Download EX PLANET E - Institute of Physics

EX
PLANET
PHYSICS
FIVE CURRICULUM LINKED PRACTICAL ACTIVITIES
FOR 11-14 YEAR OLDS
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CONTENTS
Introduction
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Activity 1: 2
The transit method
Making predictions
Activity 2: The habitable zone
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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
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Planetary orbits and spins
CREST activity:
Exoplanets: A new Earth?
Bronze Research Project
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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.
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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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THE TRANSIT METHOD
In this activity students use a lamp and polystyrene balls to model how astronomers
detect exoplanets using the transit method.
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 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 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 objectives
After 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 activity
Introduce 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
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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 activity
Students 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-curves
The 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
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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.
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.
Star
a
Taking it further
Once 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.)
Brightness %%
Brightness
100
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a
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b
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Transit duration
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g
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g
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Time (hours)
0 123 45 6
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.
100
Brightness %
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!
b
Exoplanet
Scale models of star systems
Students 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).
PLANET
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(i) Faster planet
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(ii) Bigger planet
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Time (hours)
0 123 45 6
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).
A
B
C
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FINDING EXOPLANETS: THE TRANSIT METHOD
Brightness %
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.
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Time (hours)
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The light-curve for an exoplanet called Kepler-444f. Each point on the curve was plotted by taking
the average of many measurements.
In this activity you will build a model of an
exoplanet orbiting a star to investigate how
scientists use a transit to detect exoplanets.
2. Measure the brightness of your star using
Light-Grapher software on the computer
(your teacher will show you how to do this).
What you’ll need:
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?
• 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.
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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.
Brightness %
Time
Light curve for a
bigger exoplanet
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Time
Light curve for
Taking it further The transit method is just one technique that astronomers use to search for exoplanets. Use the internet to find other ways of detecting exoplanets.
Once you have made all your predictions use your model to test them. Were the results as you expected?
Time
Light curve for a
faster exoplanet
Brightness %
Three copies of the same light curve are shown below. Make predictions about how the shape will change by drawing curves for a faster planet and
a bigger planet. Think of one more variable to test and sketch a curve for this change on light curve C.
THE TRANSIT METHOD: MAKING PREDICTIONS
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Brightness %
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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 (pages 8 and 9 respectively).
Health & Safety and Technical Notes
Old mains powered radiant heaters with bowl-fire
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 objectives
After completing this activity, students should be able to:
•
understand that the temperature of a planet depends
on its distance from its star, surface reflectively 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 activity
Introduce the idea of an exoplanet and explain that we are
interested to know whether life might exist on any of the
observed exoplanets.
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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 efficiently 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 activity
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. 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 reflects 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 figure 2a). Also ask
students how they think the graph would change for a more
powerful heater/star. Other questions to help students link
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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 zone
The habitable zone is usually defined 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
PLANET
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help them provide at least one reason why planets may
be not be at the predicted temperature; planets that reflect
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.
Taking it further
Once students have developed a better understanding
of the habitable zones, you could ask them to use the
internet to find 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.)
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.
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3d
Figure 2b
Student worksheet answers
1.
2.
(i)
Temperature decreases/goes down
(ii)
Venus
(iii)
Earth has a temperature between 0°C and 100°C / it is in the habitable zone
(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 reflects (more infrared-radiation)
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ESTIMATING TEMPERATURE: THE HABITABLE ZONE
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’.
500
Venus
Average temperature (ºC)
400
300
200
Mercury
100
Distance from Sun (millions of km)
Earth
0
-100
0 Mars
500
-200
Jupiter
1000
1500
2000
2500
3000
Uranus
Saturn
-300
3500
4000
4500
Neptune
Note: The size of the planets is
not shown to scale.
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 need to do:
What you’ll need:
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.
•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
•Safety:
GraphTake
paper
care when working with a radiant heater.
not getpaper
too close to it as you could be burned. If you
•Do
graph
are using a bulb do not look at it directly.
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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.
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.)
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(iii)
Your graph shows temperature and distance from a heater.
(i)What was the lowest temperature reached in the experiment you carried out? What do you think the lowest temperature would be for
temperature-distance graph for a star?
(ii)
2.
Taking it further A star’s habitable zone changes over time. Use the internet to find out about the lifecycle of stars. What will happen to the Sun in the future?
How will this change the habitable zone?
Can you explain why the shiny and black thermometers give different results?
Explain why the Earth is the only planet likely to sustain life.
Which planet does not fit this pattern?
(ii)
How does temperature change with distance?
(i)
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1.The graph on the instruction sheet shows the average surface temperatures of the planets and their distances from the Sun.
THE HABITABLE ZONE: TEMPERATURE-DISTANCE GRAPHS
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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.
Apparatus and Materials
Introducing the activity
•Access to a variety of light sources (filament lamp,
Introduce 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.
(per group of 2 to 4 students)
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 Notes
Refer 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’)
Learning objectives
After completing this activity, students should be able to:
•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.
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The practical activity
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. 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.
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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 demonstration
This 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.
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About the atmospheres of
exoplanets
Explain 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.
Taking it further
Students 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.
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.
The sodium flame transmits white light.
There is no shadow of the flame.
Light from a sodium lamp is shone at a There is no shadow of the flame.
Bunsen flame.
The flame transmits sodium light.
Sodium chloride is added to the flame.
The sodium flame absorbs sodium light.
A shadow of the flame can be seen on
the wall.
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UNDERSTANDING STARLIGHT: EXOPLANET ATMOSPHERES
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.
Spectrum
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.
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?
What you’ll need:
Emitting and absorbing light
•Light sources (filament lamp, fluorescent lamp,
The 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.
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.
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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.
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Observation (is there a shadow of the flame?)
Explanation
Taking it further Use the internet to research which atoms and molecules scientists search for when looking at exoplanet atmospheres.
Sodium chloride is added to the
flame.
Light from a sodium lamp is shone
at a Bunsen flame.
Sodium chloride is added to the
flame.
White light is shone at a Bunsen
flame.
Demonstration
Watch a demonstration which shows how light may be absorbed by a gas. Record your observations below.
Your explanation should include at least one of the following words: absorbs or transmits.
EXOPLANET ATMOSPHERES: ABSORBING AND TRANSMITTING LIGHT
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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.
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 (pages 16 and 17 respectively).
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.
Learning objectives
After completing this activity, students should be able to:
• measure mass and volume.
• calculate density from mass and volume.
•understand that planets can be classified according to
their densities.
Introducing the activity
Introduce 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).
14
The practical activity
You 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 steelpercentage 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.
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About planetary densities
For 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 find 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.
PLANET
PHYSICS
It is harder to find 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.
Taking it further
Students can research the densities of some known
exoplanets and identify ones that have similar densities
to Earth.
Figure 4a – Typical results
1
A
AB
C
D
E
F
Volume of steel
(cm3)
Total mass (g)
Volume of sand
(cm3)
Total volume
(cm3)
Steel
percentage by
Volume
Average
density (g/cm3)
(sand + steel )
(sand + steel )
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
2
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.
9
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).
8
3
Crust
(low density rock)
6
5
5
4
Mantle
(high density rock)
3
2
10
Density (g/cm3)
Average density (g/cm3)
7
Core
1
(iron)
0
13
0%
20%
40%
60%
Steel percentage (by volume)
80%
100%
10-70
2900
6400
Depth below surface (km)
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PLANET
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FINDING AN EARTH-LIKE EXOPLANET: PLANET DENSITY
To find 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.
Gas or Ice
80000
Rock with Iron Core
Jupiter
Saturn
60000
Radius (km)
Rock
40000
Uranus
20000
Neptune
Venus
Mars
0
0
1
2
3
4
5
Earth
Mercury
6
Note: Planets are not to scale
7
Average density (g/cm3)
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 find out how much of the
Earth is rock and how much is iron.
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
Density of steel =
mass of steel
volume of steel
What you’ll need:
4. Remove the steel from the balance.
• Balance
• Measuring cylinder
• Steel ball bearing or steel block approx.
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.
2 or 3 cm across
• Sand
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6
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Total mass (g)
Volume of steel
(cm3)
0
g
Volume of sand
(cm3)
C
g
(sand + steel )
Total volume
(cm3)
g
D
100 %
Steel
percentage by
Volume
E
Place the steel on the balance
and record the mass in cell B2
of the table
g
g
F
Average
density (g/cm3)
0.00
g
g
PLANET
PHYSICS
Plot a graph of average density against steel
percentage. Use the graph to work out the
percentage of the Earth occupied by the core
For each of your values calculate the average
density using
Total mass
Average density =
Total volume
Record your answer in column F
For each of your values calculate the
percentage of the total volume that is steel
using
Volume of steel
Steel % =
x 100%
Total volume
Record your answers in column E
For each of your values calculate the total
volume (steel + sand). Record your answers in
column D
Gradually increase the amount of sand,
recording the total mass in column B and
volume of sand in column C of the table
g
g
Add some sand and record
the total mass 0.00
and volume of
sand in cells B3 and C3
of the table
EX
Taking it further Use the internet to find out about the densities of some exoplanets. Which are likely to be gas giants? Which might be Earth-like?
(sand + steel )
AB
0.00
Place the measuring
cylinder on the balance
and zero it.
A
Record the
volume of the
steel in cell A2 of
the table
PLANET DENSITY: MODELLING THE EARTH
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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.
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)
•Marker Pen
Each student will also require a photocopy of the
instructions and worksheet (pages 20 and 21 respectively).
Health & Safety and Technical Notes
Tell students not to stare directly into the lamp.
Learning objectives
After completing this activity, students should be able to:
• 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 activity
This 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.
18
The practical activity
Explain 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 humanlike 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 orbits
On 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
EX
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,
PLANET
PHYSICS
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.
Taking it further
Students 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.
P
P
Figure 5a – An exoplanet that orbits with the same face to
its star at all times.
R
P
R
P
Q
Q
R
R
Q
Q
(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.
Figure 5b – A planet with an eccentric orbit
Whole planet
experiences summer
Whole planet
experiences winter
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LIVING ON AN EXOPLANET: DAY AND NIGHT, SEASONS
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.
N
The Earth when it is winter and
daytime in the UK.
Sunlight
S
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 find 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.
20
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.
21
EX
PLANET
PHYSICS
Taking it further Imagine that humans were sent to live on an exoplanet. Conditions would be very different from Earth. Suggest some ways in which people
could adapt to life there.
2.Astronomers have discovered that some exoplanets have orbits that are not circular. They orbit their stars in elongated ellipses. For part of the
year they are close to their star, but then their orbit takes them much farther away. Move your exoplanet in an orbit like this. Discuss what the
seasons will be like on such an exoplanet. How will its seasons be different from what we experience here on Earth?
1.Astronomers have discovered that some exoplanets orbit their star so that the same side always faces the star. Move your exoplanet round its
star in this way. Discuss whether this planet will experience day and night. Will it experience seasons?
DAY AND NIGHT, SEASONS: PLANETARY ORBITS AND SPINS
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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.
About CREST Awards
Prompts
CREST 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
The 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.
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?
Activity 1: The transit method
What does the light from the star/planet tell us about an exoplanets size,
orbit and atmosphere?
Activity 3: Exoplanet atmospheres
Prompt on student brief
What different types of stars are there?
How is your exoplanet’s star different from the Sun?
Additional prompts
Activity Links
The Sun is called a yellow dwarf; what are other star called and why?
Activity 2: The habitable zone
For your exoplanet system, is the habitable zone closer or further away from the
star than that of the Solar system? Why?
Activity 3: Exoplanet atmospheres
22
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PHYSICS
Prompt on student brief
What are conditions like on different planets?
Could the exoplanet sustain life - how could humans survive there?
Additional prompts
Activity Links
Does it have an atmosphere? Is it breathable?
Activity 3: Exoplanet atmospheres
How hot or cold is the planet?
Activity 4: Planet density
What is the planet made of? Would you be able to stand on it?
Activity 5: Day and night, seasons
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?
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.
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 Search
Useful 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 projects
For other Bronze, and also Silver and Gold CREST Award
project ideas visit www.iop.org/exoplanets
Someone with knowledge and/or experience of astronomy
and/or exoplanets would be ideal.
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EXOPLANETS: A NEW EARTH?
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?
Have you ever wondered?
Health and Safety
Are there other Earth-like planets in our galaxy?
Should you decide to carry out an experiment
or practical activity:
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?
(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)
Your task is to research exoplanets and choose
one to send a probe to. You decide to:
(d)make sure your teacher agrees with your plan
and risk assessment
•Research conditions needed to make a planet
(e)If special tools or machines are needed,
arrange to use them in a properly supervised
D&T workshop.
habitable
•Compare data available on different
exoplanets
• Recommend one exoplanet
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.
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?
24
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