Planet formation

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 an atypical case and that categories of planets that do not exist in our system are common elsewhere (e.g., the super-Earthplanets).

Source : BROKEN-LINK:Challenges in Planet Formation LINK-BROKEN Journal of Geophysical Research : Planets , 2016

Problems include :

• Dust grains orbiting newborn stars should spiral into those stars (due to gravity) rather than accreting to form planets.
  
• Notwithstanding the above, the currently understood laws of physics suggest that formation would take far longer than it has.
  
• There's no agreed theory to account for how the energy of angular momentum of the oribiting dust would somehow be lost as the planets coalesce.
  

Further reading : RSR's List of Problems with Solar System Formation

Also see : Galaxy formationplugin-autotooltip__plain plugin-autotooltip_bigGalaxy formation

There are currently two (mutually exclusive) theories regarding the formation of galaxies, which according to current models, all have a supermassive Black Hole at their centre.

In brief, the theories are that :

1) The galaxy formed around a pre-existing Black Hole which was sufficiently massive as to attract all the nearby stars.