Measuring an exoplanet transit

This activity, like Agent Exoplanet, allows you to use sample data to measure the change in brightness of a star as a planet passes in front of it. In this activity you will use SalsaJ, a free amateur photometry software program, to measure the brightness of the star by hand. You will then use a spreadsheet to plot your results as a graph and see if you can see your light curve.

 

Download SalsaJ from the European Hands On Universe Website.

Launch SalsaJ and download our lightcure plotter spreadsheet (you will need a spreadsheet application such as Excel, OpenOffice Spreadsheet or Numbers).

Download and unzip sample data for one of the extrasolar planets below:

Use the screencast below to learn how to use SalsaJ and a spreadsheet to make a light curve for a transiting exoplanet:

Below are the main steps demonstrated by the screencast:

  1. Open the first FITS file in SalsaJ
  2. Change the brightness as needed to view the image
  3. Open the finder chart (a .jpg file in the folder with the sample transit data) and compare to figure out which star is the one with a transit
  4. In SalsaJ got to Analyze -> Photometry Settings and enter your chosen star and sky radii
  5. Enter the radii in your spreadsheet (you might need them later in case you have to redo any part of this)
  6. Go to your spreadsheet and enter the name of the FITS file (or some unique part of it) in the Image Identifier column
  7. In the FITS image, go to Image -> Show Info and look for the UTSTART time. Enter this time in your spreadsheet.
  8. Got to Analyze -> Photometry, then click on the star with a transit. Enter the intensity value in your spreadsheet under Target Intensity.
  9. Choose another bright star in the image (you might even want to do several separately and add extra columns to your spreadsheet for this) and click on it. Enter the intensity value in your spreasheet under Comparison Intensity.
  10. The spreadsheet divides the Target Intensity by the Comparison Intensity to adjust for any atmospheric brightening or dimming, and plots a point on the graph.
  11. Repeat steps 6-9 for each of the remaining transit images. (You will have to adjust the brightness for each one, but the photometry radii will stay the same)
  12. You may need to double click on the Intensity axis of your spreadsheet and change the scale in order to see the light curve.

You will end up with a spreadsheet looking something like this:

The dip in the graph shows how the planet was temporarily blocking the light from the star and decreasing the observed brightness as it orbited in front of the star.

Activity time: 
45 minutes - 1 hour
Objectives: 

After doing this activity you should be able to:

  • Use SalsaJ to do photometry for an exoplanet transit
  • Use a spreadsheet to create a light curve showing how the brightness of the target star changes over time 
  • Describe how the light curve shows that as the planet goes in front of the star it blocks some of the light
Planning: 

Background articles that are relevant to this project include:

You may also want to try Agent Exoplanet.