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Mobile Planetarium Shows

Free Lesson Plans

LP7 R1 – Exoplanets
  1. Radial velocity method

How it works

This method is based on the idea that when a planet orbits its star, the planet’s gravitational pull makes the star move in a small circle rather than stay still. We can detect those small movements, or wobbles, by observing the light coming from a star.

**additional information for the kids, that are already familiar with the Doppler effect: we can detect the wobbles by looking at the star’s spectrum (we know what kind of spectrum to expect from a stationary star): if the spectrum shifts towards blue, that means the star is moving towards us, if the spectrums shifts towards red, that means that the star is moving away from us. If the red-to-blue-and-back shifts are periodical, that means the star is moving in a circle. Bingo! We found a planet!

Essential points

  • The first exoplanets were detected using the radial velocity
  • According to NASA, 64 exoplanets (that is about 18.4% of all exoplanet detections) have been discovered with this method.
  • This method works best with low-mass stars as they are more easily disturbed by the planet’s gravity and also rotate slower, meaning that the shifts of spectrum are more easily detected.
  • The method works best if the planet’s orbital plane “edge – on”, meaning it is parallel to the line of sight. Then we can detect the forward – backward motion. If the orbital plane is perpendicular to the line of sight, or “face – on”, then no forward-backward motion

can be detected.

  • The method doesn’t tell us much about the planet, only the minimal mass.
  • The method can detect planets up to to 100 light years away from the Earth.
  1. Transit photometry

Method name decoding

“Transit” means passage, the word ”photometry” comes from greek words “photo” (meaning light) and “metrio” (meaning measure).

How it works

When the planet passes in front of its star, it blocks a tiny bit of light coming from the star. And we can observe the dips in the star’s brightness!

There are lots of stars that appear to change their brightness when we observe them from Earth, we call them variable stars. Not any change of brightness is interesting to us, but only a periodic change, when the dip repeats at a certain intervals (because the planet orbits its stars periodically). And unfortunately there are many reasons for the star to change its brightness in a pattern. For example:

  1. Some stars pulsate on a periodic basis, they swell and contract due to a different processes going on inside.
  2. Some stars appear to change their brightness because there is in fact not one star but the system of two stars orbiting around the invisible point in between the two. As one star periodically passes in front of another (as seen from Earth), we observe periodical changes in brightness.
  3. And only a small portion of the detected dips is related to the transit of the planet!

 

Essential points

  • The method is responsible for 77.9% (2771 exoplanets according to NASA) of the known exoplanet detections.
  • With this method scientist can scan big areas of the sky ( with the radial velocity method thy have to explore one object at a time).
  • The method can detect planets that are much further away.
  • The percent of the events that the method mistakenly sees as a planet transit is very high so the follow-up observations are always necessary to confirm the discovery.
  • The orbit of the planet should be “edge – on” as seen by the observer from the Earth.
  • With this method we learn the size of the planet directly.

 

  1. Gravitational microlensing

Gravitational microlensing is another way to find a planet outside the Solar System.

The magic behind the method is this: gravity can bend light!

How it works

For the method to work, we need two stars, the star we study should be at the front, another star should be behind it at the background. Imagine that the light from a distant star moves towards the Earth. As it pases by our study star, it gets bent by the star’s gravity. As a result, we get to see two images of a background star instead of one!

 

Now let’s imagine there is a planet orbiting the foreground star. The planet’s gravity will also bend the light of the distant star. Just a bit.

As a result, an observer on Earth should see not two, but three images on the background star. Though the third image will be less bright than the other two.

In practice, though, the images appear so close together that it is impossible to see them as three different pictures of a star. Instead, the background star will appear to look brighter and the planet’s input to such temporary brightening will be easily detectable.

 

 

Essential points

  • Microlensing can find planets situated 1000s of light years away, near the galactic center (so often it is impossible to do the follow-up observations with other methods, as they are more shortsighted).
  • This method is more sensitive to finding smaller planets orbiting further away from their star, so the typical planet that can be found with this method will be about as heavy as Neptune.
  • For this method to work, an alignment of the two stars should happen, meaning the event will not repeat itself. Moreover, such events only happen rarely.
  • According to NASA, 51 planet have been discovered using this method.

 

  1. Direct imaging

How it works

As the name suggests, in this method we try to get a direct image of an exoplanet. Easier said, than done, as normally the dim and relatively small planet will be lost in the glare of its big and bright companion star. To solve this problem coronagraphs can be used. Coronagraphs are disks that can be placed in front of a telescope (ground-based or space telescope) to “hide” the brightest central part of a star.

They are so called because the first such attachments were used to study the corona, the outer layer of the Sun, that is otherwise lost in the light of the very bright central layers of the Sun.

To tell them apart, we now use the terms Solar coronagraphs and Stellar coronagraphs.

 

 

Essential points

  • This method works best for detecting big bright planets that orbit far away from the parent star
  • This method works best when the planet’s orbital plane is “face on” to the observer
  • According to NASA, 44 exoplanets have been detected using direct imaging

 

 

 

 

LP7 – Exoplanets

Lesson 7:

Exoplanets

Aims:

  1. To learn what exoplanets* are.
  2. To find out what methods are being used to search for exoplanets. To compare the methods effectiveness.
  3. To learn about TRAPPIST and Kepler telescopes.
  4. To “design” an exoplanet.

Age group: The lesson can be adapted for the different age groups from Y6 all through the secondary school. The technical details of the methods described in the lesson should be fully accessible to the students KS4+.

Time: 60 minutes +

Materials:

  1. Information sheet 1: How to search for exoplanets
  2. Information sheet 2: Glossary
  3. Video Radial velocity method, Transit photometry method, Direct imaging method, Gravitational microlensing method
  4. TRAPPIST online resources: TRAPPIST official page , TRAPPIST Wikipedia page, TRAPPIST – 1 Wikipedia page , NASA catalog of exoplanets
  5. Kepler online resources: Kepler official page , Kepler Wikipedia, NASA catalog of exoplanets

 

Introduction:

In this lesson, the pupils learn what an exoplanet is, they find out what methods scientists use to detect exoplanets, how these methods work and what types of planets each method can be used to find. They learn about some of the ongoing exoplanet search projects, including TRAPPIST, responsible for the discovery of seven Earth-like planets orbiting one star. They discuss why the search for exoplanets is an important part of the modern space exploration, what kind of planets we are particularly interested in and why. Then the children write down the new concepts they’ve learned during the lesson. At the end of the lesson they design an exoplanet by choosing the type of a star, the planet’s distance from the star, planet’s composition etc and try to determine what kind of life could exist on their made-up planet. If there is time, the children do a drawing/ painting of their planetary system and its inhabitants.

 

Procedure:

  1. Exoplanet
  • Ask the children what an exoplanet is, listen to their suggestions and write down the correct answer on the whiteboard. Ask the children if they think  planets near other stars actually exist. Explain, that for hundreds of years people were wondering whether the Solar System was unique and finally in early 90s astronomers got the prove that it was not, as they discovered planets orbiting other stars!

 

  • Discuss with the children why it is difficult to look for exoplanets in space and what kind of planet is the easiest to spot (Looking for exoplanets is no easy task as we try to look far into space and spot something small and dim, a planet, next to something big and bright,a star. One cannot just point the telescope to a star and find a planet next to it, the planet will be simply lost in the star’s glare, To overcome this problem scientist can either “dim” the image of the star before taking a picture or figure out what sort of “symptoms” will indicate the presence of the planet next to the star and then check the stars for such symptoms. Naturally, at first we were only finding big planets, but as the technology and data analysis methods improved, we started finding Earth-sized planets, or Super Earths, and finally even those smaller than the Earth!
  1. How to search for exoplanets
  • Tell the children, that now they are going to learn how scientists can detect an

          exoplanet in Space.

  • Give the information sheet (1) to the children. They can study it individually or in groups, depending on the class. Allow 10 minutes to complete the task.
  • When they finish, watch the video (3), one method at a time. Pause after each video to discuss the method with the class, revising the information (1) they’ve just learned. Ask questions (What kind of planet are we likely to find if we use this method: small planet / big one? orbiting close to the star / far away? How would its orbit be oriented relatively to our line of sight? etc).
  • After figuring out the advantages and disadvantages of each method, summarize, that none of the methods is perfect and astronomers should always be prepared to compromise on something. Ask the children to imagine themselves being scientists in charge of building a telescope for an exoplanet search. They can only build one instrument, as telescopes are extremely expensive and their construction is time consuming. What kind of instrument would they choose? Would it be a ground based telescope, less expensive to build and maintain but bounded by the Earth’s atmosphere? Or a space telescope? What method  would be used on the telescope? And why?
  1. Why search for exoplanets
  • Now that the children know many different (and complicated!) ways to search for exoplanets, it is time to think about why look for planets near other stars in the first place! Ask the children for their suggestions and write down the best ones on the whiteboard (our mobile planetarium visitors often come up with these brilliant ideas : 1. To find another Earth-like planet where we could move in the future if we are short on resources or before the Sun is about to die,  2. To find a planet suitable for life where aliens could live, 3. To help scientists learn more about Space and how the star systems are formed.  All three are good starting point for the further discussion!).

 

  1. Ongoing exoplanet search projects

               Tell the children, that now they are going to look at the two ongoing exoplanet  

          search projects that are actively discovering planets outside our Solar System  

          right now!

  • Divide the class into two teams: “ground team” and “space team”.
  • Explain, that each team is going to independently research about one exoplanet hunt project. “Ground team” will be learning about TRAPPIST ground – based telescope. “Space team” will be learning about Kepler space telescope. The teams can use the online resources (4) and (5), such as official telescope’s pages, wikipedia and NASA exoplanet database.
  • Make a list of “facts to look up”, that will help the teams with their research. The facts may include the basic information about the telescope, the detection method being used, the number of  planets that were discovered with the telescope, an example of an interesting exoplanet/ planetary system discovered. Allow the teams up to 20 minutes for their research and an extra 10 minutes to prepare the presentation (they can chose the format).
  • Ask the teams to share the prepared information with the rest of the class allowing for a little discussion and questions following each team’s presentation.
  1. Glossary
  • Ask the children to think of any new concepts/ words they have learned during the lesson.
  • Add the new words to the Glossary (2).
  1. Design an exoplanet

               Time for some creative work!

  • Ask the class to create an imaginary exoplanet. The work can be done individually or the whole class can take part in creating one “classroom exoplanet”.
  • Ask the children to decide on the type of a star, the type of a planet, the planet’s distance to the star, how fast it orbits/ spins around the star, whether it has any moons etc.
  • Ask the children to think of what kind of living things could possibly exist on their made-up planet(s).
  • Ask the children to draw or paint this made – up world and its inhabitants.
  • Praise the children for their hard work during the lesson.

 

     End of the lesson!

LP7 – Glossary

Exoplanet – a planet orbiting a star other than the Sun.

 

Hot Jupiter – a Gas Giant exoplanet, similar to Jupiter. Hot Jupiters orbit close to their parent star, therefore they have a very high surface temperature and short orbital period (less than 10 days).

 

Light Year – measure of distance in Astronomy. It is equal to the distance that light travels in one year, or 9.5 000 000 000 000 km.

 

Orbital plane (of a planet) –  a plane that contains the orbit of a planet around its star.

 

Radial velocity – how fast an object moves along the line of sight of the stationary observer.

Super Earth – an exoplanet with the mass between 1 and 10 of that of the Earth. Heavier planets are called Mini – Neptunes. The term does not specify the composition, proximity to the star, presence of the atmosphere etc.

Useful websites / further reading

 

www.nasa.gov -The offical NASA website

 

www.esa.int/ESA – European Space Agency

 

www.bbc.co.uk – BBC science of space

 

www.bbc.co.uk/educationKS3 Physics space topic

 

www.bbc.co.uk/education – BBC KS2 Space education resources

 

www.bbc.co.uk/education – BBC KS1 Earth and Space resources

 

www.gov.uk – UK Space Agency

 

https://principia.org.uk – Tim Peaks Journey to the ISS

 

www.kidsastronomy.com – Videos and games about the solar system

 

www.universetoday.com – Up to date news on astronomy

 

www.space.com – Great site on space exploration

 

http://en.roscosmos.ru – ROSCOSMOS is a Russian Space State Corporation

 

www.skyandtelescope.com – Full information guide to astronomy

 

space-facts.comSpace facts out of this world

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