Extrasolar Planets and their Detection

By Alexandria Janopoulos

Extrasolar planets, or planets that orbit stars other than our Sun, are found using various indirect methods. It is harder to find a planet than to find how that planet interacts with its star. For example, if you are looking for a fly on a headlight, you only see the headlight; however, if you study the headlight closely you could see a drop in brightness if the fly is interacting with the headlight. One popular method to use on finding extrasolar planets is the transit method.

The transit method is based on the observation of a star’s small drop in brightness, which occurs when the orbit of one of the star’s planets passes, or transits, in front of the star.  It determines the size, or radius, of the planet. When the planet orbits around its star, scientists can measure the reduced brightness of the star and determine the size of the planet. It determines the size of the planet because scientists know the brightness of the star. If the extrasolar planet was the size of the star you would see little or no light , and if the extrasolar planet was smaller and only took up a portion you would see more light, but not as much as before. The Kepler Space Telescope is completely devoted to using the transit method to find extrasolar planets. So, how do scientists know they are not seeing space junk in place of seeing an extrasolar planet? They know this by making sure the drop in brightness is there every orbital period, or period it takes an extrasolar planet to orbit its star.

This method can be used to isolate certain characteristics of the extrasolar planet. If we can combine detection methods, we can find out more information on the planet. Using the transit method and other detection methods together we can move forward in learning as much as we can about the extrasolar planet.

The transit method is described in the figure above.  It shows how the extrasolar planet crossing, or transiting, the star. The graph shown under the figure, shows you how the placement of the planet effects the brightness shown from the star. The y- axis, or brightness, shows the amount of light emitted by the star, and the x-axis, or time, shows how that light emitted is affected over the time the star is being observed.


1.     In the figure, planet phase 1 does not affect the brightness. Why does this happen?

2.     When would the light curve change and go back to the original brightness?

3.     Challenging Question: Referring back to the figure on the transit method, when the planet goes behind the star how would the light curve change? (Hint: If the planet was a candle and the star was a spotlight, what would happen?)


4 thoughts on “Extrasolar Planets and their Detection

  1. Wow, I was so excited to finally see this on the blog! This is very technical and very informational and I loved it! Great job! 🙂

  2. I really enjoyed reading this, and i agree with the comment above. This is very informational. Also, i thought the graph showing the placement of the planet was a good touch. 🙂

  3. Pretty cool to think that there are so many more planets to be discovered. Are there any other telescopes that use another system besides the Transit method to look for planets?

    • Check out my Radial Velocity post to see another extrasolar planet detection method. I’m not sure about the telescopes.

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