Abstract
The formation history of giant planets remains one of the most significant open questions in planetary science. I propose a new evolutionary pathway where runaway gas accretion is delayed until a core reaches approximately 100 MEarth . Under this framework, the transition to a gas-dominated composition occurs only at masses comparable to Saturn’s. This delay is likely driven by an intermediate stage of efficient heavy-element accretion, which provides enough energy to suppress rapid gas contraction. A key implication is that Saturn never reached the runaway phase, characterizing it instead as a 'failed giant planet.' This model naturally explains the divergent bulk metallicities and internal structures of Jupiter and Saturn, as well as the distinct natures of Uranus and Neptune. Furthermore, shifting the threshold for runaway accretion aligns with observed transitions in exoplanetary mass-radius relations and explains the high metallicities of intermediate-mass exoplanets.