Okay, not a planet in reality. A simulation. Still this is very fun (kinda ironic the simulation engine is phun) you can make planets with gravity and sometimes moons.

You will need

-A computer
-Some time

Step 1: Download Phun

First you need to download the simulation engine, Phun. You can do so by following this link.

http://www.algodoo.com/wiki/Download

Go to the bottom of the page and select the Phun 5.28 installer respective to your system. Then run the installer. After installed find the target path of the installer and run phun.exe.

Step 2: Setup

In phun go to the File and select New. Then open the options menu and select Advanced mode. Also under the Simulation tab uncheck Gravity.

Step 3: Making the Planet.

Now zoom in (by scrolling) until the scale bar measures 1 meter. Then use the circle tool to make a circle about the same diameter as the scale bar. Then select the circle, then select script menu on the menu that comes on the left. Set attraction to 100. Then clone the object. Then clone the group of objects. And after you have a couple circles press the space bar to run the simulation. Once they accumulate into a bunch press the space bar to pause and repeat until you have a decent sized planet. (Usually when it forms a circular shape on its own.) If you have many circles but the planet isn't circular. Use the pen tool and select all of the circles, select materials, and slide attraction down to 0 and back to 100.

Step 4: Orbit and Rotation

Use the drag tool and drag a part of your planet (preferably in the center) the way you want your planet to rotate. Orbits are much harder but can be accomplished with patience. Use the drag tool and drag a circle in a straight line. Keep trying until you finally get the circle caught in the planets gravity while having equal centrifugal force so that the circle orbits the planet.