How to Measure the Density of a Planet
While you can measure directly the size and mass of a planet, you must determine density through calculations. The density of any object is the ratio of its mass to its volume. For planets, this means that the largest planets, the gas giants of Jupiter, Saturn, Uranus and Neptune, are not necessarily the densest planets. The small and rocky planets like Earth are actually significantly denser than these larger planets. The densest planet is, in fact, the Earth.
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Instructions
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Find the mass of the planet. Astronomers calculate planetary masses by extrapolation from the gravitational forces between the planet and some other body, such as a moon or the sun. You can calculate the mass using the equation M = F x r-squared / G x m (F = gravitational force, r = the radius of the orbit, G = the gravitational constant and m = the mass of the other object), or you can find the mass on a table.
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Find the planet's diameter. Using the parallax method (the shift in apparent position when you view an object from two different locations), astronomers determine the distance from the Earth to another planet. They can observe the angular size of the planet with a telescope. With these numbers, they find the diameter of the planet using the equation: diameter = angle x distance.
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Calculate the volume of the planet. You can assume that all planets are spheres in shape, so the volume is found with the equation V = 4 x 3.14 x r-cubed, where V is the volume and r is the radius (the radius equals one-half of the diameter).
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Use the equation of density = mass / volume to find the density of the planet.
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Tips & Warnings
Double-check your units when you calculate density. Most scientific data is given in metric units, typically kilograms for mass and meters for distance.
It is very difficult to find the distance to a planet using the parallax method, and it requires multiple observations throughout a year or at different locations. For accuracy, your best choice is to use the numbers already determined.
