In most of the models I have read up on, the farther out from the centre of the solar "dust cloud", the longer it will take to form a planet. Recently I came across the claim that Neptune and Uranus are just too far out to have formed according to these models, even over the 4.5-billion-year age given to the solar system.
The following quotes were given:
‘What is clear is that simple banging together of planetesimals to construct planets takes too long in this remote outer part of the solar system. The time needed exceeds the age of the solar system. We see Uranus and Neptune, but the modest requirement that these planets exist has not been met by this model.’
Taylor, S.R., Destiny or Chance: our solar system and its place in the cosmos, Cambridge University Press, Cambridge, p. 73, 1998.
‘There have been many attempts to model the evolution of a swarm of colliding planetesimals … Safronov calculated the characteristic time-scales for planetary growth. In the terrestrial region he found timescales of 10xE7 (10,000,000) years but the time estimates increased rapidly in the outer regions of the solar system and was 10xE10 (10,000,000,000)years for Neptune—which is twice the age of the solar system.
It is clear that, in view of the large timescales found for the formation of the outer planets, a satisfactory theoretical model for the accretion of planets from diffuse material is not available at present.’
Dormand, J.R. and Woolfson, M.M., The Origin of the solar system: the capture theory, Ellis Horwood Ltd, W. Sussex, p. 39, 1989.
I was wondering about the apparent non-existance of these two planets and what others thought?