Size The size of a planet and star in which it orbits is very important to finding life. Assuming the planet in which we are speaking is orbiting a star similar to the sun, then it's will have to be in a habitable zone of a specific size, this size is from .725 to 3.0 au (1 au is distance earth is from sun) which is 108,458,456,000 m to 448,793,612,000 m respectively. Lets just assume this theoretical planet we are creating is in the middle of the habitable zone: 1.8625 au=278,626,034,000 m Sidereal Orbital Period: Time it takes a planet to make one complete orbit around sun with a non rotating reference To calculate this period we use the equation:   Where µ is GM and "G" is the Gravitational constant (6.67384E-11) and "M" is the mass of the object you are orbiting (sun=1.989E+30). and "a" is the distance from the sun (assuming perfectly circular orbit) This results in T=8.02E+7 seconds or 928 earth days (24hrs) for each rotation around the sun Now lets calculate the size of the planet: Say we want to achieve a force of gravity on an object at the surface of .9 earths gravity. That would be 8.83 m/s^2 We can then use the equation To calculate to find what our new planet' mass and radius should be. Mass=DV or density times volume, assuming this planet has similar density to earth (5540 kg/m^3) then we plug in D(4/3)πr^3 for M and get r=6,264 km just under the size of earth. This radius can then tell us the mass of this planet by finding its volume v=(4/3)πr^3 so v=1.029E+21 and mass=5.71E+24 kg And that is how you make a planet in terms of physics! background courtesy  hdw.eweb4.com Index: Introduction Habitable Zone Size Moon Home Bibliography . .