To find the average density of a planet, you can either derive your own values for mass and volume by combining equations or simply retrieve the values from a chart. Combining equations correctly is a common practice in intermediate and advanced physics and astrophysics. It is often a good indication of a student's mastery of the subject matter, too.
Things You'll Need
 Graphing calculator with accurate values for pi

Find the mass of the planet for which you want to calculate density by looking at a reputable mass chart (see Resources). Or, plug in values for the radius of the planet's orbit ("r"), the period or time it takes for one orbit ("T"), and the gravitational constant ("G") into the equation (4pi^2r^3)/(GT^2). If you have the velocity of the planet in orbit ("v"), plug the values into the equation rv^2*G.

Find the volume of the planet by consulting a reputable volume chart or using the planet's radius, which is equal to half of the diameter, in the equation (4/3)pir^3.

Divide the mass by volume to determine the average density of the planet.
Tips & Warnings
 Be sure to check your units on the final answer, as this often indicates whether you have used the equations and figures correctly. Correctly squaring, canceling and multiplying your figures will result in an answer containing a unit appropriate to density.
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