Theories of formation of close-in exo-planets

Arguably the greatest surprise in exoplanetology has been the sheer number of close-in planets detected by missions as Kepler. These planets tend to be larger than Earth, predominately terrestrial by composition, but with significant envelopes of hydrogen/helium. They are further seen in "packed", multiplanetary configurations, quite unlike that of the solar system. How did they form? I will review the leading theories of in situ formation and distant formation, followed by planet migration. These theories give different predictions for the composition, and architecture of the close-in planetary systems. I will then discuss alternative formation scenarios, such as the pebble accretion paradigm, where planet growth is fueled by a steady supply of small particles, and the hypothesis that planets start forming at special locations.