Not Galactus, but Jupiter, says New Scientist. The king of planets got to be so big because it gorged itself on super-Earths sometime in its early past:
New simulations by Shu Lin Li of Peking University in China, and colleagues, may explain why. They calculated what would happen when a super-Earth of 10 times the mass of our planet slammed into a gas giant. The rocky body flattened like a pancake when it hit the gas giant’s atmosphere, then barrelled into the giant’s core about half an hour later. The energy of the collision could have vaporised much of the core.
These vaporised heavy elements would then have mixed with the hydrogen and helium of the gas giant’s atmosphere, leaving only a fraction of the gas giant’s former core behind. This could explain not only why Jupiter’s core is so small, but also why its atmosphere is richer in heavy elements compared with the sun, whose composition is thought to mirror that of the nebula that gave birth to the solar system’s planets (arxiv.org/abs/1007.4722).
The super-Earth might have grown into a gas giant itself one day if it had not collided with Jupiter, says study co-author Douglas Lin at the University of California, Santa Cruz. “It may very well have been on its way to becoming a gas giant, but lost the race and got gobbled up,” he says.
Y’know, it was Saturn who ate Jupiter and the other gods in the old myths. I think the planets have been messing with us, man. They got us all turned around.