Largely by reading.

Posted on: October 6, 2016 at 11:17:18 CT

Astronomers and cosmologists have investigated planet creation for a long time. We've observed star formation in different stages throughout the universe and the swirling gases slowly spin towards each other. As they get closer, gravitational forces take over and begin to attract more material. Not all of it will make it into the star, but instead cool and float on its own. This material will collide with itself, and when enough of those happen, there is enough accretion to form larger formations, and once those are the size of a mountain, it has it's own (albeit weak) gravitational field. From there, if there's enough material floating around, it's just a matter of time.

Hope this helps.