How Are Asteroids Distributed Throughout the Solar System?

by Carson
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Asteroids are everywhere. They come in all shapes and sizes and orbit the Sun in various regions throughout the Solar System. But how are asteroids distributed here? Let’s find out in this article.

Distribution Chart

First, we’ll visualize the orbital elements with matplotlib, based on data from the JPL Small-Body Database.

distribution of semi-major axis and eccentricity of asteroids in the inner solar system
Figure 1: Distribution of a randomly selected sample of 20000 numbered asteroids whose semi-major axis is less than 6 AU.
Figure 2: Distribution of all numbered asteroids whose semi-major axis is between 6 and 30 AU
Figure 3: Distribution of all numbered asteroids whose semi-major axis is between 30 and 60 AU

We’re going to explain these graphs, one by one, in the following sections of this article.

Figure 1: The Inner Solar System

A version of figure 1, annotated with the asteroid categories

The Main Asteroid Belt

As you can see from figure 1, there is a very dense region of asteroids marked in blue. This is the asteroid belt, which is a relatively stable region of asteroids between the orbits of Mars and Jupiter. Even though there are already many objects, the asteroid belt should have been three or four orders of magnitude more massive (Clement et al., 2018). In fact, this region is strongly depleted, which is why the asteroids do not form into a planet — they are not massive enough. The precise evolution of this process is still an active area of research, but it is known that it mainly involves the powerful perturbations of Jupiter and Saturn.

The Hungaria Asteroids

Inside the main asteroid belt is another population of asteroids, the Hungaria asteroids. On average, they are closer than 2 AU to the Sun, and orbit inside the 4:1 resonance with Jupiter. In fact, they have eccentricities of less than 0.18 and a fairly high inclination of between 16 and 34 degrees. (Forgacs-Dajka et al, 2021). This prevents close encounters with Mars and reduces in-plane perturbations, keeping them on a stable orbit around the Sun.

The near-Earth Asteroids

Inside the Hungaria asteroids are the asteroids on planet-crossing orbits, which are the near-Earth asteroids. The number of asteroids there is very small compared to other regions of the inner Solar System. This is because the orbits there are highly unstable, in which close approaches to the planets perturb them around and ultimately cause the asteroid to escape the near-Earth space.

The Cybele and Hilda asteroids

Looking outside the main asteroid belt, there are the Cybele asteroids. They orbit outside the 2:1 resonance with Jupiter, meaning it takes more than half a Jupiter year to orbit the Sun once. They are also interior to the 3:2 resonance with Jupiter, taking no more than two-thirds of a Jupiter year to make one revolution around the Sun.

Interestingly, the 3:2 resonance, the outer border of the Cybele asteroids, is populated by a large group of asteroids — the Hilda asteroids. They orbit the Sun with a semi-major axis between 3.7 and 4.2 AU, and an eccentricity below 0.3. (Ohtsuka et al, 2008). According to the JPL Small-Body Database, there are more than 5600 asteroids there, of which more than 2500 are numbered. This starkly contrasts with other resonances (such as the 2:1 and 3:1 resonances with Jupiter), which are almost entirely devoid of asteroids. They are called the Kirkwood gaps, and we’ll discuss that in more detail later.

The Jupiter Trojans

Outside the Hilda asteroids are an empty region almost devoid of asteroids, followed by another large clump at about 5.2 AU from the Sun. They are the Jupiter trojans that orbit the Sun around the L4 and L5 points of the Sun-Jupiter system. Specifically, since their orbits are elliptical and inclined, they appear to form an ellipse in a rotating frame with Jupiter held stationary. The center of the ellipse also moves around because its orbital period is not the same as that of Jupiter. Instead, the trojan asteroids stay in sync with Jupiter by the gravitational perturbations of Jupiter, making them librate around the L4 and L5 points. It’s not easy to describe this behavior with words alone, so let’s show an animation here.

Animation of 2010 TK7 around Sun and Earth
Animation of the asteroid 2010 TK7, an Earth trojan, around a rotating frame where the Earth is held stationary. The Sun is not shown.
The Jupiter trojans have a similar relationship with Jupiter as well.
Data source: JPL Horizons
Blue: Earth; Magenta: 2010 TK7

Kirkwood Gaps

If you carefully inspect the distribution of the main-belt asteroids, you will notice a few gaps where very few asteroids reside. They are the Kirkwood gaps, which are the main resonances with Jupiter. For example, prominent gaps are found in the 2:1, 3:1, 5:2, and 7:3 resonances with Jupiter. In these regions, frequent perturbations by Jupiter increase the asteroid’s eccentricity until the orbit crosses that of an inner planet. A close approach then ejects the asteroid out of the resonances, causing a dip in the asteroid distribution within the gaps.

On the other hand, we can also see large clumps of asteroids around more stable resonances, such as the 1:1 and 3:2 resonances. These are the Jupiter trojans and Hilda asteroids, which are phase-protected by the resonance to avoid drastic perturbations.

Additionally, secular resonances can create a similar effect to the Kirkwood gaps. They occur when the asteroid’s orbital precession comes in sync with that of a planet, causing periodic perturbations that increase the eccentricity of the asteroid. Unlike the Kirkwood gaps, though, they don’t just depend on the semi-major axes of the asteroids. Instead, they also depend on other factors, such as the eccentricity and inclination of the orbit.

Distribution of 20000 randomly selected numbered asteroids with semi-major axes between 2.0 to 3.5 AU and eccentricities < 0.6
The red lines mark the Kirkwood gaps, and the text above them marks the resonances in each of these gaps.
Data source: JPL Small-Body Database

Figure 2: The Outer Solar System

A version of figure 2, annotated with the asteroid categories

In the outer Solar System, there is only one dynamical category of asteroids — the centaurs. As you can see, very few of them are in the Solar System. According to the JPL Small-Body Database, only 916 asteroids are classified as centaurs, of which 604 are numbered.

The reason for the depletion in this region is similar to that of the near-Earth asteroids, which is that the orbits of centaurs often cross that of the planets. The giant planets there can exert powerful perturbing forces on asteroids’ orbits, often exciting their eccentricities or even ejecting them out of the centaur region.

Most centaurs are icy in composition, meaning they likely formed in the outer Solar System, within the Kuiper Belt region, and then transited into the planet-crossing orbits with a Neptune encounter. However, the other way round is also a likely scenario, as outer main-belt asteroids can be pushed to the centaur region with a Jupiter encounter. While we have not spotted a Neptune encounter due to the long orbital periods involved, we have observed at least one such Jupiter encounter, which pushed the object 39P/Oterma into the centaur region.

Figure 3: The Trans-Neptunian Region

A version of figure 3, annotated with the asteroid categories

Beyond the orbit of Neptune, there is a region called the Kuiper Belt, which are filled with trans-Neptunian objects. As you can see, there are two significant clumps there. They are the plutinos and the “kernel”, respectively.

The plutinos orbit the Sun in the 2:3 resonance with Neptune, with an average distance of about 39 AU. They orbit the Sun twice every three Neptune years, so even if their orbits cross that of Neptune, they are phase-protected by the resonance. The dwarf planet Pluto, once the ninth planet, is inside this zone. Therefore, the asteroid group is named after Pluto and is thus called the plutinos.

The other condensation farther out the Kuiper Belt is more interesting and is still an active area of research today. This is nicknamed the “kernel” by researchers (not to be confused with operating system kernels) and consists of asteroids whose semi-major axis lies around 44 AU, eccentricities of less than 0.05, and inclinations of less than 5 degrees.

One of the explanations involves a “jumping Neptune” scenario, where Neptune migrated back and forth during the formation of the Solar System. According to this theory, this trapped some planetesimals in the 1:2 resonance with Neptune, and once Neptune leaves the resonance zone, the asteroids are left as they were. (Nesvorny, 2015).

Conclusion

In this article, we have explained the distribution of asteroids in three parts:

  1. The inner Solar System
  2. The outer Solar System
  3. The trans-Neptunian region

In the inner Solar System, there is an asteroid belt with Kirkwood gaps, Cybele and Hilda asteroids and Jupiter trojans outside the main-belt, and the Hungaria and near-Earth asteroids inside the asteroid belt. In the outer Solar System, there are planet-crossing centaurs on very unstable orbits. Lastly, in the Kuiper Belt, there are the plutinos and the asteroids in the mysterious “kernel”.

If you would like more content from this article, please leave your suggestions in the comment box below. Also, if you are curious and would like more information, feel free to read the research papers in the references below.

References

  1. Clement et al. (2018, November 22). “Excitation and depletion of the asteroid belt in the early instability scenario”. Retrieved January 30, 2023, from https://arxiv.org/abs/1811.07916
  2. Forgacs-Dajka et al. (2021, October 22). “A survey on Hungaria asteroids involved in mean motion resonances with Mars”. Retrieved January 30, 2023, from https://arxiv.org/abs/2110.11745
  3. Ohtsuka et al. (2008, August 16). “Quasi-Hilda Comet 147P/Kushida-Muramatsu: Another long temporary satellite capture by Jupiter.” Retrieved January 30, 2023, from https://arxiv.org/abs/0808.2277
  4. Nesvorny, D. (2015, June 19). “Jumping Neptune Can Explain the Kuiper Belt Kernel.” Retrieved January 31, 2023, from https://arxiv.org/abs/1506.06019

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