Giant planets Solar system Evolution Dissipation Naissance
The sun blows away the gas in the protoplanetary disk like a giant hair dryer. (Image source: Liu Beibei)
The sun is the center of the entire solar system. The eight major planets each hold their own orbits and revolve around the sun in peace. But astronomers have been trying to answer a question: How did the orbits of planets in the solar system become what they are now?
Researcher Liu Beibei from Zhejiang University and collaborators from the University of Bordeaux in France and Michigan State University in the United States have proposed a new model for the orbital evolution of giant planets in the solar system. This model traces the past history of giant planets, revealing a period of disharmony between them and suggesting that this history occurred before the birth of Earth. The related achievements have been published in the journal Nature recently.
The orbits of giant planets have undergone drastic changes
A long time ago, gas molecular clouds in interstellar space collapsed, and the central part formed the Sun; The residual material is baked by the sun, and the gas continuously evaporates while rotating around the sun. A flat, gas filled disk appears from here.
Planets grow inside the disk and interact with the gas inside, gradually rounding their orbits and migrating towards the sun. For the four giant planets of the solar system, Saturn, Jupiter, Uranus, and Neptune, on the outer edge of the disk, they gradually found equilibrium points through inward migration and various forces pulling, and their motion trajectories gradually became stable and regular.
Just like vehicles driving normally on highways, everyone is relatively constant in speed and maintains an appropriate distance from each other, Liu Beibei, the corresponding author and first author of the paper, told China Science Daily.
But strangely, according to this idea, the trajectories of the four giant planets located outside the solar system will not be as they are today. The distance between the four giant planets today is even farther than in the early days of the solar system, said Liu Beibei.
Astronomers speculate that during the evolution of planetary orbits, the orbits of giant planets have undergone drastic changes. Just like when a vehicle suddenly changes gears and experiences a collision or rear end collision, the distance between vehicles is disrupted and the original orderly state is disrupted, said Liu Beibei.
However, in this way, another new question arises: what caused the orbits of giant planets to undergo drastic changes?
The confusion left by famous models
Astronomers have proposed various models for the drastic changes in the orbits of giant planets, among which the most popular is the Nice model.
This model appeared in 2005. At that time, the creator of the model published three Nature papers in a row, causing a sensation in the astronomical community. The model is also named after its creator from the Blue Coast Observatory in Nice, France. However, while causing a sensation, the Nice model still leaves some confusion.
At present, astronomers believe that in the early stages of the birth of the solar system and the evolution of the gas disk to the late stage, high-energy photons of solar radiation shine directly on the gas disk, forming a strong light pressure that first blows away the gas near the sun, causing a hollow structure to appear inside the disk. Then, the light pressure gradually disperses the remaining gas in the disk from the inside out.
This process is called 'photo induced evaporation'. The sun is like a giant hair dryer, constantly blowing away the gas in the disk, said Liu Beibei.
The Nice model suggests that after the gas in the disk is dissipated, the giant planet interacts with the outer disk of stars (composed of particles with diameters ranging from several kilometers to hundreds of kilometers) and continuously exchanges orbital energy, ultimately causing the dynamic instability of the giant planet's orbit.
The model also concludes that due to the slow process of energy exchange between giant planets and distant stars, the drastic changes in the orbits of giant planets occur billions of years after the birth of the solar system, which belongs to late stage instability.
The Earth grew between 30 million and 100 million years after the birth of the solar system. If the Nice model predicts that the instability of giant planet orbits occurs after Earth's formation, the strong disturbances generated will disrupt Earth's orbit, which is inconsistent with the current observed situation, Liu Beibei said.
He stated that the dynamic instability of giant planets would disrupt the original calm of the solar system, and their powerful gravitational disturbances would force surrounding small celestial bodies to continuously collide with other planets and satellites, leaving craters on the surface of the celestial bodies. But scientists have found that lunar craters have a wide age distribution, and asteroid impact events did not suddenly increase during a certain period of time.
Key details overlooked by predecessors
The confusion left by the Nice model is also a question that Liu Beibei has been pondering. One day in 2019, Liu Beibei received an email from overseas, which facilitated his research.
We suspect that the instability of the orbits of giant planets in the solar system may occur earlier than expected by the Nice model. In the email, two international planetary dynamics experts shared their thoughts with Liu Beibei.
These two experts are Sean Raymond, a professor at the University of Bordeaux in France, and Seth Jacobson, a professor at Michigan State University in the United States. They later became collaborators of Liu Beibei's research.
Both professors have seen the paper previously published by Liu Beibei and others in Astronomy and Astrophysics. In the paper, Liu Beibei et al. investigated why many super Earth planets do not operate smoothly and orderly like planets in the solar system in other multi planet systems. They proposed a rebound idea, believing that the dissipation of gas in the protoplanetary disk caused significant changes in the planet's orbit and caused it to break away from a stationary state.
Raymond and Jacobson found the paper extremely inspiring and decided to send an email to the corresponding author Liu Beibei. Afterwards, they began to communicate through various forms such as email and meetings. As the discussion deepened, all three felt that this research was of great significance.
In the discussion, they found that previous research overlooked a key detail - during the dissipation process of gas disks, planets undergo reverse motion due to gas forces. This process is similar to play badminton. When you swing the racket to hit the ball, the badminton changes its original trajectory, bounces back and moves outward with the racket face, Liu Beibei said.
Through theoretical calculations, they found that due to the scorching heat near the inner boundary of the sun, gas quickly dissipates. When the inner boundary of the gas disk expands outward due to light induced evaporation, the planet that originally migrated inward changes its direction of motion and moves outward together with the inner boundary.
Due to the different masses of giant planets in the solar system, their rebound and outward migration rates also vary, thus breaking the originally harmonious and stable orbital distances between them.
Occurred before the formation of the Earth
Based on assumptions and theoretical calculations, they conducted simulation experiments on the evolution of giant planet orbits.
The experimental results show that according to the rebound model, the original four giant planets and another ice giant underwent a significant orbital change when dissipating the gas disk. The ice giant collided with Jupiter and was thrown out of the solar system. The final orbital distribution of the four stable giant planets is consistent with current observations.
In addition, studies have shown that the instability of giant planet orbits occurs approximately 3 to 10 million years after the birth of the solar system. That is to say, unlike the speculation of the Nice model, we believe that instability in the orbits of giant planets had already occurred before the birth of Earth, said Liu Beibei.
The orbital evolution of giant planets has a profound impact on the evolution of other planets, satellites, and small celestial bodies, as well as the origin and habitability of Earth and life. Liu Beibei believes that the new model can better explain the mass and orbital configuration of subsequent terrestrial planets, which are different from traditional models.
The anonymous reviewer believes that this model is likely a missing part of the theory of solar system evolution, and the article is novel and significant.
In the future, we will further explore the impact of the orbital evolution of giant planets on the formation of Earth and its water origin, said Liu Beibei. (Ni Sijie and Cui Xueqin)
Related paper information:
https://doi.org/10.1038/s41586-022-04535-1
