Does it portend an unknown planet?
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 did the gas giants – Jupiter, Saturn, Uranus and Neptune – end up where they are, orbiting the sun the way they do?
The research also has implications for how terrestrial planets such as Earth formed and the possibility of a fifth gas giant 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.”
The research, published in the journal Nature on April 27, offers an explanation of what happened to gas giants in other solar systems and ours.
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.
In the late 20th century, scientists began to believe that gas giants initially orbited the sun in sharp, compact, regularly spaced orbits. Jupiter, Saturn and the others, however, have long since settled into relatively oblong, skewed, and spread out orbits.
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.
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.”
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 major 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 the 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 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 ejected from our solar system during the instability, helping the remaining gas giants find their orbits.
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.
Simulations suggest a planet the size of Earth or Mars could lurk beyond Neptune
Beibei Liu et al, Early solar system instability triggered by gas disk dispersion, Nature (2022). DOI: 10.1038/s41586-022-04535-1
University of Michigan
Instability at the start of the solar system: does it portend an unknown planet? (2022, April 27)
retrieved 27 April 2022
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