Who would have guessed that from simply measuring brightness changes in a star, we could find not only whether the star has a planet, but a host of interesting properties of that planet as well. Even whether or not the planet might be inhabitable by life forms!

Using SalsaJ image processing software and telescopic images of stars, we can make measurements of star brightness, then plot a light curve (graph of brightness vs time) for a planet transit and analyze it to find out those interesting properties hadn't dreamed of knowing before before 1995.

This Investigation has five parts:

1. Plot a Transit Light Curve
2. Examine the Light Curve
3. Find the Planet's Size
4. Find the Distance of the Planet from its Star
5. Is the Planet Habitable?

Materials

• An image processing app such as 
• JS9 (online app) - https://js9.si.edu
• JS9-HOU (online app) - https://hou-js9plugins.educontinuum.org
• SalsaJ available from European HOU site
• Images — four possible sets:
• • 20 images of  HD189733 (included in SalsaJ 2.3 software as of 2016, from EUHOU Exercise "Discover an Exoplanet: The Transit Method") 
    
  • 34 images of  TrES-3
  • 19 images of star SAO 107623 (also called HD 209458)
  • 102 images of GJ436 from Bareket Observatory (Israel)
    

These are  observations of four different stars (HD189733, TrES-3, HD209458, GJ436) each containing a transit event—the dimming of the star as a planet passes in front of it. 

First let’s have another look at graph E of the light curves near the end of chapter 6.

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I. Plot a Transit Light Curve

In order for a planet transit (and corresponding dimming of the star's light) to be observed:

• The planet’s orbital plane must be in line with our view of the star (as with eclipsing binary stars). Geometrically, less than 2% of exoplanets would satisfy this condition. (See illustration of Geometry for Transit Probability on the NASA Kepler About Transits page.)
  
• The planet must be large enough for us to detect a drop in brightness.  Earth based observations can detect a drop of 1% from a transit of a Jupiter sized planet.

Each image is really 20 or so images “stacked” on one another so that “noise” in the resulting image is kept to a minimum.

Comments about each image set:

• HD189733 (20 images) - The exoplanet orbiting the star HD 189733 was discovered on October 5, 2005 by transit method in France. It was the very first star for which planet transits were observed.  The planet had already been discovered by the spectroscopic method.
  The planet is known as HD189733b.  Planets around a star are designated by a lower case letter of the alphabet, with the star taking the letter “a” and each orbiting body taking a letter in the order of their discovery.   HD189733b was the first planet discovered around star HD189733.
  The images in this set were taken with the NASA Spitzer satellite. We list this set first because a macro is automatically included with SalsaJ software that can Open all 20 images and tile them. (Images are also available by downloading this file:  20 image_spitzer 4.77 Mb . 
  
  
• TrES-3 - The TrES-3 images were provided by Todd Klaus who worked in the Kepler Science Operations Center as Lead Software Engineer at NASA Ames Research Center in Mountain View CA. The telescope used was at the Zen Observatory.
  
  
• HD209458 (19 images) - As is common HD209458 has multiple names for the same star, another being SAO 107623. This is part of the Astronomical Image Set for A Changing Cosmos, the Hands-On Universe high school course, part of Global Systems Science.
  
  
• GJ436 - The 102 GJ436 images were provided by Bareket Observatory. This set, being so large, lends itself to teams of students dividing the task into smaller batches of images and pooling the results.

For any given set of images, use the following procedure (like the Finding Supernova investigation—Chapter 6).

1. Find the time of each image from the Image Header Info. Find the difference, in minutes, between the observation time and the time of the first image (i.e. 10/20/2001, 3:06 UT for HD209458).
   
   
2. Identify the correct star on the image. Use your judgment or a finder map. For all the image sets except for one (HD 209458), there is a special labelled star finder image with reference stars marked. For HD 209458 (SAO 107623), it's the star that is the brightest. Take some Counts measurements using the Aperture tool to make sure you've found the brightest star.
   
   
3. Find reference star(s). The bright star about 45° to the upper right of HD 209458 (x = 564, y = 266) can be a reference star. Even better reference stars are at (x = 185, y = 181) or (x = 311, y = 276).

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II. Examine the Light Curve

DURATION

TRANSIT DEPTH

Transit Depth is the dip in the light curve.  This is the drop in brightness of the star as a planet passes in front of it.

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III. Find the Planet’s Size

The transit depth (TD) is related to the size of the planet in a very simple way: the area of light blocked when the planet transits is exactly the area of the apparent disk of the planet. So, the ratio of area of planet disk to star disk should directly determine the drop in brightness.  

    TD = (area planet)/(area star)
    Since area = πr ² ,
    TD     = (πr_planet ²)/(πr_star ² )
    TD    = (r_planet /r_star )²

Size Matters The size of the planet gives us crucial information about its possible habitability. It’s a little like the Goldilocks story. If the planet is too small (like Mercury or Mars), it will not have enough gravity to hold on to an atmosphere—gas molecules will escape the planet over a time-span of not many years in the lifetime of the planet-star system. If the planet is too large, it will retain a huge amount of atmosphere and have crushing atmospheric pressure, like the giant planets Jupiter and Saturn.