The research, published in the journal Nature on April 27, offers an explanation for what happened to gas giants in other solar systems and our own, namely Jupiter, Saturn, Uranus and Neptune.
Stars are born from massive, swirling clouds of cosmic gas and dust. After our sun became active, the early solar system was still filled with a disk of primordial gas that played a vital role in the formation and evolution of planets, including gas giants.
In the late 20th century, scientists began to believe that gas giants initially orbited the sun in orderly, compact, and regularly spaced orbits. Jupiter, Saturn and the others, however, have long since settled into relatively oblong, twisted and elongated orbits.
So the question for researchers now is “Why?”
In 2005, an international team of scientists offered an answer to this question in a trio of landmark Nature articles. The solution was originally developed in Nice, France, and is known as the Nice model. He posits that there was instability between these planets, a chaotic set of gravitational interactions that ultimately set them on their current paths.
“It was a tectonic shift in how people thought about the early solar system,” Jacobson said.
The Nice Model remains a major explanation, but over the past 17 years scientists have found new questions about what triggers Nice Model instability.
For example, the instability of the gas giant was originally thought to have occurred hundreds of millions of years after the dispersal of this primordial gas disk that gave rise to the solar system. But more recent evidence, including some found in moon rocks recovered by Apollo missions, suggests it happened more quickly. It also raises new questions about the evolution of the inner solar system that houses Earth.
Working with Beibei Liu from Zhejiang University in China and Sean Raymond from the University of Bordeaux in France, Jacobson helped find a solution related to how the instability started. The team came up with a new trigger.
“I think our new idea could really ease a lot of tension on the ground, because what we’ve come up with is a very natural response to when the instability of the giant planet happened,” Jacobson said.
The idea grew out of a conversation Raymond and Jacobsen had in 2019. They speculated that the gas giants could have gone their current path because of the way the primordial gas disk evaporated. This could explain how the planets spread out much earlier in the evolution of the solar system than the Nice model originally postulated and perhaps even without the instability to push them there.
“We wondered if the Nice model was really necessary to explain the solar system,” Raymond said. “We had the idea that the giant planets could eventually expand through a ‘bounce’ effect as the disk dissipates, perhaps without becoming unstable.”
Raymond and Jacobsen then approached Liu, who pioneered this idea of the rebound effect through extensive simulations of gas disks and large exoplanets (planets from other solar systems) orbiting near their stars.
“The situation in our solar system is slightly different because Jupiter, Saturn, Uranus and Neptune are spread out in wider orbits,” Liu said. “After a few iterations of brainstorming sessions, we realized that the problem could be solved if the gas disk dissipated from the inside.”
The team found that this inside-out dissipation provided a natural trigger for Nice’s model instability, Raymond said.
“We ended up strengthening the Nice model instead of destroying it,” he said. “It was a fun illustration of testing our preconceptions and following the results wherever they lead.”