How Have Instabilities Shaped Our Solar System?

The Solar System might look stable from the outside and will likely remain so for billions of years. But do you know that many different instabilities have played a role in shaping our familiar Solar System? Let’s explore those events in this article.

We owe our existence to a gravitational instability that happened 4.5 billion years ago. At that time, the material near the Solar nebula was unevenly distributed. This could be caused by some outside events, such as a supernova explosion nearby. Some regions became better at attracting nearby material than others as they carry more mass. This triggers an instability, and eventually, the asymmetry is strong enough for accretion to take place.

As the center of mass quickly attracts material, it collects so much matter that hydrogen fusion begins in the core. This forms a star. The remnants around that star then coalesce into planets, moons, and asteroids. This theory is the well-known nebular hypothesis, and we have mentioned it many times on our website. But researchers were still fascinated. They wanted to learn about the mysteries behind how the Solar System settled into its current configuration. Let’s talk about it in the following sections of this article.

If you are curious, please visit this page for more information about star formation.

Given that planets are in nearly circular and coplanar orbits (i.e., their eccentricity and inclination are close to zero), it would be reasonable to expect that they all formed harmoniously from the nebular hypothesis. When the stellar leftovers accrete within a disk, the materials stay in these neat orbits, so you might think the planets have been there this way from their formation until the present. But scientists had seen some problems with this model.

Specifically, there are some quirks in the Solar System that we can’t easily explain if everything has been neat and in order since the beginning. For example, the Moon has been hypothesized to form via an interplanetary collision — a Mars-sized body colliding with the Earth. But if everything had been in a relatively stable state, these collisions couldn’t happen. Moreover, the mass of Mars is anomalously tiny, only ~10% of that of Earth. But traditional simple models produce replicas of Mars that are about as massive, if not more massive, than Earth. Furthermore, the objects in the asteroid belt are “excited” — they have high eccentricities and inclinations in their orbits. There are also much fewer asteroids than we can ever expect in a stable model.

These problems were a clear pointer to the fact that the Solar System was unstable and experienced massive instabilities when the planets are still forming. Let’s explore the theories and the problems in the rest of the article.

Over the years, researchers have been searching for the best possible model that creates a replica of our Solar System. Eventually, two main theories came out, addressing different problems for the formation of the planetary system. One is the Nice Model, and the other is the Grand Tack hypothesis. Let’s talk about them one by one in the following sections.

The Nice Model depicts a giant planet instability at the beginning of the Solar System. In the early Solar System, the particles in the protoplanetary disk formed spiral density waves around planets, deflecting them via gravity assists. Slowly and steadily, these particles change the planet’s velocity vectors and, thus, its orbital shape and size. This is known as planetary migration, which is the key to triggering the planetary instability in the Nice Model.

After a few hundred million years of slow migration, Jupiter and Saturn crossed the 2:1 resonance. That means, when Jupiter orbits the Sun twice, Saturn orbits once. This resonance caused repeated gravitational perturbations between the two planets, triggering an instability. In the end, Uranus and Neptune scattered outward into their current locations. The icy asteroids, which were initially farther than Neptune from the Sun, were also scattered everywhere, becoming the members of the Kuiper Belt, the centaurs, and the Jupiter trojans.

Later, another model of the giant planet instability was released. This is the Grand Tack hypothesis, and it was theorized to occur just a few million years after the Solar System started forming, and when the giant planets hadn’t even been fully assembled yet. Jupiter, the first planet to form, initially formed at around 3.5 astronomical units (AU) from the Sun. But as the planet migrated, it reached a location of about 1.5 AU from the Sun. With its powerful gravity, it started to suck up nearby material. This truncated the planet-forming region at about 1 AU, scattering the asteroids in the asteroid belt, and decreasing the amount of material that can go into Mars.

But how did it leave the 1.5-AU zone and reach its current orbit, 5.2 AU from the Sun? As Saturn was forming, it also migrated inward. As Saturn has a lower mass, it migrated faster than Jupiter. Eventually, Jupiter and Saturn reached a 3:2 resonance, where Jupiter orbits three times as Saturn orbits twice. The instabilities that resulted from the resonance pushed the two planets back to where they are today.

As good as the Nice Model and the Grand Tack scenarios are, there are still problems that we haven’t resolved yet. For example, the formation scenarios might require low-probability events that are difficult to replicate in actual scenarios. Or they might even fail to reproduce some constraints in our Solar System. Therefore, as researchers can access more computing resources to test different cases, they have improved the theories in the past few years.

For example, recent evidence suggested that an early instability was more likely instead of the late instability suggested by the Nice Model and in explaining the Late Heavy Bombardment. (de Sousa et al, 2019; Deienno et al, 2017; Nesvorny et al, 2021). In fact, a research paper from 2022 suggested that the trigger was the inside-out dispersion of the gas disk just a few million years after the Solar System began forming. (Liu et al, 2022).

They have also explored the ways that the instability should happen. For example, a 2012 research paper explored large patches of the parameter space of the instabilities, including adjusting the resonances and the number of planets. In the end, they found that an instability involving five planets works best, and the best combination of resonances is a 3:2 chain between all five planets. In fact, at least one of the simulations managed to satisfy all constraints imposed in the model. Note that there are still some problems with this model, which might require low-probability events to resolve, but this mode of instability is one of the best ones that have been proposed in recent research. (Nesvorny and Morbidelli, 2012).

As good as the aforementioned model was, replicating the Solar System architecture was still a huge challenge, even under relaxed constraints. In fact, of the four constraints proposed in the paper, there is only a 4% chance that this model meets three of them. While progress was made in the last ten years, a model that can consistently emulate the Solar System most of the time remains elusive. The existing models might require low-probability events to work, or it might not be feasible to obtain the initial conditions at all.

There are alternate models to explain the oddities of the Solar System. For example, to address the low mass of the asteroid belt and of Mars, some researchers have explained that the corresponding regions of the protoplanetary disk might just be almost empty from the beginning. But these theories usually only go for one problem at a time. And thus to entirely explain the Solar System using these models, many unusual conditions might need to be satisfied, which makes it even more improbable than an instability that explains everything.

What does this mean? There are two possibilities: 1) researchers might one day develop new models that work much better than the current ones or 2) the Solar System’s general architecture requires a low-probability event. If the latter is true, it will be hard to know how the planets are put into place.

In this article, we’ve mentioned how instabilities have shaped the Solar System. Without an instability at the beginning, the Sun and the planetary system might not form in the first place. And without the turbulent action of the planets when they were still developing, it’s hard to get the current architecture of the Solar System that we see today. If you are interested in the topic and would like to learn more about them, refer to the articles and research papers in the references below.

1. de Sousa et al. (2019, December 19). “Dynamical evidence for an early giant planet instability”. Retrieved June 22, 2023, from https://arxiv.org/abs/1912.10879
2. Deienno et al. (2017, February 7). “Constraining the giant planets’ initial configuration from their evolution: implications for the timing of the planetary instability”. Retrieved June 22, 2023, from https://arxiv.org/abs/1702.02094
3. Nesvorny et al. (2021, January 6). “The Role of Early Giant-planet Instability in Terrestrial Planet Formation”. Retrieved June 22, 2023, from https://iopscience.iop.org/article/10.3847/1538-3881/abc8ef
4. Liu et al. (2022, May 4). “Early Solar System instability triggered by dispersal of the gaseous disk”. Retrieved June 22, 2023, from https://arxiv.org/abs/2205.02026
5. Nesvorny, D., Morbidelli, A. (2012, September 13). “Statistical Study of the Early Solar System’s Instability with 4, 5 and 6 Giant Planets”. Retrieved June 22, 2023, from https://iopscience.iop.org/article/10.1088/0004-6256/144/4/117
6. (2018, April 24). “The Nice Model” . Retrieved June 22, 2023, from https://lucy.swri.edu/2018/04/24/Nice-Model.html
7. Kruesi, L. (2012, September 24). “Understanding the Nice model”. Retrieved June 22, 2023, from https://www.astronomy.com/science/understanding-the-nice-model/
8. (2011, August 19). “Jupiter’s “Grand Tack” Reshaped the Solar System”. Retrieved June 22, 2023, from https://astrobiology.nasa.gov/news/jupiters-grand-tack-reshaped-the-solar-system/