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Planet Orbiting a Star

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Summary of Current Information
A. The Parent Star
B. The Exoplanet

IV. Calculating the Size of the Exoplanet

A careful calibration and analysis of the original photometric data has shown that the current exoplanet caused a drop in light intensity from the parent star. With this data in hand, the size of the exoplanet can be calculated. Since all planets are large enough for gravity to pull them into a spherical shape, we will simply calculate the radius of the exoplanet as a measure of its size.

The ratio of the light being blocked by the transit of the exoplanet to the total light usually reaching the photometer is equal to ratio of the cross-sectional areas of the exoplanet and the parent star. Since:
  (area of a circle) = pi * radius2
the percentage drop in light from the star as the exoplanet transits the star is simply equal to the ratio of the squares of the radii of the exoplanet and the star.

On the last page, the radii of stars of different spectral types were given. Thus the radius of the present star is already entered in the form below.

(Note: 1 solar radius = 109 Earth radii.)

  drop in light output = (exoplanet radius)2 / (star radius)2
solar radii * 109)2
(Press the calculate button to estimate the radius of the exoplanet.)

Press the "Next Page" button to continue your analysis.


Notes on Estimating the Radius of the Exoplanet

As usual, there are some assumptions built into our calculations. For example, the chance that the exoplanet is moving straight across the center of the star as measured from the Kepler spacecraft is not very good. However, an excellent starting assumption is that the parent star radiates uniformly from its whole disk. Thus wherever the exoplanet makes a transit across the star we assume we see the same percentage reduction in light coming from the star. This leads us to the formula used above.

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Simulation Authors: Richard L. Bowman (Bridgewater College) and David Koch (Kepler Mission)

Maintained by: Richard L. Bowman rbowman@bridgewater.edu (2002-04; last updated: 19-Apr-04)