A colossal frontal collision between Jupiter and a still-forming planet in the early solar system about 4.5 billion years ago could, according to a study in this week's magazine, explain surprising readings from NASA's Juno spacecraft nature,
Astronomers from Rice University and Sun Yat-sen University in China say their direct collision scenario may explain Juno's previously puzzling gravitational measurements, indicating that Jupiter's core is less dense and longer than expected.
"That's puzzling," said rice astronomer and study co-author Andrea Isella. "It indicates that something has happened that has upset the core, and this is where the gigantic influence comes into play."
Isella said leading theories of planet formation suggest that Jupiter began as a dense, rocky or icy planet that later gathered its dense atmosphere from the original disk of gas and dust that our sun produced.
Isella expressed skepticism when the lead author of the study, Shang-Fei Liu, first introduced the idea that the data could be explained by a huge impact that moved Jupiter's core and blended the dense contents of its core with less dense layers of it. Liu, a former postdoctoral fellow in Isella's group, is now a member of Sun Yat-sen's faculty in Zhuhai, China.
"It sounded very unlikely to me," Isella recalled, "like a one trillion chance, but Shang-Fei convinced me after shearing that this is not unlikely."
The research team carried out thousands of computer simulations and found that a fast-growing Jupiter may have disrupted the orbits of nearby "planetary embryos," protoplanets that were in the early stages of planet formation.
Liu said the calculations included estimates of the collision probability under different scenarios and the distribution of impact angles. In all cases, Liu and colleagues found a probability of at least 40% that Jupiter would swallow a planetary embryo for the first few million years. In addition, Jupiter mass-produced "strong gravitational focusing," making frontal collisions more common than grazing.
Isella said the collision scenario became even more convincing after Liu delivered 3D computer models that showed how a collision would affect Jupiter's core.
"Because it's dense and contains a lot of energy, the impactor is like a bullet that goes through the atmosphere and hits the core head-on," said Isella. "Before the impact, you have a very dense core, surrounded by atmosphere, the direct impact distributes things and dilutes the core."
Impacts at a grazing angle could cause the impacting planet to become trapped in gravity and gradually sink into Jupiter's core. Liu said that smaller planetary embryos are about as massive as the earth would disintegrate in Jupiter's thick atmosphere.
"The only scenario that resulted in a core density profile equivalent to that of Juno today is a direct impact on a planetary embryo that is about ten times more massive than Earth," Liu said.
Isella said the calculations suggest that even if these effects occurred 4.5 billion years ago, "it could take many, many billions of years for the heavy material to revert to a dense core under the circumstances proposed by the newspaper sets. "
Isella, who also works on the NASA-funded Rice CLEVER Planets project, said the impact of the study extends beyond our solar system.
"There are astronomical observations of stars that could be explained by such events," he said.
"This is still a new area, so the results are far from solid, but as some people have searched for planets around distant stars, they sometimes see infrared emissions that disappear after a few years," said Isella. "One idea is that if you look at a star while two rocky planets clash and shatter head-on, you could create a cloud of dust that absorbs and releases the starlight, so you're kind of struck. Now you have that cloud of dust, the light And after a while the dust dissolves and this emission disappears. "
The Juno mission was designed to help scientists better understand Jupiter's origin and development. Launched in 2011, the spaceship has instruments to map the gravitational and magnetic field of Jupiter and to study the deep internal structure of the planet.
Other co-authors of the study are Yasunori Hori from the Astrobiology Center of Japan, Simon Müller and Ravit Helled from the University of Zurich, Xiaochen Zheng from Tsinghua University in Beijing, and Doug Lin from the University of California at Santa Cruz and Tsinghua University in Beijing.
The research was supported by NASA (80NSSC18K0828), the National Science Foundation (AST-1715719) and the Swiss National Science Foundation (200021_169054).