Zhejiang University researchers demystify the "childhood" of the solar system, new models to supplement the "loopholes" of predecessors

Liu Beibei team.　Offered by Zhejiang University

Hangzhou, April 28 (Xie Panpan Ke Yineng) How was the planet born? How did the solar system evolve? When people look into the depths of space, they always sigh. On April 28, the reporter learned from Zhejiang University that the results of the new model of the orbital evolution of giant planets in the solar system proposed by Zhejiang University have been published in Nature, through the study of the big brothers in the planetary family - Jupiter, Saturn, Neptune, Uranus, from their dynamic changes "pipe peeping leopard", revealing the "childhood" of the solar system. The new model also complements the "loopholes" of previous research.

The study was jointly proposed by Liu Beibei, a researcher at the School of Physics of Zhejiang University, and Professor Raymond of the University of Bordeaux in France, and Professor Jacobson of Michigan State University in the United States, to propose a new model of the orbital evolution of giant planets in the solar system.

They point out that in the early days of the solar system, the protoplanetary disk was vaporized by sunlight, and the dissipation of gas in the disk from the inside out induced the reshaping of the giant planet's orbit and caused dynamic instability. This result was published in Nature on April 27, Beijing time, and Researcher Liu Beibei is the first and corresponding author of the paper, and the School of Physics of Zhejiang University (hereinafter referred to as "Zhejiang University") is the first unit.

Scholars believe that at the beginning of the birth of the solar system, clouds of gas molecules in interstellar space collapsed, the central part formed the sun, and the residual material rotated around the star to form a flat protoplanetary disk. This period is also known as the solar system's gas disk period. During the gas disk period, the four giant planets of the solar system, such as Saturn, Jupiter, Uranus, and Neptune, migrated into an orbital resonance state.

However, the orbital distribution of the four giants is now more open, and the giants have also been detached from the original resonance state, and scholars believe that the orbits of the giants have undergone dynamic upheavals.

"Imagine a viaduct with normal vehicle circulation, if there is a collision and rear-end collision of vehicles, the entire driving order will be disrupted." Liu Beibei said that tracing the causes of dynamic instability in the early solar system is a problem of great concern to the academic community.

The most popular description of the evolution of giant planets in the solar system is the Nice model, named after the model's creator from the Côte d'Azur Observatory in Nice, France.

The Nice model believes that orbital instability occurs hundreds of millions of years after the birth of the solar system, when the gas of the protoplanetary disk is dissipated, and the interaction between the giant planet and the outer planetary disk (composed of stars with a diameter of several kilometers to hundreds of kilometers) constantly exchanges orbital energy, which eventually frees the planet from the resonance bondage and causes dynamic instability. Because the energy exchange during this process is very slow, orbital instability falls under the "late instability" of hundreds of millions of years after the birth of the solar system.

Liu's team proposed that the dissipation of gas disks could be used to explain the evolution of planetary orbits, a factor that had not been taken into account in previous models.

Liu Beibei pointed out that previous studies ignored the gas disk dissipation process that the planet is reversed by the force of the gas. "In the late stages of the evolution of the gas disk, the high-energy photons of solar radiation directly hit the planetary disk, and the strong light pressure formed first blew away the gas close to the sun, and a hollow structure appeared inside the planetary disk. Subsequent light pressure gradually disperses the remaining gas in the disk from the inside to the outside, and the mass of the planetary disk decreases with the expansion of the boundary inside the disk outward, a process known as the photoevasion of the planetary disk. Liu Beibei said that at this time, the sun is like a giant hair dryer, constantly "blowing" the gas in the disk.

Through theoretical calculations, Liu Beibei's team found that due to the rapid dissipation of gas at the inner boundary, the planet is subjected to an outward gas force at that place, which is very different from the inward force that the planet receives at other locations in the disk. When the inner boundary of the gas disk expands outward from photo-induced evaporation, the planets that originally migrated inward change their direction of motion and move outward together with the inner boundary. "The process is like playing badminton, swinging the ball, the badminton ball changes its original trajectory, bounces off and moves outward with the beat surface." Giant planets migrate outward at different rates due to their different masses, breaking the original orbital resonance state and causing dynamic instability.

"Our study shows that the kinetic instability caused by this process immediately follows the dissipation of the gas disk, occurring about five to ten million years after the birth of the solar system." Unlike the Nice model, our model has giant planet orbital instability occurring earlier. Liu Beibei said.

Using the traditional Nice model and the "rebound" model proposed by Liu Beibei's team to deduce the "childhood" of the solar system, the most obvious difference is that dynamic instability occurs sooner or later, the former is considered to be "late instability", while the latter is considered "early instability".

"We can find new evidence from the age of lunar craters." According to Liu Beibei, the unstable dynamics of giant planets will break the original calm of the solar system, and their strong gravitational disturbance forces the surrounding small bodies to constantly crash into other planets and moons, leaving craters on the surface of the star. "Lunar craters have a wide age distribution, and asteroid impact events naturally decay over time, which is more self-consistent with our team's early instability model research."

In addition, the orbits of terrestrial planets also support the "bounce" model of Liu Beibei's team.

According to observations, the primordial Earth formed in the protoplanetary disk stage and eventually grew up 30 million to 100 million years after the birth of the solar system. If instability occurs before the Earth is fully formed, the giant planet's orbital turbulence has the potential to trigger a large collision event, inducing the original Earth to collide with a Martian-sized object, gradually forming the current Earth-Moon system. "And the Nice model predicts that instability occurs after the Formation of the Earth, the Earth cannot become the Earth it is today." Liu Beibei said, "Early dynamic instability is more in line with the records from other celestial bodies in the solar system about the timing of asteroid impacts." The new model could also better explain the mass and orbital configurations of subsequently formed terrestrial planets, which are all advantages that distinguish them from traditional models. ”

The Nature reviewer commented on the study: "The model is likely to be a missing component in the theory of solar system evolution, and the article is novel and significant. ”

Liu Beibei said that in the future, the team will further explore the impact of the orbital evolution of giant planets on the formation of the Earth and its water origin. (End)