Instability in the Early Solar System – Implications for the Mysterious “Planet 9”

All stars, including our sun, are born from a cloud of dust and gas. This cloud can also seed planets that will orbit around the star. Credit: NASA/JPL-Caltech

The instability at the beginning of the solar system

A new explanation of why our solar system is the way it is – and why others are too.

Michigan State University’s Seth Jacobson and his colleagues in China and France have unveiled a new theory that could help solve a galactic mystery about the evolution of our solar system. Specifically, how the gas giants — Jupiter, Saturn, " data-gt-translate-attributes="[{" attribute="">Uranusand Neptune – end up where they are, orbiting the sun as they do?

The findings have ramifications for how terrestrial planets like Earth developed, as well as the possibility of a fifth gas giant planet lurking 80 billion miles away.

“Our solar system has not always looked like it does today. Over its history, the orbits of the planets have changed dramatically,” said Jacobson, an assistant professor in the Department of Earth and Environmental Sciences at the College of Natural Sciences. “But we can understand what happened.”

Hypothetical early solar system

An artist’s rendering shows a hypothetical early solar system with a young star picking its way through the gas and dust left over from its formation. This compensation action would affect the orbits of the gas giants orbiting the star. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

The research, published in the journal Nature on April 27, 2022, offers an explanation of what happened to gas giants in other solar systems and our own.

It’s a beautiful model

Massive, swirling clouds of cosmic gas and dust give rise to stars. The early solar system was still filled with a disc of primordial gas when our sun ignited, and it played an important role in the formation and evolution of planets, including gas giants.

In the late 20th century, scientists began to believe that gas giants initially orbited the sun in sharp, compact, and evenly spaced orbits. Jupiter, Saturn, and the others, however, have long since settled into relatively oblong, misaligned, and splayed orbits.

Seth Jacobson

MSU Assistant Professor Seth Jacobson

So the question for researchers now is, why?

In 2005, an international team of scientists offered an answer to this question in a trio of landmarks Nature papers. The solution was originally developed in Nice, France, and is known as the Nice model. He posits that there was instability among these planets, a chaotic set of gravitational interactions that ultimately set them on their current paths.

“It was a tectonic shift in how people thought about the early solar system,” Jacobson said.

The Nice Model remains a major explanation, but over the past 17 years scientists have found new questions to ask about what triggers the instability of the Nice Model.

For example, the instability of the gas giants was originally thought to have taken place hundreds of millions of years after the dispersal of that disk of primordial gas that gave rise to the solar system. But new evidence, including some found in moon rocks recovered by Apollo missions, suggests it happened faster. It also raises new questions about the evolution of the inner solar system that houses Earth.

Sean Raymond University of Bordeaux

Sean Raymond, astronomer at the University of Bordeaux.

Working with Beibei Liu of Zhejiang University in China and Sean Raymond of the University of Bordeaux in France, Jacobson helped find a solution that has to do with how the instability started. The team came up with a new trigger.

“I think our new idea could really ease a lot of tension on the ground, because what we’ve come up with is a very natural response to when the instability of the giant planet happened,” Jacobson said.

The new trigger

The idea originated from a conversation Raymond and Jacobsen had in 2019. They speculated that the gas giants may have been placed on their current path due to the way the primordial gas disk has evaporated. This could explain how the planets spread out much earlier in the evolution of the solar system than the Nice model originally postulated and perhaps even without the instability to push them there.

“We wondered if the Nice model was really necessary to explain the solar system,” Raymond said. “We had the idea that the giant planets could eventually spread out through a ‘bounce’ effect as the disk dissipates, perhaps without ever becoming unstable.”

Beibei Liu

Beibei Liu, research professor at Zhejiang University.

Raymond and Jacobsen then contacted Liu, who pioneered this rebound effect idea through extensive simulations of gas disks and large exoplanets — planets from other solar systems — that orbit near their stars.

“The situation in our solar system is slightly different because Jupiter, Saturn, Uranus and Neptune are spread out in wider orbits,” Liu said. “After a few iterations of brainstorming sessions, we realized that the problem could be solved if the gas disk dissipated from within.”

The team found that this inside-out dissipation provided a natural trigger for Nice’s model instability, Raymond said.

“We ended up strengthening the Nice model rather than destroying it,” he said. “It was a fun illustration of testing our preconceptions and following the results wherever they lead.”

With the new trigger, the image at the beginning of the instability is the same. There is still a rising sun surrounded by a cloud of gas and dust. A handful of young gas giants orbit the star in sharp, compact orbits through this cloud.

“All solar systems are formed in a disk of gas and dust. It’s a natural byproduct of star formation,” Jacobson said. “But when the sun ignites and begins to burn its nuclear fuel, it generates sunlight, heating the disk and eventually blowing it from the inside out.”

This created a growing hole in the gas cloud, centered on the sun. As the hole grew, its edge crossed each of the gas giant orbits. This transition leads to the required instability of the giant planet with very high probability, according to the team’s computer simulations. The process of moving these large planets to their current orbits is also moving rapidly relative to the original Nice Model timeline of hundreds of millions of years.

“The instability occurs as soon as the sun’s gaseous disk dissipates, limited to a few million years to 10 million years after the birth of the solar system,” Liu said.

The new trigger also leads to the mixing of materials from the outer solar system and the inner solar system. Earth’s geochemistry suggests that such mixing must have occurred while our planet was still forming.

“This process will really shake up the inner solar system and Earth can grow from this,” Jacobson said. “It’s pretty consistent with the observations.” Exploring the link between instability and the formation of the Earth is a future work topic for the group.

Finally, the team’s new explanation also applies to other solar systems in our galaxy where scientists have observed gas giants orbiting their stars in configurations similar to those we see in ours.

“We are just one example of a solar system in our galaxy,” Jacobson said. “What we show is that the instability happened in a different, more universal and more consistent way.”

Planet 9 seen from space

Although the team’s paper doesn’t highlight it, Jacobson said the work has implications for one of the most popular and sometimes heated debates about our solar system: How many planets does it have?

Currently, the answer is eight, but Nice’s model turns out to work slightly better when the early solar system had five gas giants instead of four. Unfortunately, according to the model, this extra planet was ejected from our solar system during the instability, helping the remaining gas giants find their orbits.

Artist's illustration of the ninth planet

An artist’s conception of Planet 9. Credit: ESO/Tom Ruen/nagualdesign

In 2015, however, Caltech researchers found evidence that there could still be an undiscovered planet on the outskirts of the solar system about 50 billion kilometers from the sun, about 47 billion kilometers further than Neptune.

There is still no concrete proof that this hypothetical planet – dubbed Planet X or Planet 9 – or the “extra” planet in the Nice model actually exists. But, if they do, could they be one and the same?

Jacobson and his colleagues couldn’t answer that question directly with their simulations, but they could do the next best thing. Knowing that their instability trigger correctly reproduces the current picture of our solar system, they could test whether their model works best by starting with four or five gas giants.

“For us, the result was very similar whether you start with four or five,” Jacobson said. “If you start with five, you’re more likely to finish with four. But if you start with four, the orbits end up matching better.

Either way, humanity should soon have an answer. The Vera Rubin Observatory, which should be operational by the end of 2023, should be able to spot Planet 9 if it is there.

“Planet 9 is super controversial, so we didn’t highlight it in the paper,” Jacobson said, “But we like to talk about it with the public.”

It’s a reminder that our solar system is a dynamic place, still full of mysteries and discoveries to be made.

Reference: “Early Solar System Instability Triggered by Gas Disk Scattering” by Beibei Liu, Sean N. Raymond, and Seth A. Jacobson, April 27, 2022, Nature.
DOI: 10.1038/s41586-022-04535-1

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