Although there are significant outstanding issues, our understanding of the formation of the Sun's planetary system is well developed. In particular, the «planetesimal accretion model», where most of the solids in the protoplanetary disk were in solid bodies of roughly 1 km or larger, offers a foundation for understanding the formation of the terrestrial planets, the outer planets, and the populations of small bodies seen in the present day Solar System.
However, planetary systems come in a variety of architectures, with a large fraction falling into a class known as Systems with Tightly-spaced Inner Planets (STIPS). Such configurations lend themselves to discovering details of the planets such as their mass, radii, and composition. Hence these systems may provide the best constraints on the hypothesis that the formation theory of our own Solar System is applicable to other planetary systems.
