Extrasolar Planets: Finding What We Can't See
Lesson 16 of 16
Objective: Students use a simulator to determine how astronomers find extra-solar planets.
In Going Full Circle on Gravity - Day 1 and Day 2, students apply the centripetal force equation and Newton's Universal Law of Gravity to derive the formulas for circular orbits. In this activity, students apply their knowledge of orbits and Newton's Laws of Motion to construct an explanation as to how scientists are able to discover hundreds of planets outside our solar system without ever seeing them. Students use a PhET gravity simulator to observe the motion of planets around a central star. With careful observation, one can see that the star moves in response to the pull of the orbiting planet. Armed with this knowledge, students then read about the different techniques astronomers use to detect extra-solar planets.
Since students are constructing an argument as to why scientists know about exoplanets, NGSS Science Practice 7: Engaging in argument from evidence is applied. This activity is in the context of NGSS Space Systems HS-ESS1-4, where students use computational representations to predict the motion of orbiting objects in a solar system. Science Practice 2: Developing and using models is also applicable here. Students also have to read about and present to the class specific methods of how exoplanets are found. This incorporates NGSS Science Practice 8: Obtaining, evaluating, and communicating information.
When class starts, displayed on the whiteboard is an internet page from NASA called Planet Quest. It displays a count of how many extra-solar planets (a.k.a. exoplanets) have been discovered. I define what exoplanets are for the students: planets that circle other stars, far away from our solar system.
The first exoplanet was discovered in 1992 and then refer to the number that is on the PlanetQuest website. Here is the shocking thing... we can't see these planets! Even with all of our technology and with the best telescopes in the world, we cannot see these planets.
I then take out a piece of paper with a small black dot in the center. The dot is just barely visible to the students in the back of the room. I then say to the class: trying to see an exoplanet is like trying to see the dot, with me walking across the room, down the hallway, getting into my car and driving 10 miles down the road. Even if the line of sight between us is clear, that black dot would be impossible to see, even with the world's most power telescope. This is for two reasons: 1) The planet is tiny compared to the distance it is away from us and 2) The planet doesn't emit light, it only reflects a tiny bit of light from its companion star.
Today, I tell the students, you are to explain how scientist and astronomer claim to have discovered hundreds of exoplanets without being able to see them!
I hand out the Gravity Simulator worksheet and every student gets his or her own sheet. Students work together in groups of 3 with one computer per group. Also a benefit of working in groups is students can discuss possible answers with their peers. Though the observations are easy to make, constructing the explanations to the questions is a challenge. The worksheet instructs the student to navigate their web browser to a PhET simulator called My Solar System. This is a great resource to explore and is demonstrated here with the Phet Video Demo.
Students open up and run the PhET simulator which defaults to a single star and planet. As the simulator runs, because the planet is 5% the mass of the star, the star wobbles back and forth as the planet orbits. This is the key observation that I want students to see. They should first notice that information and they use it to answer questions on the worksheet such as:
- Using the formula for orbital period, how can scientist then make a good guess about a planets distance from its star?
- Astronomers can also estimate the mass of the planet orbiting the star. How do you think they do that?
The answer to the first question is in the period formula for orbital motion. The period is measured based on the rate of the wobble and the stars mass can be estimated based on its brightness and color (this information is given to students on the worksheet). Solve this equation for the radius.
The answer to the second question is that by measuring the amount of the starts wobble and applying Newton's Third Law, you can estimate the mass of the planet.
Time permitting students can move to some of the more advanced questions such as: All the stars of our galaxy (and many others) are orbiting the galactic center. Does there need to be a central mass that all those stars orbit? Explain using examples from the simulator. This question has a scaffold with students observing binary star motion where there is no central mass. The next activity, students read about the specific methods used to discover exoplanets.
Once students have completed the worksheet the groups are to read about and present to the class specific techniques used to discover exoplanets. NASA provides a good interactive for this. Students navigate to the "Interactives" section of the NASA Planet Quest and open up the "5 ways to find a planet" activity. As a group they choose one of the ways to find a planet and inform me so that I can cross that off the list so there is no overlap. Other groups must choose a different "way". I also require students to use at least one other reputable source (e.g. www.NASA.gov, or www.ESA.int or it could be a news source such as NYTimes or The Washington Post) to explain their way to the rest of the class.
To present, each group uses the Planet Quest interactive activity and explains what they learned on how we discover exoplanets. Given they only had 10-15 minutes to prepare, it is a casual presentation, with all the team members standing at the front of the room. It is good practice for students to stand in front and talk. Everyone in the group is expected to say something and they are told this as they prepare. But I don't grade this presentation, it is simply exposing students to being in front of their peers, talking about something without the high stakes of a grade involved.
For homework, students learn about the search for exoplanets that could have liquid water as scientist believe this is required for life. I hand out the Goldilocks Homework sheet. Students read the article and write a 3-2-1 on it: 3 things they have learned, 2 things they found interesting and 1 question it brings up. This assignment is collected and graded next class period.