  The existence of Uranus and Neptune presents severe difficulties for the core
accretion model for the formation of ice giant planets. We suggest an
alternative mechanism, namely disk instability leading to the formation of gas
giant protoplanets, coagulation and settling of dust grains to form ice/rock
cores at their centers, and photoevaporation of their gaseous envelopes by a
nearby OB star, as a possible means of forming ice giant planets.
