A colossal, head-on collision between Jupiter and a massive newborn planet about 4.5 billion years ago could explain surprising readings from NASA’s Juno spacecraft, according to a new study.

Researchers say the new impact scenario can explain Juno’s previously confusing gravitational readings which suggest that Jupiter’s core is less dense and more extended that expected.

“This is puzzling,” says Andrea Isella, an astronomer at Rice University. “It suggests that something happened that stirred up the core, and that’s where the giant impact comes into play.”

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 says.

He says he was skeptical when lead author Shang-Fei Liu first suggested the idea that a giant impact that stirred Jupiter’s core—mixing the dense contents of its core with less dense layers above—could explain the data. 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 says, “like a one-in-a-trillion probability. But Shang-Fei convinced me, by shear calculation, that this was not so improbable.”

Planetary embryos

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

The calculations included estimates of the probability of collisions under different scenarios and distribution of impact angles, Liu says. In all cases, the researchers found 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.

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 says. “Before impact, you have a very dense core, surrounded by atmosphere. The head-on impact spreads things out, diluting the core.”

Juno evolution

Impacts at a grazing angle could result in the impacting planet becoming gravitationally trapped and gradually sinking into Jupiter’s core. Liu says 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 says.

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 says.

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

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

“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 re-emits 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.”

Scientists designed the Juno mission to help better understand Jupiter’s origin and evolution. The spacecraft, which launched in 2011, carries instruments to map Jupiter’s gravitational and magnetic fields and probe the planet’s deep, internal structure.

The paper appears in Nature.

Additional coauthors are from the Astrobiology Center of Japan; the University of Zurich; Tsinghua University in Beijing; and the University of California, Santa Cruz. NASA, the National Science Foundation, and the Swiss National Science Foundation supported the work.

Source: Rice University

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