Seth Jacobson of Michigan State University and colleagues in China and France have revealed a new theory that could help solve a galactic puzzle about how our solar system evolved. Specifically, how did the gas giants – Jupiter, Saturn, Uranus, and Neptune – end up orbiting the Sun as they do?
Research also has implications for how terrestrial planets Like Earth formed, the possibility of a fifth gas giant lurks 50 billion miles in the distance.
“for us Solar System You didn’t always look the way you do today. “Over the course of their history, the orbits of the planets have changed drastically. But we can find out what happened,” said Jacobson, assistant professor in the Department of Earth and Environmental Sciences at the College of Natural Sciences.
Research published in the journal temper nature On April 27th, an explanation of what happened gas giants In other solar systems and ours.
It’s a beautiful model
Stars are born from huge clouds of cosmic dust and gas. Once our sun ignited, the early solar system was still filled with a primitive disk of gas that played an essential 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 neat, compact, and evenly spaced orbits. But Jupiter, Saturn, and others have long settled in relatively elongated, skewed and scattered orbits.
So the question for researchers now is “why?”
In 2005, an international team of scientists proposed an answer to this question in three milestones temper nature Leaves. The solution was originally developed in Nice, France and is known as the Nice Model. Presumably there is instability between these planets, a chaotic set of gravitational interactions that ultimately set them on their current paths.
“This was a fundamental shift in the way people thought in the early solar system,” Jacobson said.
The Ness model is still a leading interpretation, but over the past 17 years, scientists have found new questions to ask about the causes of the instability of the Ness model.
For example, it was originally thought that the gas giant’s instability occurred hundreds of millions of years after the dispersal of the primordial gas disk that gave birth to the solar system. But more recent evidence, including some found in moon rocks recovered by the Apollo missions, suggests this happened more quickly. This also raises new questions about how the inner solar system that represents Earth evolved.
Working with Beibei Liu of Zhejiang University in China and Shawn Raymond of Bordeaux University in France, Jacobson helped find a solution regarding how the instability began. The team suggested a new trigger.
“I think our new idea can relieve a lot of the tensions in this area because what we proposed is a very natural answer when the giant planet instability occurred,” Jacobson said.
The idea began with a conversation that Raymond and Jacobsen had in 2019. They theorized that the gas giants might have been set on their current paths because of how the primordial gas disk evaporated. This could explain how planets proliferated much earlier in the evolution of the Solar System than the Ness model originally assumed 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 came up with the idea that giant planets It can be propagated by a ‘rebound’ effect as the disc dissipates, possibly without instability at all.”
Next, Raymond and Jacobsen reached out to Liu, who pioneered this idea of a bounce-back effect through extensive simulations of gas disks and large exoplanets — planets in other solar systems — orbiting close to their stars.
“The situation in our solar system is a little different because Jupiter, Saturn, Uranus and Neptune are distributed in wider orbits,” Liu said. “After repeated brainstorming sessions, we realized that the problem could be resolved if the gas disc dissipated from the inside out.”
Raymond said the team found that this inside-out dissipation provides a natural incentive for the instability of the Nice model.
“We ended up promoting the Nice model rather than destroying it,” he said. “This was a fun illustration to test our preconceptions and follow the results wherever they are.”
With the new launcher, the image at the beginning of the instability looks the same. There is still a nascent sun surrounded by a cloud of gas and dust. A handful of young gas giants orbit the star in neat, compact orbits through that cloud.
“All solar systems form in a disk of gas and dust,” Jacobson said. “It’s a natural byproduct of how stars form.” “But when the sun lights up and starts burning its nuclear fuel, it generates sunlight, heats the disk, and eventually blows it away from the inside out.”
This created a growing hole in the sun-centered cloud of gas. As the hole grew, its edge swept through each of the gas giants’ orbits. This transition leads to the much needed instability of the giant planet with a very high probability, according to a computer simulation conducted by the team. The process of moving these large planets into their current orbits is also moving quickly compared to the original Ness model timeline of hundreds of millions of years.
“The instability occurs early when the gaseous disk of the Sun dissipates, and remains within a few million years to 10 million years after the birth of the solar system,” Liu said.
The new trigger also mixes materials from the outer solar system and the inner solar system. Earth’s geochemistry suggests that such mixing should occur while our planet is still in the midst of the formation process.
“This process would really excite the inner solar system and the Earth could grow from that,” Jacobson said. “This is very much in agreement with the notes.” Exploring the relationship between instability and land formation is the topic of the group’s future work.
Finally, the team’s new interpretation also applies to other solar systems in our galaxy where scientists have observed gas giants orbiting their stars in configurations like what we see in our group.
“We are just one example of a solar system in our galaxy,” Jacobson said. “What we are showing is that the instability happened in a different way, a more global, more consistent way.”
Planet 9 from outer space
Although the team’s paper doesn’t emphasize this, 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 the Nice model did a little better when it was early solar system Five gas giants instead of four. Unfortunately, according to the model, this extra planet was shed from our solar system during the instability, helping the remaining gas giants find their orbits.
However, in 2015, Caltech researchers found evidence that there may be a hitherto undiscovered planet orbiting the fringes of the solar system about 50 billion miles from the sun, about 47 billion miles from Neptune.
There is still no concrete evidence for the existence of this hypothetical planet – nicknamed Planet X or Planet 9 – or the “extra” planet of the Nice model. But, if they did, could they be one and the same?
Jacobson and his colleagues couldn’t answer that question directly with their simulations, but they can do the next best thing. knowledge Instability The trigger correctly reproduces the current picture of our solar system, and they can test whether their model works best starting with four or five gas giants.
“For us, the result was very similar if you started with four or five,” Jacobson said. “If you start with five, you are more likely to end up with four. But if you start with four, you end up matching the orbitals better.”
Either way, humanity should have an answer soon. The Vera Rubin Observatory, due to be operational by the end of 2023, should be able to detect Planet 9 if it is present there.
“Planet 9 is very controversial, so we didn’t stress it in the paper, but we love talking about it with the public,” Jacobson said.
It’s a reminder that our solar system is a dynamic place, still full of mysteries and discoveries waiting to be discovered.
Beibei Liu et al, Early Solar System instability resulting from gaseous disk scattering, temper nature (2022). DOI: 10.1038 / s41586-022-04535-1
Michigan State University
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