Heavy Metal Jupiters
It is thought that gas giant planets like Jupiter and Saturn nucleated around much smaller "metal" (that is, anything heavier than helium) cores that gravitationally accreted gas from a surrounding disk.​ The critical core mass to initiate runaway gas accretion from the disk is estimated at several times the mass of the Earth.
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However, recently several giant exo-planets were detected with much higher metallicities, containing up to a hundred times the mass of the Earth in metals. One way to form these heavy-metal Jupiters is by successive planetary mergers during runaway gas accretion.
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See Ginzburg & Chiang (2020) for details.
Mass fraction of heavy elements (metals) Z compared to the Sun as a function of the planet's mass M. Planetary mergers can explain metal cores that are dozens of times the mass of the Earth.