What was the “childhood” of the solar system like?

How did the planets form? How did the solar system evolve?

Teacher. LIU Beibei from Zhejiang University School of Physics, in collaboration with Professor Sean Raymond of the University of Bordeaux in France and Professor Seth Jacobson of Michigan State University in the United States, proposed a new model for the evolution of the orbits of the giant planets of the solar system. They pointed out that the scattering of the Sun’s gaseous protoplanetary disk likely triggered the instability of the giant planets.

LIU Beibei (middle) and his students

This groundbreaking discovery was published in Nature April 27.

The planetary orbits of the solar system that we see today are significantly different from what they were in its “infancy”. At the start of the solar system, a concentration of interstellar dust and hydrogen gas called a molecular cloud contracted. The proto-Sun formed at the hot, dense center and the rest of the cloud formed a swirling disk called the solar nebula. This period is also known as the gas disc period. Scientists believe that the four giant planets – Jupiter, Saturn, Uranus and Neptune – migrated to a chain of orbital resonances.

Four giant planets migrated in a chain of orbital resonances.

Today, however, the orbits of the giant planets seem to be more widely distributed, and these planets have deviated from their original resonances. The orbital structure is thought to have been sculpted by an episode of dynamic instability among the giant planets.

The Nice Model, originally developed by an international team of scientists in 2005 in Nice, France, is the most popular model explaining the evolution of the solar system. He postulates that the orbital instability occurred hundreds of millions of years after the formation of the solar system. As the protoplanetary disk dispersed, the giant planets interacted with the outer stellar disk to exchange orbital energy and were eventually freed from resonances, thereby triggering dynamic stability. Due to the extremely slow process, this orbital instability is considered a “late instability”.

Together with Jacobson and Raymond, Liu proposed that disk dispersion could explain the evolution of planetary orbits, a factor not taken into account in previous models.

The idea started with a conversation Raymond and Jacobsen had in 2019. They then contacted Liu, who pioneered the idea of ​​the rebound effect through extensive simulations of gas disks and large exoplanets – planets from other solar systems – that orbit close to their stars.

“In the late phase of the gas disk, the high-energy photons emitted from the Sun directly hit the planetary disk, the strong light pressure blew the gas near the Sun, and a hollow structure formed inside the planetary disk. planetary disk. The subsequent slight pressure gradually dispersed the gas remaining in the upside-down disk, and the mass of the disk decreased with the outward expansion of the disk. This process is known as photoevaporation”, said Liu, “The Sun was like a giant hair dryer, constantly ‘blowing’ gas into the disc.”

Theoretical calculations by Liu and his team showed that due to the rapid dissipation at the inner edge of the disk, the planet was subject to the outward gas force, which differed greatly from the inward forces at the others. places on the disc. When the inner rim of the disk extended outward, the planet migrated outward instead of migrating inward. This mechanism is called “rebound”. Due to their different masses, the giant planets would migrate outwards at different speeds, thus breaking the original state of orbital resonance and triggering dynamical instability.

Pattern of rebound effects, similar to the game of badminton, where the shuttlecock changes course and bounces outward with the face of the racket

The dynamic instability of the first orbits caused the four giant planets and one other ice giant planet to undergo substantial orbital variations during the dissipation of the gas. The giant ice planet was catapulted out of the solar system after its near collision with Jupiter. The final orbital configuration of the four giant planets is consistent with what we observe today.

The giant planets find themselves close to their current orbit after the instability triggered by the dispersion of the disk of gas.

“Our study indicates that the dynamical instability following gas disk dispersion occurred about five to ten million years after the formation of the solar system, earlier than the Nice model suggests,” said Liu.

“We are able to find new evidence for the age of lunar craters,” Liu said. “The dynamic instability of the giant planets could perturb the rest of the solar system, and their powerful gravitational disturbances could force small bodies around them to continually collide with other planets and moons, leaving craters on the surface. of celestial bodies. Lunar craters differ significantly in age. Asteroid impact events naturally decrease over time, which is also consistent with the early instability model study proposed by our team.

“Early dynamic instability is more consistent with the timing of asteroid impacts in other celestial bodies in the solar system. Our model can also provide a deep understanding of the masses and orbital configurations of terrestrial planets,” Liu said.

“In the future, our team will further explore the impact of changing orbits of giant planets on the formation of Earth and the origin of water,” Liu added.

Photo credit: LU Shaoqing and LIU Beibei

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