Original: Assuming a spherically symmetrical planet ( a reasonable approximation ) , the strength of this field at any given point is proportional to the planetary body 's mass and inversely proportional to the square of the distance from the center of the body .
Simplified: Assuming a perfectly round planet , the strength of this field at any location is proportional to the mass of the planet and inversely proportional to the square of the distance from the center planet 's mass .