The Kepler-discovered Systems with Tightly-packed Inner Planets (STIPs), typically with several planets of Earth to super-Earth masses on well-aligned, sub-AU orbits may host the most common type of planets, including habitable planets, in the Galaxy. They pose a great challenge for planet formation theories, which fall into two broad classes: (1) formation further out followed by inward migration; (2) formation in situ, in the very inner regions of the protoplanetary disk.
The actual categories are such:
1. Rocky, gaseous, plasmatic bodies as mutually exclusive, and only plasmatic ones evolving.
2. Rocky, gaseous, plasmatic bodies as not mutually exclusive, with plasmatic and gaseous ones evolving towards becoming rocky.
They pose a great challenge to planet formation theories... well duh!!