Jupiter may have been struck by a huge protoplanet long ago, spreading the heavy material inside the planet's core across a much wider area inside the planet than we'd typically expect to find. That's the current theory of researchers examining data beamed back by the NASA probe Juno, which is designed to take measurements of Jupiter's gravitational field to intuit the structure and distribution of material inside the gas giant.
Measurements of Jupiter back in 2017 revealed that the planet's core is large and poorly defined. What we'd typically expect -- and this is true for both rocky planets and gas giants -- is layers of increasing density, with the densest material packed at the center of the planet, where temperatures are hottest. Instead, Jupiter's core is irregular and potentially diluted, meaning the heavy elements within it have spread out and are now distributed through a much larger proportion of the total planet.
Scientists have been working to explain this finding(Opens in a new window) for the past few years and they've come up with a model that fits both the data and what we know about the early days of the solar system. The Nice model and Grand Tack hypothesis both posit that Jupiter essentially 'swept' the solar system in the early days of its existence. Interactions with Jupiter and Saturn cleared the asteroid belt of most of the remaining planetesimals, particularly Jupiter. These shifts in orbits and orbital eccentricity may have caused the Late Heavy Bombardment or even the creation of Earth's Moon.
What the research team proposes is that Jupiter was struck by a massive protoplanet during this hectic early period -- something about eight times the mass of Earth, surrounded by about two masses worth of hydrogen and helium. Jupiter weighs about 318x what Earth does, so being hit by this kind of mass contributes a remarkably small amount of total energy. What the impact does do, however, is scramble Jupiter's core -- diffusing the elements within it and spreading them out over a much larger area.
Interestingly, this research suggests the impact would have to be head-on, and with a fairly large object (though still small compared to Jupiter itself). Impactors of one Earth mass or less will disintegrate in the atmosphere of Jupiter before even reaching the core. Glancing impacts don't possess enough energy to disrupt or dilute the core in the observed fashion. The only way to create conditions sufficient to lead to the gravitational distribution of material that we see is to smash Jupiter's core with an impactor of nearly equivalent mass (to the core, not the entire planet), with a great deal of energetic mixing occurring thereafter.
Researchers are still exploring other methods that would have led to this unusual result, but the idea of Jupiter being smacked by some truly enormous rocks in the early days of the solar system isn't crazy. Scientists believe the protoplanetary disc of our own star originally contained a great deal more material than is present today. If the Nice model of planetary formation is accurate, an enormous number of planetesimals, protoplanets, and even dwarf planets were flung into the void by the gravitational interactions of the early planets and their moons. We know the Kuiper Belt used to be a reservoir for these objects -- Triton (a moon of Neptune) and Pluto are virtually identical. The fact that Triton is now a moon of Neptune, acquired via unknown processes, is evidence of just how much things were bouncing around a few billion years back.
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