Key Stage 4 – Planet detector

Pupil worksheet

Life beyond Earth

Conditions on Earth are perfect for life. Might other planets support living things?

As far as we know, there is no life on the other seven planets of our solar system. But does life exist on planets that orbit other stars in our galaxy, the so-called exoplanets? Scientists, and people like you, are working hard to find out.

So how do we find exoplanets? And how do we know whether an exoplanet might support life?

Conditions for life

Almost all living things of life on Earth need water. If we assume that water is also vital for life beyond Earth, then a habitable exoplanet needs water, or at least the elements to make it from.

The water must be in its liquid state. This means that the temperature on the surface of the planet must be just right – too hot, and water exists as vapour; too cold, and any water will be in the form of ice.

Three factors affect the surface temperature of an exoplanet, including the temperature of its star, its distance from this star, and whether or not the planet has an atmosphere to insulate it.

Scientists look for life-supporting exoplanets around a star in its habitable zone, also called the Goldilocks zone. This is the range of distances from the star which could include planets on which any water might be liquid.

Your task – Finding exoplanets

In this activity you will find out – and teach others – about one of three ways of detecting an exoplanet against the dazzling backdrop of its parent star.

Prepare

Read the information about your detection method (transit, wobble or direct imaging). If you have time, explore the web links.

Use the materials provided to make a model to help you explain this method to other students.

Plan what you will say and do to explain your detection method. Use sections A, B and C of the Teaching plan form to guide your planning.

Write a short activity to test the understanding of the students you will teach. This could be a gap fill or similar. Do this on a separate piece of paper. You may be able to get it photocopied so that everyone you teach can have their own copy.

Teach

Follow your plan to teach other students about your detection method.

Learn

Listen and watch as other students teach you about the other two detection methods. Carry out their activities to check your understanding.

www.oxfordsparks.net/planet

Key Stage 4 – Planet detector

Teaching plan

www.oxfordsparks.net/planet

Section AName of detection method (circle one) transit wobble direct imaging

Objective

Section BLabelled diagram of our model.

How we will use our model to help us to explain our detection method.

www.oxfordsparks.net/planet

Does the model have any shortcomings? If so, what are they?

Section CWhat we will say to explain how our detection method works.

Section DHow we will check that students have understood what we taught them.

Key Stage 4 – Planet detector

Detecting exoplanets: the transit method

When an object passes in front of a light source, part of the light is blocked. We notice this when a cloud moves past the Sun, or a person walks in front of a light.

In June 2012 Venus passed in front of the Sun. All over the world people observed this, the transit of Venus.

www.oxfordsparks.net/planet

Scientists use the Kepler space telescope to observe other stars. The telescope orbits the Sun, trailing behind Earth. It continuously monitors the brightness of 100,000 stars.

Sometimes the telescope detects a dip in the brightness of the light from a star. This could be because the star has a planet. When the planet passes in front of, or transits, the star it blocks a tiny fraction of light from that star.

If the brightness dips regularly scientists can be more confident that the star has a planet. The planet blocks light from the star every time its orbit takes it between the star and the telescope.

The change in brightness tells us about the planet size. The time between transits tells us its orbital time and allows us to estimate its temperature. http://kepler.nasa.gov/Mission/QuickGuide/howKeplerFindsPlanets/ www.astronomynotes.com/solfluf/s12.htm (see the animation)

Key Stage 4 – Planet detector

Detecting exoplanets: the wobble method

The light emitted by every star is different. You can see these differences when you look at a star through a spectrometer, which splits light into its separate colours. Every star has a different pattern of black lines, called absorption lines.

These show the frequencies of light absorbed by the elements in the star.

As a planet orbits its star, gravitational forces pull the two objects towards each other. The star also moves in a tiny orbit around its centre of mass. You can imagine this as a parent holding hands with their child, and spinning the child around.

When the planet moves in its orbit away from us, the star moves towards the observer. This compresses its light waves slightly, so the absorption lines move towards the blue end of the spectrum. This is blueshifting.

When the planet moves in front of the star, the star moves away from us. The light waves from the star spread out, so the absorption lines move towards the red end of the spectrum. This is redshifting.

This is an example of the Doppler effect, which also changes how an observer hears the pitch of sound from an ambulance as it approaches and passes.

www.astronomynotes.com/solfluf/s12.htm (see the animation)http://planetquest.jpl.nasa.gov/page/methods

Key Stage 4 – Planet detector

Detecting exoplanets: direct imaging

Telescope technology has improved greatly. Today, the Hubble Space telescope searches directly for planets around nearby stars.

www.oxfordsparks.net/planet

Credit: NASA

Credit: NASA

The telescope detects light reflected by an exoplanet from its star. But the reflected light is difficult to see next to the dazzling brightness of the star.

Astronomers use various techniques to overcome this problem: Coronography uses a masking device attached to the telescope to block out

light from the star. This makes objects around the star easier to see. In space the masking device could be in the form of a starshade which shades

telescope mirrors from the light of a star, rather like a giant beach umbrella.

Digitally reducing the amount of light appearing in an image from a star.

Direct imaging techniques work best for big, hot Jupiter-like planets, and for rogue planets like sol-i that float freely

through space instead of orbiting a star.

This image shows light reflected from three planets orbiting a star 120 light-years away. The star is called HR8799. It was taken by the Hale Telescope using a coronagraph to block out light.

http://planetquest.jpl.nasa.gov/page/methods

Image: NASA/JPL-Caltech/Palomar Observatory

www.oxfordsparks.net/planet