What fate befell Uranus and Neptune compared to Jupiter and Saturn regarding gas accretion?

Answer

They became "failed cores" because the gas supply vanished before they reached the critical mass.

The dissipation of the primordial gas, which acts as a deadline for gas accretion, meant that Uranus and Neptune's cores did not grow large enough rapidly enough to capture the surrounding hydrogen and helium envelopes.

What fate befell Uranus and Neptune compared to Jupiter and Saturn regarding gas accretion?
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