Jupiter became the solar system's biggest planet by consuming its chief rival, a massive rocky planet ten times bigger than Earth. New discoveries suggest Jupiter and Saturn learned a lesson from their mythological namesakes, "eating" any planet that opposed them.
Both Jupiter and Saturn began life as rocky planets that were at least a few times more massive than Earth, which would make them so-called "Super-Earths." Their greater size made them big enough to trap the nebula gas that swirled around them, creating the huge atmospheres that made them the gas giants we know today.
For that model to work, Jupiter and Saturn should have rocky interiors that are roughly the same size, but recent measurements revealed that wasn't the case. Jupiter's core is only about two to ten times the mass of Earth, while Saturn is much bigger, maybe 15 to 30 times the mass of our planet. There's only one possible explanation - but it paints a grim picture of just how violent and cutthroat the ancient solar system really was.
If another Super-Earth planet smashed into Jupiter, the gas giant's immense atmosphere would have flattened the rocky intruder, then sent it hurtling towards the core shortly afterward. The collision between the flattened Super-Earth and Jupiter's core would have vaporized both the planet and much of Jupiter's core, kicking up lots of heavy elements into the gaseous atmosphere.
This model would explain pretty much everything we know about Jupiter. It explains why it's so much more massive than the rest of the gas giants, why its atmosphere has a greater mix of heavy elements than that of the Sun, and why its core is so small.
Saturn, on the other hand, perfected a more gentle method of adding to its atmosphere. Instead of taking on its heavy elements in one massive collision, it instead gobbled up a bunch of smaller objects that were less massive than Earth. (For mythology buffs, it should be pointed out that there is currently no evidence that Saturn ate five such objects, then tried and failed to eat Jupiter, which it then vomited up. But I'm not seeing any evidence against it, either.)
These objects would have had much the same effect as Jupiter's Super-Earth, throwing an unusual amount of heavy elements into the atmosphere. But crucially, they wouldn't have damaged Saturn's core and might even have added to its mass, which would explain why its core is now so much bigger than that of Jupiter.