Caption: 
Southwest Research Institute scientists propose a new model for the formation of compact exoplanetary systems that contain multiple rocky planets in tight orbits around their star. In this model, planets begin to form in regions of a disk around a young star that are fed by an ongoing infall of gas and small grains. Growing planets collect rocky material while gradually spiraling inward through interactions with surrounding gas. As a planet gains mass, its inward migration accelerates. This process yields a compact planetary system with a planets-to-star mass ratio consistent with observed compact exoplanetary systems.