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How to Measure the Density of a Planet

The physical density of any object is simply its mass divided by its volume; density is measured in units such as pounds per cubic foot, grams per cubic centimeter or kilograms per cubic meter. When calculating the density of a planet, look up its mass and radius, the latter of which is the distance from the surface to the center. Because planets are roughly spherical, calculate the volume of a sphere using the radius. Then divide the mass by the volume of the sphere to get the density.

Things You'll Need

Scientific Calculator

Instructions

- 1
  Find the planet̵7;s mass and diameter. For example, Earth̵7;s mass is about 6,000,000,000,000,000,000,000,000 kg and its radius measures 6,300 km.
- 2
  Enter the radius in the calculator. Multiply by 1,000 to convert kilometers to meters. Cube this number by pressing the "x^3" key; alternatively, you can press the "x^y" key, enter the number three and then press "equals." Multiply by the number pi -- or 3.1416 -- multiply by four and then divide by three. Store the result by pressing the "M+" or other memory key. The figure you see is the planet̵7;s volume in cubic meters. To continue the example, 6,300 km times 1,000 meters/km = 6,300,000 meters. Cubing it gives 250,000,000,000,000,000,000. Multiplying by pi times 4/3 yields 1,047,400,000,000,000,000,000 cubic meters.
- 3
  Key the planet̵7;s mass into the calculator. Press the divide key, then recall the volume figure stored in the calculator̵7;s memory. Press the equals key. This result is the planet̵7;s density in units of kilograms per cubic meter. In our example, dividing 6,000,000,000,000,000,000,000,000 kg by 1,047,400,000,000,000,000,000 cubic meters results in a density of about 5,730 kg per cubic meter.

Astronomy