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The artistic imagination of the original planetary chart. Image source: the planets in the NASA,Pat Rawlings solar system seem to orbit the sun forever, but billions of years ago, the solar system actually experienced violent shocks that greatly changed the orbit of the planets. The nice model can explain this huge migration, but recently a new guess challenged the model, which was proposed 17 years ago and has become mainstream today.

4.6 billion years ago, a huge swirl of gas and dust wrapped around the newborn sun, and the solar system was born. The initially formed sun is surrounded by a protoplanetary disk of dense gas and dust. Earth-like planets (Mercury, Venus, Earth and Mars) and giant planets (Jupiter, Saturn, Uranus, Neptune) gradually formed in this gas disk. Since the end of the last century, scientists have believed that the early giant planets of the solar system formed in compact and evenly spaced orbits. However, observations show that their orbits are slightly skewed and scattered. Why do the orbits of planets shift from uniform to dispersed? When did this migration take place? This is an important issue related to the formation of the solar system.

In 1984, scientists suggested that the collision between the microplanetary belt on the periphery of the protoplanetary disk and the giant planet may be the cause of this phenomenon. These microplanets first collided with the outermost Neptune, some of which were scattered outward, most of them entered the inside of Uranus's orbit, and Neptune's orbital angular momentum increased and moved outward. Such collisions occurred one after another on Uranus and Saturn, causing their orbits to shift outward. For the more inward Jupiter, most of the microplanets shooting at it are ejected outward, so Jupiter's orbit is slightly closer to the sun.

After A. Morbidelli and H.F. Levison, scientists have gained some inspiration in the exploration of exoplanets. At first, scientists discovered a class of gaseous giant planets that are very similar to Jupiter, but they are very close to the stars and have short orbital periods, some of which are even less than 10 days. These planets are called "hot Jupiter". Then scientists discovered gaseous giants with longer orbits, and they noticed that most of their orbits were far from the circle, as if they had been stretched.

This phenomenon is thought to be caused by "dynamic instability": gaseous giant planets near stars usually produce more than one, and when the protoplanetary disk of the galaxy dissipates, these giant planets are pushed under the influence of gravity and slightly change each other's orbits. If the orbits are close enough, or even cross, the giant planets will meet and disperse again and again due to gravity. The end result is usually one or more planets being ejected. At this point, the orbit of the surviving planet will not be a regular circle. At least 75% of exoplanetary systems are the product of this "dynamic instability". Scientists believe that such instability may be another possible cause of orbital oscillations of planets in the solar system.

In 2005, a new model for the evolution of the solar system, the Nice Model (Nice model), combined the collision and dynamic instability of the microdisk into a coherent theory. This model speculates that there was a fifth giant planet in the early solar system, but hundreds of millions of years after the protoplanetary disk dissipated, the giant planet was ejected from the solar system because of dynamic instability with Jupiter. At the same time, a microplanetary belt about 20-30 times the mass of Earth outside the orbit of Neptune causes the orbits of Uranus and Neptune to shift outward. Neptune's orbit is getting closer to the edge of the microplanetary belt, and other giant planets are scattered to their current positions.

The Nice model simulates images of giant planets and microplanetary belts, with dark blue as the orbit of Neptune and light blue as the orbit of Uranus. The left shows that before Jupiter and Saturn resonate at 2:1, the model predicts that Uranus and Neptune have a 50% chance of swapping positions; the middle shows Neptune's orbital changes and microplanetary scattering into the solar system; in the right, Neptune's orbit moves outward to the edge of the microplanetary belt. Image source: the dynamic simulation results of the AstroMark Nice model explain many astronomical observations and have been widely accepted in the past decade. However, according to the time speculated by this model, this instability will destroy fully formed Earth-like planets, including Earth. Other studies, including lunar rock samples taken by the Apollo mission, also suggest that planetary migration may have occurred earlier.

In April, a paper published in the journal Nature proposed a new model of changes in planetary orbits, suggesting that the evolution was probably caused by the dissipation of the early sun's protoplanetary disk from the inside out. Liu Beibei of Zhejiang University, Sean Raymond of the University of Bordeaux (University of Bordeaux) in France and Seth Jacobson of Michigan State University (Michigan State University) collaborated on the study. "this new theory can help us alleviate the existing conflicts in this field, because it is a very natural answer to the instability of giant planets," Jacobson said. "

The researchers speculate that earlier, solar nuclear fusion ignited and released high-energy radiation, causing the protoplanetary disk to evaporate from the center, creating a hole around the sun. Just like the hole in the middle of a doughnut gets bigger and bigger, the doughnut itself gets thinner and thinner until it disappears. The hole gradually expands outward, and the giant planet it sweeps is pulled outward by the "doughnut", and the orbit of the giant planet moves out with it. The exception is Jupiter, which is so massive that it can itself create faults on the gas disk, offsetting the pulling force caused by the dissipation of the gas disk. Saturn is not so lucky, it is pulled outward by the gas disk, close to the orbit of the ice giant planets (Uranus and Neptune). According to the simulation, there is a high probability that this will cause dynamic instability and cause ice giant planets to be ejected by Saturn. The orbit of the giant planet then became uniform and reached its present position.

A schematic diagram of the instability caused by the dissipation of the gas disk, with the gray part indicating the gas disk, chronologically from top to bottom. Source: Sean Raymond, planetary images from NASA and, more importantly, according to this new model, dynamic instability can cause asteroids and comets to hit a still-forming Earth, which had not yet produced primitive life. "this process stirs up the inner solar system, and the Earth can form from it," Jacobson said. "it fits very well with current observations." The relationship between the formation of the earth and this instability is also a topic that the research team wants to continue to explore in the future.

At the same time, this theory also applies to other star systems in the Milky way. Although this dynamic instability has had a great impact on the solar system, it is still weak compared with that outside the solar system. Fortunately, because Jupiter escaped the pull of the gas disk, Jupiter and Saturn did not get close in the process, otherwise, like many giant planets outside the solar system, Jupiter's orbital eccentricity would be 5-10 times what it is today (the smaller the eccentricity, the closer the orbit is). In this case, the earth cannot be formed. Because her components were involved in the sun before the earth was formed.

Reference article:

Https://nautil.us/were-it-not-for-cosmic-good-fortune-we-wouldnt-be-here-18446/

Https://www.eurekalert.org/news-releases/951041

Https://planetplanet.net/2022/06/30/the-giant-planet-instability-the-nice-model/

Reference paper:

Https://www.sciencedirect.com/science/article/abs/pii/0019103584901015?via%3Dihub

Https://www.nature.com/articles/nature03539#Abs2

Https://www.nature.com/articles/s41586-022-04535-1

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), compiled by Meng Fanqiong revision: Wang Yu

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