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A. The Parent Star 
B. The Exoplanet 
The mass of the exoplanet can be determined from an estimate of its density. Plotting the densities of the planets in our solar system as a function of their radii may be informative.
A. Model #1
A loglog plot of the densities and radii of the planets in our solar system produces the graph at right. The line indicates an approximate best fit. However, many of the planets do not fit the curve well. The average density of an object is defined by: 
To use the data from the graph, locate the radius of this exoplanet on the graph and read its associated density from the curve. Enter this density in the formula below.
(Data for Earth: density = 5500 kg/m^{3}, mass = 5.98 X 10^{24}kg, radius = 6.378 X 10^{6}m)
mass of exoplanet = density * (4/3 * pi * radius^{3}) 
B. Model #2
A straight forward plot of the densities of the planets in our solar system with the average distance they are from the Sun produces the graph at right. The line indicates an approximate best fit. As before, not all of the planets fit well on this curve. To use this data locate the semimajor axis of this exoplanet on the graph and read its associated density from the curve. Enter this density in the formula below to calculate a second estimate of this exoplanet's mass. 
mass of exoplanet = density * (4/3 * pi * radius^{3}) 
Notes on Estimating the Mass of the Exoplanet
The models used here to estimate the density of a planet are very simplistic. It is assumed that the density depends only upon one variable (either the size, i.e., radisu, of the planet or its distance from the Sun. Even for our solar system, this cannot be the whole story, as it can be seen from the rough attempts to draw a bestfit line to the plots above.
A more complete model for the density of a planet should at least include:
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Simulation Authors: Richard L. Bowman (Bridgewater College) and David Koch (Kepler Mission)
Maintained by: Richard L. Bowman rbowman@bridgewater.edu (200204; last updated: 19Apr04)