Instability at the beginning of the solar system: Does it herald an undiscovered planet?

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

Michigan State University’s Seth Jacobson and his Chinese and French colleagues have unveiled a new theory that could help solve a galactic mystery regarding the evolution of our solar system. Specifically, how did the gas giants – Jupiter, Saturn, Uranus and Neptune – end up where they are, orbiting the sun?

This research also has implications for how terrestrial planets such as Earth formed and whether a fifth gas giant lurks 80 billion miles in the distance.

“Our solar system has not always looked the way 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 Science. “But we can understand what happened.”

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

It’s a beautiful model

Stars are born from massive, swirling clouds of cosmic gas and dust. After our sun was lit, the early solar system was still filled with a disk of primordial gas that played a vital role in the formation and evolution of planets, including gas giants.

At the end of the 20th century, scientists began to believe that gas giants initially orbited the sun in sharp, compact, regularly spaced orbits. Jupiter, Saturn, and the other planets, on the other hand, have long adopted relatively oblong, skew, and spread out orbits.

The question that researchers are asking today is therefore “why?” “.

In 2005, an international team of scientists offered an answer to this question in a trio of landmark Nature articles. The solution was originally developed in Nice, France, and is known as the Nice Model. She posits that there was instability between these planets, a chaotic set of gravitational interactions that ultimately set them on their current path.

“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 take place hundreds of millions of years after the dispersal of the disk of primordial gas that gave rise to the solar system. But more recent evidence, including some found in moon rocks recovered by Apollo missions, suggests it happened more quickly. It also raises new questions about how the inner solar system that houses Earth has evolved.

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

“I think our new idea could really ease a lot of tension in the field, because what we’ve come up with is a very

An artistic depiction shows a hypothetical early solar system in which a young star is making its way through the gas and dust left over from its formation. This cleaning action would affect the orbits of the gas giants orbiting the star. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

The new primer

The idea grew out of 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 instability pushing 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 perhaps expand by a ‘bounce’ effect as the disk dissipates, perhaps without ever becoming unstable. »

Raymond and Jacobsen then turned to Liu, who pioneered this rebound effect idea through extensive simulations of gas disks and large exoplanets – planets in other solar systems – that orbit near their star.

“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 the inside out. »

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

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

With the new trigger, the image of the beginning of the instability is the same. There is always 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 disc 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 out from within. »

This created a growing hole in the gas cloud, centered on the sun. As the hole grew, its edge swept the orbit of each of the gas giants. This transition drives the required instability of giant planets with very high probability, according to the team’s computer simulations. The process of moving these large planets to their current orbits is also fast compared to the original Nice Model timeline, which called for hundreds of millions of years.

“The instability occurs at the start of the dissipation of the sun’s gaseous disk, constrained to be between a few million years and 10 million years after the birth of the solar system,” Liu said.

This new trigger also causes materials from the outer solar system to mix with the inner solar system. Earth’s geochemistry suggests that such mixing must have occurred while our planet was still forming.

“This process is really going to stir up the inner solar system and the Earth can grow out of that,” Jacobson said. “This is quite 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 dwell on that point, Jacobson said the work has implications for one of the most popular and sometimes heated debates about our solar system: How many planets are there? he ?

Currently the answer is eight, but it turns out that Nice’s model worked slightly better when the early solar system had five gas giants instead of four. Unfortunately, according to the model, this extra planet was knocked out of our solar system by a hammer blow during the instability, helping the remaining gas giants find their orbit.

In 2015, however, researchers at Caltech found evidence that there may still be an undiscovered planet that lies on the outskirts of the solar system, some 80 billion kilometers from the sun, or 47 billion kilometers from more than Neptune.

There is still no concrete proof of the existence of this hypothetical planet – dubbed Planet X or Planet 9 – or the “additional” planet of the Nice model. But, if they exist, could they be one and the same planet?

Jacobson and his colleagues couldn’t directly answer this question with their simulations, but they were able to do the closest thing to it. 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 end up 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 exists.

“Planet 9 is super controversial, so we didn’t push that point in the paper,” Jacobson said, “but we like to talk about it with the public. »