Mysterious ‘Instability’ In the Cosmos Puzzled Scientists for Decades. Now, We Have an Answer

Our solar system is relatively tranquil at this point in time, with planets that follow predictable orbits around the Sun, but these cosmic surroundings weren’t always so calm.

 Scientists have long suspected that during the infancy of our solar system, tumultuous instabilities dramatically shifted the orbits of the gas giants—Jupiter, Saturn, Uranus, and Neptune—and may have even straight-up kicked a fifth mysterious planet out into the interstellar wilderness. However, the cause of this instability, which has also been observed in other star systems, has remained a matter of debate

Now, scientists led by Beibei Liu, a physicist at Zhejiang University in China, have proposed a new mechanism that can explain how these giants ended up in their distant orbits, and can even account for some of the puzzling features of the solar system’s innermost rocky worlds, such as Earth and Mars. 

Whereas previous models have proposed that the orbital instabilities arose after the evaporation of the cloudy primordial disk that birthed our solar system, Liu and his colleagues ran 14,000 simulations that suggest this evaporating cloud was, itself, the driver of the turbulent effects that led to the familiar planetary configuration we live in today.

“The Solar System’s orbital structure is thought to have been sculpted by an episode of dynamical instability among the giant planets,” said the team in a study published on Wednesday in Nature. “However, the instability trigger and timing have not been clearly established.”

“Here we use dynamical simulations to show that the giant planets’ instability was probably triggered by the dispersal of the gaseous disk,” As the disk evaporated from the inside out, its inner edge swept successively across and dynamically perturbed each planet’s orbit in turn.”

In other words, as the Sun began to shine for the first time some 4.6 billion years ago, its heat and energy pushed the protoplanetary cloud of gas and dust further out into the solar system. This process occurred no more than ten million years after the solar system was born, when its rocky worlds were still cooking, and its outer gas giants were emerging in neat compact orbits within the gassy disk, much closer to the Sun than they are today. 

But as the cloud of dust moved outward, its inner edge caught the gas giants in its tide, causing their orbits to go awry and get more spread out. Liu’s team modeled this process, which they call the “rebound effect” using differing numbers of gas giants, including an early solar system that had five giant worlds, instead of four. The simulations predicted that this extra planet was gravitationally ejected from our system by instabilities caused by the dispersal of the primordial gas disk. 

Some scientists have actually proposed that the solar system contains a hidden planet in its outer reaches, a hypothetical world known as Planet Nine. While Liu and his colleagues model this outcome, they did not find the simulation with five gas giants was any more likely than the one involving the four huge worlds we know today

In addition to reconstructing the position of the giant planets, the results may also explain how Mars ended up so much smaller than Earth. As the disk evaporated through the embryonic inner planets, it may have disrupted the red planet as it formed, leading to its reduced mass. 

Moreover, the new study has implications well beyond our solar system, as the team notes that almost all the star systems that are observed beyond Earth are similarly shaped by orbital instabilities. In this way, unlocking the enigmatic origins of our local solar neighborhood could help us understand distant alien worlds across our galaxy, the Milky Way.

“The rebound effect may explain why dynamical instabilities appear to be nearly ubiquitous in exoplanetary systems,” Liu and his colleagues concluded. “Thus, the rebound effect during disk dispersal may be a nearly universal process affecting not just our Solar System but planetary systems across the Galaxy.”

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