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 exact trigger and timing of this type of instability, which has also appears to have occurred 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. 

A previous hypothesis, known as the Nice model after the French city where it originated, proposed that the orbital instabilities arose after the evaporation of the cloudy primordial disk that birthed our solar system. Now, Liu and his colleagues present results from 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, according to a study published on Wednesday in Nature. 

Study co-authors Seth Jacobson, a planetary scientist at Michigan State University, and Sean Raymond, an astronomer at the Laboratoire d'Astrophysique de Bordeaux in France, first started developing this new explanation a few years ago.

“While the evidence for a giant planet instability in the solar system is clear, both Sean and I knew that there was mounting evidence that the instability must have taken place much earlier than originally hypothesized in the 2005 Nice model,” Jacobson said in an email. 

Jacobson pointed to two main paradigm shifts about the early solar system that laid the groundwork for the new study. First, evidence suggests that a glut of ancient impact craters on the Moon may have been caused by a longer period of bombardment, rather than a short-lived pulse of collisions, as previously believed. Second, the Nice model suggests the instability occurred after the formation of rocky planets like Mars and Earth, but new research implies that these inner worlds would have been much more disrupted by this event if that were the real timeline.

“These two realizations over the last 15-ish years provoked us to question whether a different giant instability triggered earlier in solar system history could explain many of the same phenomena as the original Nice model,” Jacobson explained. “Sean and I were then thinking about what could be an alternative trigger and Sean identified Beibei Liu's work on planet-disk interactions near the magnetospheric cavity as potentially worth examining more closely as an analog to what might have happened during disk photoevaporation.”

This evaporation was sparked some 4.6 billion years ago when the Sun began to shine for the first time, prompting its heat and energy to push the cloud of gas and dust further out into the solar system. This process occurred within ten million years of the solar system’s birth, when its rocky worlds were still cooking, and its outer gas giants were emerging in neat compact orbits within the same plane of 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 already proposed that the solar system contains a hidden planet in its outer reaches, a hypothetical ice giant known as Planet Nine. Jacobson noted that he has “always been partial to the Planet Nine hypothesis” because of these early instability models, and added that “passing nearby stars could perturb that ice giant onto a distant orbit, like that hypothesized for Planet Nine.”

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. Jacobson pointed out that only about five percent of star systems are arranged in the kind of resonant compact structure predicted by models, revealing a gap between our expectations and real observations of outer space.

“Other works, most importantly those of Andre Izidoro [an astronomer at Rice University], have then shown that dynamical instabilities must have occurred in these systems to explain how they go from where theory predicts they should be to where they are observed,” Jacobson said. “It could be that in each system the dynamical instability was triggered by a different mechanism, but the rebound effect that we discovered in this paper is nearly universal and reasonably could have caused instability in the ~95% of planetary systems that we see it in.”

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.

Update: This article has been updated with comments from co-author Seth Jacobson.