The early solar system was a crowded place. Unfathomable numbers of small particles jostled with one another in the dusty disk that surrounded the nascent Sun. Collisions among these particles gradually built larger objects, and eventually planets formed. Every one of these collisions produced heat, so objects in our solar system were born hot. The rocky planets grew even warmer when heavy elements sank to form their cores, releasing gravitational energy in the process.Astronomy magazine subscribers can read the full column and view the movies for free. Just make sure you're registered with the website.