“The origin of planets is a vast, complex, and still quite mysterious subject. Despite decades of space exploration, ground-based observations, and detailed analyses of meteorites and cometary grains (the only space samples available in our laboratories), it is still not clear how the planets of the solar system formed. The discovery of extrasolar planets has added confusion to the problem, bringing to light evidence that planetary systems are very diverse, that our solar system is not atypical case and that categories of planets that do not exist in our system are common elsewhere (e.g., the super-Earthplanets).
Source: Challenges in Planet Formation Journal of Geophysical Research : Planets , 2016
Problems include :
• Dust grains orbiting newborn stars should spiral into those stars rather than accrete to form planets.
• The currently understood laws of physics suggest that formation would take far longer than it has.
• There's no agreed account for how the energy of angular momentum would somehow be lost as the planets coalesce.
Further reading : RSR's List of Problems with Solar System Formation