What is the difference between a star and a planet, as in why does one form instead of the other?
Mass. The difference is simply how much matter coalesced to form that particular body. The largest planet in our solar system is of course Jupiter by a long way. However the smallest star is about 80 times heavier. To be considered a star it has to be hot enough to sustain hydrogen fusion. The heat comes from the matter coalescing in the first place and it is directly related to the mass of the body. Below 80 Jupiter masses the temperature never gets high enough to allow hydrogen fusion to occur.
