A science rocket sent to Planet Jupiter collected information on that giant gas planet’s gravitational field. This has led, through the use of modeling, to the idea that Jupiter was smacked into by another planet early in its life. Bottom line: There should be heavy stuff (heavier than the gasses that make up gas planets) only in a compact core in the middle of the planet, over which the gasses that make it a gas planet accreted. The gravity information seems to suggest that heavy material is distributed more widely.

From the abstract of the paper just out in Nature:

The Juno mission1 has provided an accurate determination of Jupiter’s gravitational field, which has been used to obtain information about the planet’s composition and internal structure. Several models of Jupiter’s structure that fit the probe’s data suggest that the planet has a diluted core, with a total heavy-element mass ranging from ten to a few tens of Earth masses (about 5 to 15 per cent of the Jovian mass), and that heavy elements (elements other than hydrogen and helium) are distributed within a region extending to nearly half of Jupiter’s radius. Planet-formation models indicate that most heavy elements are accreted during the early stages of a planet’s formation to create a relatively compact core and that almost no solids are accreted during subsequent runaway gas accretion. Jupiter’s diluted core, combined with its possible high heavy-element enrichment, thus challenges standard planet-formation theory. A possible explanation is erosion of the initially compact heavy-element core, but the efficiency of such erosion is uncertain and depends on both the immiscibility of heavy materials in metallic hydrogen and on convective mixing as the planet evolves. Another mechanism that can explain this structure is planetesimal enrichment and vaporization during the formation process, although relevant models typically cannot produce an extended diluted core. Here we show that a sufficiently energetic head-on collision (giant impact) between a large planetary embryo and the proto-Jupiter could have shattered its primordial compact core and mixed the heavy elements with the inner envelope. Models of such a scenario lead to an internal structure that is consistent with a diluted core, persisting over billions of years. We suggest that collisions were common in the young Solar system and that a similar event may have also occurred for Saturn, contributing to the structural differences between Jupiter and Saturn

From the press release:

HOUSTON — (Aug. 14, 2019) — A colossal, head-on collision between Jupiter and a still-forming planet in the early solar system, about 4.5 billion years ago, could explain surprising readings from NASA’s Juno spacecraft, according to a study this week in the journal Nature.

Astronomers from Rice University and China’s Sun Yat-sen University say their head-on impact scenario can explain Juno’s previously puzzling gravitational readings, which suggest that Jupiter’s core is less dense and more extended that expected.

“This is puzzling,” said Rice astronomer and study co-author Andrea Isella. “It suggests that something happened that stirred up the core, and that’s where the giant impact comes into play.”

Isella said leading theories of planet formation suggest Jupiter began as a dense, rocky or icy planet that later gathered its thick atmosphere from the primordial disk of gas and dust that birthed our sun.

Isella said he was skeptical when study lead author Shang-Fei Liu first suggested the idea that the data could be explained by a giant impact that stirred Jupiter’s core, mixing the dense contents of its core with less dense layers above. Liu, a former postdoctoral researcher in Isella’s group, is now a member of the faculty at Sun Yat-sen in Zhuhai, China.

“It sounded very unlikely to me,” Isella recalled, “like a one-in-a-trillion probability. But Shang-Fei convinced me, by shear calculation, that this was not so improbable.”

The research team ran thousands of computer simulations and found that a fast-growing Jupiter can have perturbed the orbits of nearby “planetary embryos,” protoplanets that were in the early stages of planet formation.

Liu said the calculations included estimates of the probability of collisions under different scenarios and distribution of impact angles. In all cases, Liu and colleagues found there was at least a 40% chance that Jupiter would swallow a planetary embryo within its first few million years. In addition, Jupiter mass-produced “strong gravitational focusing” that made head-on collisions more common than grazing ones.

Isella said the collision scenario became even more compelling after Liu ran 3D computer models that showed how a collision would affect Jupiter’s core.

“Because it’s dense, and it comes in with a lot of energy, the impactor would be like a bullet that goes through the atmosphere and hits the core head-on,” Isella said. “Before impact, you have a very dense core, surrounded by atmosphere. The head-on impact spreads things out, diluting the core.”

Impacts at a grazing angle could result in the impacting planet becoming gravitationally trapped and gradually sinking into Jupiter’s core, and Liu said smaller planetary embryos about as massive as Earth would disintegrate in Jupiter’s thick atmosphere.

“The only scenario that resulted in a core-density profile similar to what Juno measures today is a head-on impact with a planetary embryo about 10 times more massive than Earth,” Liu said.

Isella said the calculations suggest that even if this impact happened 4.5 billion years ago, “it could still take many, many billions of years for the heavy material to settle back down into a dense core under the circumstances suggested by the paper.”

Isella, who is also a co-investigator on the Rice-based, NASA-funded CLEVER Planets project, said the study’s implications reach beyond our solar system.

“There are astronomical observations of stars that might be explained by this kind of event,” he said.

“This is still a new field, so the results are far from solid, but as some people have been looking for planets around distant stars, they sometimes see infrared emissions that disappear after a few years,” Isella said. “One idea is that if you are looking at a star as two rocky planets collide head-on and shatter, you could create a cloud of dust that absorbs stellar light and reemits it. So, you kind of see a flash, in the sense that now you have this cloud of dust that emits light. And then after some time, the dust dissipates and that emission goes away.”

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