Jodrell Bank Discovery Centre Big Science: Big Telescopes
Exoplanets: Lesson Plan
A lesson on detecting exoplanets for Key Stage 4 pupils (and high-achieving year 9 pupils).
This lesson plan has been developed by the Jodrell Bank Discovery Centre as part of the Science and Technology Facilities Council’s (STFC) Science and Society Large Award project Big Science: Big Telescopes.
This lesson plan is free for teachers to download and share.
This lesson is designed to excite and inspire pupils by engaging them with examples of the ‘Big Science’ carried out with the ‘Big Telescopes’ funded by STFC.
Some of the Big Telescopes with funding from STFC include the VLT (Very Large Telescope), ALMA (Atacama Large Millimetre/sub-millimetre Array), e-MERLIN (the UK's facility for high resolution radio astronomy observations), E-ELT (European Extremely Large Telescope) and SKA (Square Kilometre Array).
The Lovell telescope at Jodrell Bank, part of e-MERLIN. Introduction
Exoplanets are a very current and intriguing area of astronomy. An exoplanet refers to a planet orbiting around another star in our galaxy (i.e. a star which is not the Sun). It is estimated there are more than 200 billion stars in our Milky Way galaxy1. It is now thought that most of these stars have planets orbiting around them2. The vast majority of these exoplanets cannot be imaged directly. This is because planets are much smaller and dimmer than their parent stars, which completely outshine them. In order to detect exoplanets astronomers must use indirect methods; looking for clues in the star’s light that suggest exoplanets may be present. As of August 2013, scientists have detected 929 exoplanets3.
In this lesson your pupils will be introduced to the use of telescopes to do astronomy across the whole electromagnetic spectrum and the advantages of big telescopes. They will then explore the transit method of detecting exoplanets (either practically, or using real astronomical data) and use mathematics to calculate some of the properties of their exoplanet. The lesson concludes by considering the big question: are we alone in the universe?
Artist’s impression of an exoplanet
Translate advantages of big telescopes to the human eye.
Comprehend that visible light and radio waves show different phenomena.
Ask pupils to guess how many planets there are in our Milky Way galaxy.
Define term ‘exoplanet’ as a planet that orbits another star in our galaxy.
Pupils could record their guesses in a number of ways, e.g. post-it notes, mini-whiteboards. They could discuss in groups or answer individually.
It is currently not known how many planets there are in the Milky Way, but it is estimated to be upwards of 50 billion. As of August 2013, astronomers have detected 929.
Some pupils may be unfamiliar with the term ‘galaxy’.
A galaxy is a collection of many stars, bound by gravity. Our galaxy is the Milky Way and it contains around 200 billion stars.
Introduce contents of lesson (and objectives if required).
In Part 1 pupils will learn about the use of telescopes in astronomy generally and why big telescopes are advantageous.
In Part 2 pupils will focus on one particular area of current astronomy research; exoplanets. They will collect (or be given) data and analyse it mathematically to calculate some properties of an exoplanet.
Why do we need so many telescopes?
Teacher-led definition of some of the concepts used in the lesson.
Light travels through space as an electromagnetic wave, but there are other EM waves travelling through space.
Objects can emit any EM waves, such as radio waves, microwaves, infrared waves, etc… These other EM waves are identical to light, except they are a different size. A slinky could be used at this point to demonstrate waves with different wavelengths4. [This links to GCSE physics specifications]
This is Centaurus A; a galaxy around 13 million light years from Earth. When we look at it with visible light, we see an ordinary galaxy, but when we look with radio waves we see great blasts of material being thrown out the top and bottom of the galaxy.
This suggests there is an active supermassive black hole at the centre of Centaurus A5, which we wouldn’t know if we only looked in visible light. It’s only by looking at all the waves that we get the full picture about what is going on, so we need lots of different types of telescopes to look at the different waves.
Why build big telescopes?
Teacher-led examples of how this data is collected in real life.
The Lovell telescope at Jodrell Bank (in Cheshire, UK) is a radio telescope, built 1952 - 1957. It is still used today and at 76m in diameter, it is the third largest steerable telescope in the world, but why does it need to be so big?
Larger telescopes can collect more light and therefore see dimmer objects (that is, objects that are further away), but they also create sharper images6.
With modern technology (supercomputers and fibre-optic data networks) it is preferable to make many linked smaller telescopes, rather than single large ones. The Square Kilometre Array (SKA) will be the largest radio telescope in the world, built of over 3000 smaller dishes, spread across the deserts of Australia and South Africa7. It will act like one giant telescope thousands of kilometres wide! A telescope that size would be impossible to build as one giant dish! The SKA will be so powerful, it will be able to detect organic molecules in space.
Two short questions which test pupils’ understanding of science concepts.
This mini plenary assesses the first two learning objectives: Translate advantages of big telescopes to the human eye. Comprehend that visible light and radio waves show different phenomena.
Pupils could discuss answers in groups, before providing verbal feedback, or could ‘think-pair-share’. Pupils could answer individually on whiteboards, or post-it notes.
If pupils have not achieved the objectives, peer help could be used, or some other consolidation activity could be used (see ‘Additional resources’).
Lower: May find it easier to draw their answers. Or, teachers could provide multiple choice options that pupils vote on.
Higher: Pupils could use this activity as written exam practice; peer assessment could be used to give feedback on their answers.
What do we need big telescopes for? One of the things astronomers are currently using them for is hunting for exoplanets. An exoplanet is a planet orbiting around another star in space. We can’t see these directly, because they are so small compared to their star, which emits a massive amount of light. Astronomers need to look for changes in the star’s light to show a planet is there. These changes are tiny, so very sensitive, big telescopes are required to detect them.
Lower: May require a step-by-step set of instructions on how to complete the practical activity, which they can refer to during the practical.
Higher: May be able to collect results from open questions such as, “how can the equipment on the table be used to simulate an exoplanet orbiting a star?” They perhaps could also construct their own results table, rather than using the supplied worksheet.
One method to detect exoplanet is the transit method. This is what the practical activity will represent. This involves looking for a tiny, regular ‘dip’ in a star’s light as a planet passes in front of it.
See ‘Practical instructions’ for how to set up and run the practical activity. If not completing the practical activity, see ‘Using supplied results’.
Pupils to complete practical activity.
Pupils can record their results using the supplied pupil worksheet. See ‘Reading the data’ for information on how pupils should read off their results from the data they collected.
In the practical activity, pupils should complete questions 1-5 of their worksheet.
This assess the third learning objective: Interpreting data [either from practical activity, or provided information] to collect meaningful results.
Pupils could be placed in mixed ability groups, for peer support.
Analysing results 1
Pupils use a mathematical equation to analyse their results and determine how far away from its star their exoplanet would be.
Firstly, to make their results more realistic, pupils should multiply their measured orbital period (in seconds) by 100 days (e.g. 2 seconds becomes 200 days). Then, by dividing by 365.25, they have the fraction of an Earth year their planet takes to complete one orbit of its star.
This activity requires pupils to rearrange an algebraic equation. Some pupils may require support with this.
Pupils could arrange themselves by self assessment into a confident and un-confident group; so that extra support can be given to those who require it.
Pupils now use a simplified version of Kepler’s third law8 to calculate how far away from its star their exoplanet is. Please note this relationship only works if the orbital period is measured in Earth years and the distance is measured in AU (‘Astronomical Units’: 1 AU is the distance between the Sun and the Earth9). Also, this simplified version of Kepler’s third law only works for planets orbiting the Sun. Pupils should assume that their star is identical to the Sun.
This activity assesses the fourth learning objective: Mathematically analysing results to conclude properties of an exoplanet.
Once they have estimated a value for the distance, pupils can compare their exoplanet with our Solar System, to get an idea of where their exoplanet would lay.
Analysing results 2
Pupils use a mathematical equation to analyse their results and determine the diameter of their exoplanet.
Pupils have already calculated B, the fraction of starlight blocked by their exoplanet when it passes in front of their star.
This fraction will be equal to the ratio of the area of the star’s surface blocked by the exoplanet (or simply, the area of the exoplanet) to the area of the star.
By using this ratio and substituting in the equation for the area of a circle, pupils can now calculate r, the radius of their exoplanet.
Pupils should once again assume their star is identical to the Sun, by using the radius of the Sun as R, the radius of their star.
This activity assesses the fourth learning objective: Mathematically analysing results to conclude properties of an exoplanet.
This activity also requires pupils to rearrange an algebraic equation, more difficult than the last.
Before attempting this activity, pupils could reassess their confidence at rearranging algebraic equations. Some may wish to switch groups, depending on how difficult or easy they found the previous activity.
Pupils can then compare the diameter of their exoplanet with that of the Earth and Jupiter, to get an idea of the size of their exoplanet.
Linking practical and results to the big question: are we alone?
We have seen how astronomers detect exoplanets and how they use the results to find out what they are like. By August 2013, 929 exoplanets have been discovered and more are being found all the time.
There’s one in particular that made the headlines in 2011. It is a planet orbiting the star Kepler-22, which is 620 light years away, near the constellation Cygnus the swan. Kepler-22 is a star that is the same size, mass and temperature as the Sun.
Using the same method you have used today, astronomers detected an exoplanet planet around this star (the planet is called Kepler-22b). They calculated that this planet is only twice the size of Earth and it orbits its star at the same distance that the Earth orbits the Sun.
Are we alone in the universe? What do you think?
Please note that with current technology it would be impossible to image Kepler-22b and make out the planet’s surface. However, with new big telescopes it will soon be possible to detect the presence of oxygen or water in the planet’s atmosphere. If found, these could be key indicators of life.
Extension activity (optional)
Set of questions which ask pupils to evaluate their analysis.
These questions assess the final learning objective: Evaluating analysis method to identify underlying assumptions and their potential impacts on the conclusions.
Students could write, or verbally discuss their answers to these questions.
Higher: These questions could be provided to pupils who finish the previous activities more quickly.
This practical simulates the transit method of detecting an exoplanet orbiting a star. Pupils will measure their ‘star light’, looking for a regular dip when their ‘exoplanet’ passes in front of their ‘star’. Their results will be in the form of a lightcurve, showing a regular drop in light.
We recommend group sizes of three or four for this practical, though this obviously depends on equipment availability and the nature of the group.
Light level sensor, preferably connected to a data logger
Bulb, preferably with diffusing globe (see picture to the right)
Ball attached to string
Retort stand, bosshead and clamp
Stopwatch (if not using data logger)
Method for setting up:
Connect the light level sensor to the datalogger (if used).
Point the light level sensor at the lamp.
Attach the string to the clamp, such that the ball is free to swing in circles.
Place the clamp directly over the lamp, so that the ball circles around the lamp.
Adjust the height of the clamp so that the ball passes in between the lamp and the light sensor (see picture below).
Using different sized balls and different lengths of string will generate a variety of results across the class.
Pupils should be instructed to pull the ball outwards, until the string is taut, then drop the ball, so it swings with a regular circular motion. Pupils should take their readings before the ball slows down too much, due to air resistance.
It may be necessary to turn off the classroom lights and/or draw the curtains to reduce the ambient light level when readings are being taken. If you are doing this, please make sure you are aware of the health and safety implications and take all necessary precautions.
If using data-logging software, the lightcurve created should show a periodic dip in light levels, like the one below10. Pupils can read the time taken for one orbit along the x-axis (red arrow). The period for the example below would be around 2.2 seconds.
Pupils can then read the fraction of light received from the star when the exoplanet is in the way from the y-axis (green arrow). From this, they can calculate B, the fraction of light blocked by their exoplanet. In the example below, the fraction of light still detected when the exoplanet is in front of the star is 0.995. Therefore, B = 0.005.
Pupils will probably find their planets are much larger than Jupiter since the Sun is 10 times wider than Jupiter, yet the lamps used in the experiment may not be 10 times wider than the balls. This goes to illustrate that it is a lot easier to find larger exoplanets (e.g. ones larger than Jupiter) than exoplanets that are smaller (for example, Earth sized).
If not using data-logging software, pupils will need to manually time the number of seconds between each dip in the light using a stopwatch. They will also need to record the light levels displayed on the sensor at normal levels and during a dip. Repeat readings should be taken and an average calculated.
Using the supplied results
The supplied results come from real astronomical data taken by NASA’s Kepler space telescope11. The star observed is Kepler-22 and the dip in the star’s light is caused by the planet Kepler-22b passing in front of the star and blocking a small portion of its light. This is the same star and planet as mentioned in slides 18-20 of the presentation.
Pupils should notice the large spread in readings on the original data (below). This is due to random errors in the measurements, such as dust in interstellar space blocking some of the starlight.
Pupils should first use the corrected data on side A (below) to read off the time interval between each dip in the star’s light. This is marked by the red triangles. This is how long it takes for Kepler-22b to complete one orbit. Pupils should find the time for one orbit (T) to be around 290 days.
Pupils should then use the zoomed-in data on side B (below). The graph shows combined data from the three dips observed from side A. Pupils should read off the fraction of starlight detected when the exoplanet passes in front of the star. This is around 0.9995. Therefore the fraction of starlight blocked by the exoplanet (B) is around 0.0005.
Additional resources on big telescopes
Full 50 minute documentary about current big optical telescopes http://www.youtube.com/watch?v=QeobrudynUE
Square Kilometre Array Official Animations http://www.skatelescope.org/media-outreach/videos/
More information about current and future big telescopes, including school resources http://www.bigtelescopes.org.uk/
Star Gazing Live video demonstrating how to make your own small telescope http://www.bbc.co.uk/programmes/p00n6zkf
Use of Images
All images used in this lesson's presentation have been released under a creative commons license. Every effort has been made to credit all images used. Where images do not display credits, this is because this information could not be found. If you believe an image has been used incorrectly or has been mis-credited, please email Jamie Sloan on the address shown below and I will be happy to amend the presentation.
3 You can find out the most up-to-date number at http://exoplanet.eu/catalog/
4 Animation of this demonstration: https://www.youtube.com/watch?v=3BN5-JSsu_4
5 An active black hole means the black hole is currently swallowing matter. Some material around the black hole gets accelerated by the huge gravitational field and blasted out as huge jets travelling at nearly the speed of light. More details on black holes at http://curious.astro.cornell.edu/blackholes.php
6 The explanation for this is A-level physics, so it is not addressed here, but it can be found at http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/cirapp.html
7 More information on SKA can be found at http://www.skatelescope.org/
8 Also called the law of periods: http://hyperphysics.phy-astr.gsu.edu/hbase/kepler.html#c6; More about Kepler at http://en.wikipedia.org/wiki/Johannes_Kepler
9 Definition of the AU http://neo.jpl.nasa.gov/glossary/au.html
10 Image taken from http://exoplanetmusings.files.wordpress.com/2011/12/00009a.jpg
11 Data taken from http://kepler.nasa.gov/Mission/discoveries/kepler22b/