Purpose: This activity, designed for in-class or virtual learning, encourages students to research properties and detection methods of planets outside our solar system and to use fictitious data to graph and calculate the length and depth of transit, the planet’s radius and its distance from its host star.

Procedural overview: After reading “This is the first picture of a sunlike star with multiple exoplanets,” students will research exoplanets, what they are and how they are discovered. Then students will divide into groups or work individually in class to plot a light curve of a transiting exoplanet from fictitious data, to gather information about the exoplanet and its movement in its solar system. A version of this article also appears in the August 29, 2020 issue of Science News, with the headline “A weird solar system cousin makes it photographic debut.”

Approximate class time: One hour

Supplies:

Computer with internet access

Virtual space for remote discussions and data sharing

Graph paper and pencils (optional)

Calculator

The Hunt for Other Worlds student guide

Detecting Exoplanets answer key for teachers

Directions for teachers:

In this activity, students will learn about exoplanets, how they have been discovered and what scientists have learned about them. Students will use fictitious data to plot a light curve of a transiting exoplanet and then use their plots to gather information about the exoplanet and its movement in its solar system.

Want to make it a virtual lesson? Discussions can be conducted via Zoom, Skype or other suitable chat programs, while data can be shared via e-mail or through Google Docs, Sheets or Slides. In the interest of time, students should do their research on exoplanets and answer the related questions as homework before completing their worksheets in class.

For a possible extension, students could use the data, their light curves and their calculations to help create models of the exoplanet. Students could explain which properties of the exoplanet and exoplanet-star system they cannot model using the information provided, what other information they would need to find those properties and how to find that information (such as using a different method to observe the exoplanet). If time allows, students could present their models to the class.

Background

By the autumn of 2020, there were about 4,300 confirmed extrasolar planets, or “exoplanets,” discovered in around 3,200 planetary systems around other stars. More than 5,400 additional candidate exoplanets have been proposed by NASA programs alone. Many other groups around the world are also searching for and proposing candidate exoplanets.

Most of the known exoplanets are large gas giants, ranging from about the size of Neptune to several times the size of Jupiter. Scientists have found a smaller number of “super-Earths,” which are planets that are thought to be larger than Earth and to have a combination of terrestrial and gaseous conditions. A handful of smaller, rocky planets also have been discovered that are similar in size to Earth or smaller. As of September 2020, no extraterrestrial life has been found on an exoplanet.

About 98.8 percent of confirmed exoplanets have been discovered using four methods. The transit method looks for a slight dip in the amount of light coming from the star, which indicates that a body is crossing between the star and Earth. The radial velocity method looks for a wobble in the star’s motion directly toward and away from Earth, which indicates another celestial object is tugging on the star. Microlensing looks for a change in the light coming from a distant star when it is behind a nearer star, which bends light like a lens and makes the distant star appear brighter. If the lens star has an exoplanet, the distant star appears even brighter, a hint that an exoplanet is present. Planets can also be directly imaged, but this can be very difficult because the amount of light reflected by the planet is typically very small compared with the star it is orbiting. Direct imaging can be performed after the star’s light has been blocked or by thermal imaging.

A few other methods that have led to exoplanet discoveries include astrometry, transit-timing variations, eclipse-timing variations and pulsar timing.

More detailed information can be found in resources such as:

NASA Exoplanet Exploration, including 5 Ways to Find a Planet

What are exoplanets? from EarthSky

How to Search for Exoplanets, from The Planetary Society

Homework reading and questions for students

Before class, students will read about exoplanets and discovery methods using the following or similar resources:

NASA Exoplanet Exploration, including 5 Ways to Find a Planet

What are exoplanets? from EarthSky

Your Guide to Exoplanets, from The Planetary Society

The questions below will help the students complete their research on exoplanets and understand what they are seeing in the data and their graph.

Note that the student sample answers provided will be based on information current as of September 2020; the number of known exoplanets is growing quickly, and common methods of detection may change with technological advancements. Please use current data and information when grading student answers.

1. What is an exoplanet?

An exoplanet is a planet that is in orbit around another star outside our solar system.

2. What was the first exoplanet to be discovered? When and how was it discovered?

The first confirmed exoplanet discovery was in 1992 and consisted of two terrestrial planets orbiting pulsar PSR B1257+12. They were discovered by timing the pulses of radiation emitted by the pulsar.  The 2019 Nobel Prize in physics was awarded for the 1995 “discovery of an exoplanet orbiting a solar-type star.” The name of that exoplanet is 51 Pegasi b.

3. Describe the four main methods of exoplanet detection.

Radial velocity: The wavelengths of light from the star change as the star moves slightly toward us (blueshift) and slightly away from us (redshift).

Transit: The amount of light visible from a star drops slightly as the planet crosses in front and blocks some of the light that reaches Earth.

Direct imaging: Host stars are much brighter than the exoplanets that go around them. To directly image an orbiting exoplanet, astronomers use techniques to block the star’s light so the planet can be seen.  Exoplanets have been imaged directly in the visible light part of the electromagnetic spectrum and in the infrared part of the electromagnetic spectrum.

Microlensing: Changes in the light from a far distant star behind an exoplanet’s host star can indicate the presence of an exoplanet.

4. To date, approximately how many exoplanets have been discovered? You may notice different numbers such as “confirmed” or “candidates.” What is the difference between them?

Answers should reflect current values — values will change with time as more exoplanets are discovered. As of September 2020, there were about 4,300 confirmed exoplanets and about 5,400 candidate exoplanets. Confirmed exoplanets mean that the data have been verified and do show an exoplanet. The data of candidate exoplanets show an anomaly that may indicate an exoplanet, but it has not yet been verified.

5. About how many have been discovered by each detection method?

Answers should reflect current values — values will change with time as more exoplanets are discovered. In September 2020, about 3,200 of the confirmed exoplanets had been discovered using the transit method. About 800 exoplanets were discovered using the radial velocity method. About 50 exoplanets were discovered using direct imaging. About 100 exoplanets were discovered using microlensing. The rest were discovered by other methods, including astrometry, transit-timing variations and eclipse-timing variations.

6. What is the approximate range in mass of confirmed exoplanets?

Answers should reflect current values — values will change with time as more exoplanets are discovered. As of September 2020, exoplanets range from 0.02 times Earth’s mass to more than 8 times Jupiter’s mass.

7. What are the nearest and farthest discovered exoplanets? What method(s) were used in their discovery?

Answers should reflect current values — answers may change with time as more exoplanets are discovered. As of September 2020, the nearest known exoplanet is Proxima Centauri b, which was discovered with the radial velocity method and is about 4 light-years away. The farthest known exoplanet is SWEEPS-4b, which was discovered with the transit method and is about 27,700 light-years away.

8. What is the habitable zone around a star? Is it the same for each star?

It is the range of orbits around a star where liquid water could exist on the surface of a planet, and the planet could potentially have conditions suitable for life. Each star has a different habitable zone based on its temperature.

9. Have any exoplanets been found in the habitable zone of their host stars?

Yes, exoplanets have been discovered that are within their stars’ habitable zones.

10. During class you will act like an astronomer who has gathered data while studying a potential exoplanet. You will use this data to create a graph showing how the amount of light measured changes over time. Which value will go on the x-axis and which will go on the y-axis? How do you know?

Time goes on the x-axis because it is the independent variable: It is consistent and unchanged by experimentation. The amount of light goes on the y-axis because it is the dependent variable: It will change over time depending on circumstances.

Group discussion about exoplanets

Use the following questions to lead your students as a class or in small groups through a brief discussion on what they learned in their pre-class readings. Again, note that the student sample answers provided will be based on information current as of September 2020. The number of known exoplanets is growing quickly, and common methods of detection may change over time, so be sure to use current data and information when grading student answers.

1. Which of these detection methods has been the most successful for discovering exoplanets? Why do you think that is?

The transit method has found most known exoplanets. This is probably the most successful method because a survey can watch the same patch of sky for long periods of time and is able to catch the slight changes in brightness when the exoplanet transits its star.

2. In addition to studying the target stars that might host an exoplanet, astronomers also use several nearby stars to gather comparison data. Why do they do this?

Astronomers observe the brightness of several stars near the target star to correct for any potential fluctuations in the data, such as from atmospheric disturbances or changes in sky brightness.

3. Most of the exoplanets discovered early on were very large, including some that are several times the mass and radius of Jupiter. Why do you think the larger exoplanets were discovered first?

The larger exoplanets were easier to spot because they caused greater disturbances visible from Earth. They could have blocked more of the light from their host stars as they transited, or they caused their host stars to have larger wobbles than smaller planets, due to greater gravitational pulls.

Detecting exoplanets

Ask students alone or in small groups to pretend to be astronomers who have collected and processed the fictitious data found on their Hunt for Other Worlds student guide, which they will use to create a light curve of a transiting exoplanet orbiting the star GSC 00522-01199 in the Delphinus constellation. Students will then analyze their light curves and use the provided data to answer the questions in the Detecting Exoplanets worksheet, found in their student guide. For worksheet questions and answers for teachers, please refer to the Detecting Exoplanets answer key.

Possible extension

A possible extension to this activity would be to have students create a model of the exoplanet’s star system. They should use the data, their light curves and their calculations to create their models. They should also explain which properties of the exoplanet and exoplanet star system they cannot model using the information provided. Students should include other information they will need to find those additional properties (such as using a different method to observe the exoplanet). If time allows, students can present their models to the class.