The idea that Uranus and Neptune formed at their present orbital radii by the accretion of planetesimals has some serious difficulties. One of these is that:
A)these planets contain abnormally large amounts of hydrogen and helium, suggesting they were formed further out in the solar system.
B)we do not expect planetesimals to exist that far out in the early solar system.
C)evidence from other stars suggests that solar nebulae do not last long enough to form a massive planet at that distance by the accretion of planetesimals.
D)planetesimals at this distance would be expected to have far more iron than we have postulated for the composition of Uranus and Neptune.

Question

Quizplus · Accepted Answer

Evidence from other stars suggests that solar nebulae do not last long enough to form a massive planet at Uranus and Neptune's distance by the accretion of planetesimals. This suggests that another mechanism, such as gravitational instability or migration, may have been responsible for their formation.

The Idea That Uranus and Neptune Formed at Their Present