ns w

ould

be

very

diff

eren

t fro

m E

arth

. Sug

gest

som

e w

ays

in w

hich

peo

ple

co

uld

adap

t to

life

ther

e.

22

About CREST AwardsCREST is an easy-to-run STEM enrichment scheme in the UK, endorsed by UCAS for use in personal statements. It allows 11-19 year olds to build skills and demonstrate personal achievement in creative STEM (science, technology, engineering and maths) project work supporting their curriculum-based learning. CREST is run by the British Science Association, accredits over 35 other national schemes and offers tangible benefits to both students and teachers. CREST Awards can now also be used toward a ‘skill’ section in the Duke of Edinburgh’s Award scheme at the corresponding level. Find out more at: www.britishscienceassociation.org/crest

PromptsThe student brief (page 24) gives some triggers to start students thinking. They should realise that each trigger implies several items to compare. Encourage students to identify possibilities themselves and refer them to the relevant activities (see activity links below). However, if still necessary additional prompts such as the below might be given to point students in suitable directions.

EXOPLANETS: A NEW EARTH?

In order to obtain a Bronze CREST Award students should spend approximately 10 hours researching exoplanets and choose one that they think may be suitable for humans to live on.

CREST ACTIVITY

TEACHER NOTES

Prompt on student brief

How do astronomers investigate planets outside our solar system?

How was your exoplanet discovered?

Additional prompts Activity Links

What methods are there for detecting exoplanets?

What does the light from the star/planet tell us about an exoplanets size, orbit and atmosphere?

Activity 1: The transit method

Activity 3: Exoplanet atmospheres


What different types of stars are there?

How is your exoplanet’s star different from the Sun?


The Sun is called a yellow dwarf; what are other star called and why?

For your exoplanet system, is the habitable zone closer or further away from the star than that of the Solar system? Why?

Activity 2: The habitable zone


23

Suggestions for supporting students Students must research and select information for themselves. However, they may need some direction from you to identify suitable sources of relevant information at an appropriate level. Though primarily based on secondary sources, the research project is likely to be more meaningful if the students if the student includes some practical work. This could build on some of the Exoplanet Physics activities. One possibility is for two students to undertake their projects – one research, the other practical – working independently, but coming together, to share mutually useful information and activities.

Although Bronze Award students are not expected to have an official Mentor for their project, access to expert advice makes students feel their work is important. Also, if the topic is not in your area of expertise, you may find a Mentor valuable. Your CREST Local Coordinator may be able to suggest suitable contacts. Alternatively you can use the Institute’s web-site to source a physicist: www.iop.org/engaging_physicists.

Someone with knowledge and/or experience of astronomy and/or exoplanets would be ideal.

Discuss with students how they will manage their time (after school clubs, working during lunch hours, homework). Agree a completion date with them.

Students should decide their focus, although this may alter in the light of experience as the project progresses.

Internet SearchUseful search terms: exoplanet – habitable planet – main sequence star

Or try:

planetquest.jpl.nasa.gov

kepler.nasa.gov

exoplanets.org or exoplanet.eu

phl.upr.edu

Alternative CREST projectsFor other Bronze, and also Silver and Gold CREST Award project ideas visit www.iop.org/exoplanets

EX PHYSICSPLANET EX

PHYSICSPLANET

PHYSICSPLANETEX

PHYSICSPLANETEX


What are conditions like on different planets?

Could the exoplanet sustain life - how could humans survive there?


Does it have an atmosphere? Is it breathable?

How hot or cold is the planet?

What is the planet made of? Would you be able to stand on it?

Is the gravity stronger or weaker than on Earth?

How long is a day on the planet, what sort of seasons does it experience?

What conditions are needed to make a planet habitable for humans?

What type of equipment might humans need to live on the exoplanet?


Activity 4: Planet density

Activity 5: Day and night, seasons

24

Have you ever wondered?Are there other Earth-like planets in our galaxy?

You might like to imagine yourself in a situation such as… You are part of a team of astronomers that is planning a mission to send a probe to an exoplanet. You want to find out what conditions are like on the surface to see if humans may be able to live there one day. But with so many exoplanets to choose from which one would you select for the mission?

Your task is to research exoplanets and choose one to send a probe to. You decide to:

• Research conditions needed to make a planet habitable

• Compare data available on different exoplanets

• Recommend one exoplanet

Health and SafetyShould you decide to carry out an experiment or practical activity:

(a) find out if any of the substances, equipment or procedures are hazardous

(b) assess the risks (think about what could go wrong and how serious it might be)

(c) decide what you need to do to reduce any risks (such as wearing personal protective equipment, knowing how to deal with emergencies and so on)

(d) make sure your teacher agrees with your plan and risk assessment

(e) If special tools or machines are needed, arrange to use them in a properly supervised D&T workshop.

Note: Your teacher will check your risk assessment against that of your school. If no risk assessment exists for that activity, your teacher may need to obtain special advice. This may take some time.

The search for a new Earth

Astronomers have discovered thousands of exoplanets in our galaxy. Some of them orbit stars much hotter than our Sun; others planets go around stars that are cooler. Astronomers think there may be billions of exoplanets in our galaxy. Do any of them have life? Could we live on them one day?

EXOPLANETS: A NEW EARTH?

BRONZE RESEARCH

PROJECT BRIEF

Some things to think about:

• How do astronomers investigate planets outside our solar system?

• How was your exoplanet discovered?

• What different types of stars are there?

• How is your exoplanet’s star different from the sun?

• What are conditions like on different planets?

• Could your exoplanet sustain life – how could humans survive there?

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