The instability at the beginning of the solar system

Date:
April 27, 2022
Source:
Michigan State University
Summary:
Michigan State University's Seth Jacobson and colleagues in China
and France have unveiled a new theory that could help solve a
galactic mystery of how our solar system evolved. Specifically,
how did the gas giants -- Jupiter, Saturn, Uranus and Neptune --
end up where they are, orbiting the sun like they do? The research
also has implications for how terrestrial planets such as Earth
were formed and the possibility that a fifth gas giant lurks 50
billion miles out into the distance.



FULL STORY ========================================================================== Michigan State University's Seth Jacobson and colleagues in China and
France have unveiled a new theory that could help solve a galactic mystery
of how our solar system evolved. Specifically, how did the gas giants -- Jupiter, Saturn, Uranus and Neptune -- end up where they are, orbiting
the sun like they do?

==========================================================================
The research also has implications for how terrestrial planets such as
Earth were formed and the possibility that a fifth gas giant lurks 50
billion miles out into the distance.

"Our solar system hasn't always looked the way that it does today. Over
its history, the orbits of the planets have changed radically," said
Jacobson, an assistant professor in the College of Natural Science's
Department of Earth and Environmental Sciences. "But we can figure out
what's happened." The research, published in the journal Natureon April
27, offers an explanation for what happened to gas giants in other solar systems and ours.

It's a Nice model Stars are born from massive, swirling clouds of cosmic
gas and dust. Once our sun ignited, the early solar system was still
filled with a primordial disk of gas that played an integral role in
the formation and evolution of the planets, including the gas giants.



==========================================================================
In the late 20th century, scientists began to believe that the gas giants initially circled the sun in neat, compact, evenly-spaced orbits. Jupiter, Saturn and the others, however, have long settled into orbits that are relatively oblong, askew and spread out.

So the question for researchers now is, why? In 2005, an international
team of scientists proposed an answer to that question in a trio
of landmark Nature papers. The solution was originally developed in
Nice, France and is known as the Nice model. It posits that there was
an instability among these planets, a chaotic set of gravitational
interactions that ultimately set them on their current paths.

"This was a tectonic shift in how people thought about the early solar
system," Jacobson said.

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



==========================================================================
For example, it was originally thought that the gas giant instability
took place hundreds of millions of years after the dispersal of that
primordial gas disk that birthed the solar system. But newer evidence, including some found in moon rocks retrieved by the Apollo missions,
suggests it happened more quickly.

That also raises new questions about how the interior solar system that's
home to Earth evolved.

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

"I think our new idea could really relax a lot of tensions in the field
because what we've proposed is a very natural answer to when did the
giant planet instability occur," Jacobson said.

The new trigger The idea started with a conversation Raymond and Jacobsen
had back in 2019.

They theorized the gas giants may have been set on their current paths
because of how the primordial gas disk evaporated. That could explain
how the planets spread out much earlier in the solar system's evolution
than the Nice model originally posited and perhaps even without the
instability to push them there.

"We wondered whether the Nice model was really necessary to explain the
solar system," Raymond said. "We came up with the idea that the giant
planets could possibly spread out by a 'rebound' effect as the disk
dissipated, perhaps without ever going unstable." Raymond and Jacobsen
then reached out to Liu, who pioneered this rebound effect idea through extensive simulations of gas disks and large exoplanets -- planets in
other solar systems -- that orbit close to their stars.

"The situation in our solar system is slightly different because
Jupiter, Saturn, Uranus and Neptune are distributed on wider orbits,"
Liu said. "After a few iterations of brainstorm sessions, we became aware
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 the Nice model instability, Raymond said.

"We ended up strengthening the Nice model rather than destroying it,"
he said.

"This was a fun illustration of testing our preconceived ideas and
following the results wherever they lead." With the new trigger, the
picture at the beginning of the instability looks the same. There's
still a nascent sun surrounded by a cloud of gas and dust. A handful of
young gas giants revolve around the star in neat, compact orbits through
that cloud.

"All solar systems are formed in a disk of gas and dust. It's a natural byproduct of how stars form," Jacobson said. "But as the sun turns on and starts burning its nuclear fuel, it generates sunlight, heating up the
disk and eventually blowing it away from the inside out." This created
a growing hole in the cloud of gas, centered on the sun. As the hole
grew, its edge swept through each of the gas giants' orbits. This
transition leads to the requisite giant planet instability with very
high probability, according to the team's computer simulations. The
process of shifting these large planets into their current orbits also
moves fast compared with Nice model's original timeline of hundreds of
millions of years.

"The instability occurs early as the sun's gaseous disk dissipated,
constrained to be within 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 material from the outer solar system and the inner solar system. The Earth's geochemistry suggests
that such a mixing needed to happen while our planet is still in the
middle of forming.

"This process is really going to stir up the inner solar system and
Earth can grow from that," Jacobson said. "That is pretty consistent
with observations." Exploring the connection between the instability
and Earth's formation is a subject of future work for the group.

Lastly, the team's new explanation also holds for other solar systems
in our galaxy where scientists have observed gas giants orbiting their
stars in configurations like what we see in our own.

"We're just one example of a solar system in our galaxy," Jacobson
said. "What we're showing is that the instability occurred in a different
way, one that's more universal and more consistent." 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 occasionally
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
works slightly better when the early solar system had five gas giants
instead of four. Sadly, according to the model, that extra planet was hammer-thrown from our solar system during the instability, which helps
the remaining gas giants find their orbits.

In 2015, however, Caltech researchers found evidence that there may yet be
an undiscovered planet tooling around the outskirts of the solar system
some 50 billion miles from the sun, about 47 billion miles farther out
than Neptune.

There's still no concrete proof that this hypothetical planet --
nicknamed Planet X or Planet 9 -- or the Nice model's "extra" planet
actually exist. 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 their instability trigger correctly reproduces the current picture of our solar system, they could test whether their model works better starting with
four or five gas giants.

"For us, the outcome was very similar if 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 have an answer soon. The Vera
Rubin Observatory, scheduled to be operational by the end of 2023,
should be able to spot Planet 9 if it is out there.

"Planet 9 is super controversial, so we didn't stress it in the paper," Jacobson said, "But we do 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 waiting to be made.


========================================================================== Story Source: Materials provided by Michigan_State_University. Original
written by Matt Davenport. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Beibei Liu, Sean N. Raymond, Seth A. Jacobson. Early Solar System
instability triggered by dispersal of the gaseous disk. Nature,
2022; 604 (7907): 643 DOI: 10.1038/s41586-022-04535-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220427154058.htm

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