Detecting Exoplanets by the Transit Method © Center for Astronomy Education University of Arizona 2014

The Kepler Orrery From the Caltech IPAC Exoplanet Archive:

The Kepler Telescope Image by NASA: spacecraft-cross-section.html

The Transit Method When a star is orbited by an exoplanet, and the exoplanet moves between us and the star, some of the star’s light will be blocked, resulting in a decrease in the observed brightness of the star. Telescopes (such as the Kepler telescope) can measure this decrease in observed brightness.

How It Looks If you could zoom way in and look at a star being transited by a planet:

Step By Step Think about how the light from the star is affected at each point.

The Important Graph Astronomers observe the brightness of a star over a period of time and make a graph of the observed brightness vs. time. Changes in the brightness of the star on this graph indicate the presence of an exoplanet orbiting the star.

How many of the below graphs correctly show how a star’s observed brightness will change over time if it is orbited by an exoplanet? A.Only one B.Two C.Three D.All four Brightness Time Brightness Time Brightness Time Brightness Time

Real Light Curves Data from the Faulkes Telescope Project:

Planet Features There are two important properties of the planet that we can find by this method: The size of the exoplanet The time it takes the exoplanet to complete an orbit around its parent star

Dip Features Dip Depth – how much light is blocked – Deeper dip = larger planet Dip Width – how long the transit takes – Wider dip = longer orbital period Dip Sides – these are sloped! Dip Bottom – this is flat.

Orbit Times The time from the beginning of one dip to the beginning of the next dip is the time it takes the planet to complete one orbit. The time between dips is longer than the duration of the dips themselves.

Kepler’s Third Law If two planets are orbiting the same star, the planet that is further away moves slower. The planet that is further away takes longer to complete an orbit.

Repeating Dips If there is a single pattern of repeating dips that are all the same size and shape, with the same time between dips, then these dips are most likely caused by a single exoplanet.

The graph below shows how the observed brightness of a star changes over time because it is orbited by an exoplanet. Which location (A-D) in the exoplanet’s orbit corresponds to the time indicated by “X” on the graph? 3 12 Brightness Time (months) x A planet’s orbit star to Earth B D C

A graph of brightness vs. time for a star orbited by an extrasolar planet is given below. Which property of the dips is NOT affected by the time it takes the planet to complete an orbit around its star? A. The spacing between dips B. The depth of the dips C. The slopes of the sides of the dips D. The widths of the bottoms of the dips

When a star is orbited by multiple exoplanets, you can determine how many exoplanets there are from the pattern of dips in the graph of the star’s observed brightness vs. time. This pattern of dips also allows you to determine the orbital periods and relative sizes of the planets from the graph. Each exoplanet causes a unique repeating series of dips in the graph. Brightness Time Multiple Exoplanets

Multiple Exoplanets – No Overlap

Multiple Exoplanets – Overlapping

Overlapping, Step-by-Step When two exoplanets transit their star at the same time, how will their overlapping dips appear on the graph of the star’s observed brightness vs. time?

Multiple Exoplanets – Overlapping dips

Lecture Tutorial – Detecting Exoplanets with the Transit Method (handout) Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don’t understand what the Lecture Tutorial is asking, ask one of us for help.

Debrief Let’s address any questions you have about the Lecture-Tutorial.

The graph below shows how the brightness of a star changes over time due to the fact that it is orbited by an extrasolar planet. How does the planet’s orbital period around its star compare to the orbital period of the Earth around the Sun? A. It is greater B. It is less C. They are equal D. Not enough information 3 12 Brightness Time (months)

You observe two identical stars (Star A and Star B), that are the same distance from Earth. Each graph below shows how the observed brightness of Star A or Star B changes over time. Which star is orbited by the larger exoplanet? A.Star A is orbited by a larger exoplanet. B.Star B is orbited by a larger exoplanet. C.Both extrasolar planets are the same size. D.There is not enough information to tell Brightness Time (months) Brightness Time (months) Star A Star B

You observe two identical stars (Star A and Star B), that are the same distance from Earth. Each graph below shows how the observed brightness of Star A or Star B changes over time. Which star has the exoplanet that orbits at a larger distance from its parent star? A. Star A’s exoplanet orbits at a larger distance. B. Star B’s exoplanet orbits at a larger distance. C. Both exoplanets orbit at the same distance. D. There is not enough information to tell Brightness Time (months) Brightness Time (months) Star A Star B

Star A has a small planet orbiting at a larger distance, and Star B has a large planet orbiting at a smaller distance. How should the graphs change to be correct? A.The time between dips in Graph A should be less than in Graph B. B.The sides of the dips in Graph A should be vertical. C.The dips in Graph A should be wider than in Graph B. D.The dips in Graph A should be deeper than in Graph B Brightness Time (months) Brightness Time (months) Star A Star B

The observed brightness vs. time for a star is represented by the graph below. Which of the following statements is true? A.The star is orbited by two planets with different sizes and different orbital periods. B.The star is orbited by two planets with the same size but different orbital periods. C.The star is orbited by two planets with different sizes and the same orbital period. D.The star is orbited by three planets with different sizes and different orbital periods.

If the graph below represents the brightness of our Sun as seen by an alien astronomer, which pair of bodies in our Solar System could produce the observed pattern of dips? A.Jupiter and Neptune B.Earth and Mars C.Neptune and Pluto D.Mars and Jupiter

Kepler Is Still Hunting Image by NASA: #.U9__3fldUnA

Thank You NASA! The development of these materials was funded through the generous contributions of NASA's Exoplanet Exploration Program.