How Round are the Planets?

based on 6 ratings
Author: Barry Eitel

Grade Level:

High School


Astronomy, Physics, Geometry


This experiment examines if space objects become round after reaching a certain mass.

Research Questions:

-How spherical are the planets in the solar system?

-Does mass affect shape in space?


Have you ever wondered why all the planets are round, and how they got that way? Knowing how objects react in space is very important to astronomers as they discover new parts of the universe. If there is a correlation between mass and shape, astronomers could get an idea of an unknown object’s weight just by looking at it.


  • Resource listing volumes and surface areas of planets and other objects in space (moons, comets, asteroids, etc.)
  • Photos of each planet and object
  • Calculator
  • Spreadsheet program

Experimental Procedure:

  1. Look at the pictures of all the space objects. On a scale of 1-10, rate how round each of the objects are.
  2. Mathematically, sphericity is the ratio of the surface area of a perfect sphere with the same volume as the object and the surface area of an object, and can be expressed as S = As/Ap. The surface area (As) of a sphere with the same volume of an object, can be calculated as π^⅓(6V)^⅔.
  3. Using a listing of the volumes and surface areas of the space objects, calculate sphericity with the equation.
  4. All of the results should be between 0 and 1, with 1 being a perfect sphere. Note down the differing sphericities.
  5. Compare the mathematical results with your subjective ratings. How close were you?
  6. Compare the sphericity results with the mass of the objects.
  7. Analyze and graph your data. Does there seem to be a correlation between mass and sphericity (does mass become greater/smaller as sphericity moves to 1)? If there is a correlation, is there an equation that can be derived to explain this connection?

Concepts: weight, mass, planets, sphericity

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