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Measuring an exoplanet

Created: 2024-10-24

Author(s):
Anastasia Kokori (UCL, CSED Center for Space Exochemistry Data)

Exoplanets are really far away! How do we manage to detect them and get information about their nature? Particularly, how do we know how big or small they are? In this activity you will use real data of a telescope to measure the size of an exoplanet, just as astronomers do.
Note: This practical activity introduces students to the field of exoplanets. It can also be performed as an introduction to the ExoClock project, where people from all over the world observe exoplanet transits to support ESA's Ariel space mission. To learn more, visit: www.exoclock.space

Materials

As part of the attached material you will find:

A presentation, to be used in the classroom during the activity (Presentation.ppt and Presentatioon.pdf). Please note that there are notes on each slide;
A handbook for the teacher (TeacherHandbook.pdf);
A spreadsheet with the light curve of WASP-10b, to be printed out and distributed (lightcurve.pdf);
A questionnaire, to be distributed before the activity (Survey_before.pdf);
A questionnaire, to be distributed after the activity (Survey_after.pdf).

You will also need:

A computer (for the presentation)
A projector (for the presentation)
Rulers
Pencils
A printed version of the spreadsheet with the exoplanet light curve (see attachments: lightcurve.pdf)

astroedu-measuring-exoplanet-light_curve

Image 1: the spreadsheet to be distributed.

Goals

Exoplanets are a fascinating topic, through which students can:

Better understand the distances in our Universe and appreciate the Earth’s position in the cosmos (our only home)
Learn about the different types of planets (gas and rocky planets)
Understand the limitations of modern technology and methods (we can only observe certain details of exoplanets and therefore can’t get all the information about them)

Learning Objectives

Students will

learn how astronomers detect exoplanets through the transit method
learn how to apply the mathematical formula to measure the size of the exoplanet
familiarise themselves with the analysis of exoplanet data
get introduced to statistics and error propagation

Background

Extrasolar Planets

An extrasolar planet, or exoplanet, is a planet outside the Solar System, orbiting a star other than the Sun. Exoplanets are very far from Earth, and they do not shine their own light but only reflect the light from the star. One can only very rarely distinguish the planets from the star in one image, yet scientists have understood how they may prove the existence of an exoplanet.

Light curve and transit method

A light curve is a graph that shows the brightness of an object over some time. In the transit method, astronomers measure the drop in the brightness of the star as the planet passes in front of it. While the planet transits the star, we receive less light than before. This light drop is detectable by telescopes. The larger the planet, the more light it blocks, and the more significant the dip. Larger planets are easier to detect than smaller ones.

A-light-curve-showing-the-transit-method-of-deteting-exoplanets

Image 2: the light curve of an exoplanet orbiting around its star. Credits: NASA Ames

NOTE: the teacher should read the power point presentation (there are notes on each slide) and familiarise themselves with the topic of exoplanets. You can also read the complimentary pdf document for additional background .

Full Description

Before the activity

Print the light curve spreadsheets (one for each group).

Prepare the classroom for projection: we suggest using the power point presentation throughout the activity. Be aware that under each slide there is a description of what we suggest the teacher should say.

Divide students into teams of 2 to 5 members (depending on the number of students).

Introduction to exoplanets

Introduce the topic of exoplanets by using the power point presentation (slides 1-4).

Image 3: the activity in classroom at 2 different moments. Credits: Anastasia Kokori

Introduction to light curves and the transit method

Introduce the transit method: astronomers use this method to discover exoplanets. Explain that the images from telescopes are all from the host star and not the planet directly (slides 5-6). Explain that from these images, astronomers trace a light curve, a graph that shows the brightness of the star over time. In the transit method, astronomers measure the drop in the brightness of the star as the planet passes in front of it. While the planet transits the star, we receive less light than before.
You can show examples of different exoplanets and different light curves (slides 7- 8).
You can then show the equipment that has been used (from slides 9-12) and explain how we use telescopes and cameras for these measurements. In this way, students can get an example of the astronomical equipment required for an exoplanet transit observation.
Explain that each data point they see in the light curve represents the brightness of the star in a single image (slide 13). Students can get an idea of the high number of images needed to get the final light curve of the exoplanet.
Show the light curve in slide 14 and explain that this is an exoplanet light curve that represents a real scientific measurement. This particular one is from exoplanet WASP-10b and the observation has been performed with a small telescope (11 inches) and a camera.
Starting from slide 15, you can distribute one copy of the printed spreadsheet to each group and tell the students that they will now discover how to measure the size of the planet using this light curve.
Point out to the students that in the light curve of the spreadsheet, single observations are not a point but a vertical line (slide 17). This is due to the fact that all real measurements have some uncertainty. Uncertainty means how much confidence we have in our measured values. There is no infinitely precise measurement, as the instruments that take the measurements are limited. Uncertainties arise from several factors, such as the instrument itself, the Earth’s atmosphere moving in front of the telescope, or the star itself.

Measuring the size of the exoplanet

Explain that we will now measure the size of the exoplanet from the depth of the pit of the light curve: the deeper the curve, the larger the planet (slides 15-17).
Give the following scientific task to the students: ask them to measure with the ruler the length and the depth of the exoplanet transit from WASP-10b as shown in slides 20 and 21.

Image 4: measuring the pit of the light curve. Credits: Anastasia Kokori

Ask the students to use the mathematical formula shown on slide 18 to calculate the depth of the exoplanet transit and so find out the size. See slide 22 for an example of this calculation. Note that the radius of the host star WASP-10 is RS =550.000 km
Using the formula with their measurements, students should find a result of a Radius of about 92000km for WASP-10b. Ask each team to share their results, and write on the board the different measurements by each team.

Discussing the results

It is likely that the groups will have slightly different measurements: discuss with them why. Explain that to decrease the uncertainties, additional observations are needed. See slides 23 and 24 for two different results.
Discuss with the students the nature of exoplanet WASP-10b in comparison with other planets of our Solar System (slide 25), and explain that WASP-10b is a giant planet.
The activity can end by sharing some extra facts about this planet as shown in the next slides.
Slide 28 also highlights the important use of small telescopes, as this light curve was taken by using a small telescope as the image shows. Small telescopes can have significant contributions to space telescopes (this can be another lesson/ task).
OPTIONAL: you can ask the students to look up on the internet for some more facts about WASP-10b and share them in the class.

Image 5: Comparison of best-fit size of the exoplanet WASP-10 b with the Solar System planet Jupiter, as reported in the Open Exoplanet Catalogue (2015-11-14). Credits: Aldaron.

Evaluation

The teacher can distribute the attached questionnaires before and after the activity (attached documents: survey before and survey after).

Curriculum

The activity can be linked with mathematics and physics

Additional Information

This practical activity introduces students to the field of exoplanets. It can also be performed as an introduction to the ExoClock project, where people from all over the world observe exoplanet transits to support ESA's Ariel space mission. To learn more, visit: www.exoclock.space

The author welcomes teachers or students who have used the activity to get in touch for questions and feedback (filled questionnaires will be very useful). Please contact Anastasia Kokori for any feedback: anastasia.kokori[@]gmail.com