Mysteries of the asteroid belt

Most of the asteroids in the Solar System are located between 2.1 and 3.3 AU from the Sun and constitute a population called the main belt (1 AU = one astronomical unit, corresponding to the mean distance between Earth and the Sun, or roughly 150 million kilometers). The main belt is outside Mars that is located at 1.5 AU, and interior to Jupiter that is 5.2 AU from the Sun. This post is about two of the largest mysteries of the main belt – why does it contain such a small mass and why are the orbits of asteroids around the Sun so extreme?


The Asteroid (21) Lutetia with a diameter of about 100 kilometers, imaged with the camera OSIRIS onboard the European spacecraft Rosetta.
Original image:

The total mass of main belt asteroids is only about 0.0005 Earth masses, which means that one would need about 2000 asteroid belts to build a planet of Earth’s size. If we also consider that Mars only has 0.1 Earth masses, it means that the region between about 1.3-5.0 AU from the Sun only contains roughly a tenth of an Earth mass, although there should have been several Earth masses there. Where has all of this mass gone? Furthermore, the orbits of main belt asteroids are characterized by rather high eccentricities (their orbits are clearly elliptic) and inclinations (they often tilt significantly with respect to Earth’s orbital plane). We say that the population is dynamically hot, which is different from the large bodies in the region – Venus, Earth, Mars and Jupiter – that all have more or less circular orbits in almost the same plane, which makes them dynamically cold. How have the asteroids obtained these strange orbits?

There are currently two different scenarios that seek to explain both why the area between Earth and Jupiter is almost empty, and why most bodies there are dynamically hot. Both scenarios are very dramatic. If we could decide which of them that is correct, an important but yet unwritten part of the history of our Solar System could be clarified. By describing these scenarios in detail we also touch upon some other important questions – how is natural science conducted, what does it take to prove a scientific theory, and what is the difference between unfounded opinions and scientific hypotheses? However, before addressing these questions it is necessary to describe how the asteroids formed, and how the asteroid belt looks like today.

Planetesimals in the Solar Nebula

About 4.57 billion years ago, something happened that frequently takes place in our galaxy, the Milky Way – a part of a molecular cloud (see an earlier post about the interstellar medium) contracted due to its self-gravity and formed a starless core – a cold lump of gas and dust that measured 10,000 AU across. It may have maintained an equilibrium configuration for a long time, up to one million years, before it collapsed further and formed the protosun – a structure from which the Sun formed – and the Solar Nebula, a flat and warm cloud of gas and dust that revolved around the protosun. This final collapse could have happened spontaneously, but it is more likely that it was initiated by the explosion of a nearby type II supernova – such supernovae produce a cocktail of short-lived radioactive isotopes such as aluminum-26, iron-60, chlorine-36, manganese-53, and calcium-41, whose decay products are found in the meteorites that impact Earth.

We can see such young, newly collapsed systems around us and they are called class 0 or I protostars, depending on how far they have come in their evolution. These phases last a couple of hundred thousand years in total. The hot environment in the Solar Nebula close to the protosun was suitable for creating a kind of grain cluster called calcium-aluminum-rich inclusions or CAI. They consist of different minerals like melilite (a mix of åkermanite and gehlenite) and fassaite that are rich in calcium and aluminum. These can be found in meteorites and can be dated through radiometric methods – it is the age of these grains of 4.57 billion years that we define as the age of the Solar System. The measurement of time in the Solar System use CAI as a reference – a given moment in time is clocked as a certain numbers of years “after CAI”.

Also other types of particles formed at lower temperature, that were rich in oxygen, silicon, magnesium, iron, and sulphur, such as amoeboid olivine aggregates (AOA) and agglomeratic olivines (AO). All these particles have typical sizes of 0.01-1 centimeters, are often very porous, and there are reasons to believe that the vast majority of grains stopped growing at this size due to a phenomenon called the bouncing barrier – if two particles of these kinds collide with each other, the probability that they will stick to each other and build something bigger, is very low.

A fraction of these CAI, AOA and AO managed, in spite of the difficulties, to merge early on into boulders measuring decimeters or meters across. These could, in turn, merge to form even larger bodies – planetesimals – with sizes measuring hundreds of kilometers. The planetesimals that formed to within one million years after CAI contained sufficient amounts of radioactive aluminum-26 to get heated to the point that they melted. Thereby the grains were destroyed, and the minerals broken down into their atomic constituents. The gravity of the planetesimal forced heavy elements like iron, nickel and sulphur to sink towards the center of the body, where they formed a nucleus rich in metal and sulfides. On top a mantle formed that contained lighter elements – mostly oxygen, silicon, and magnesium – recombining to form minerals like olivine and pyroxene. Possibly, an outer crust formed that consisted of the lightest minerals, like pyroxene and feldspar – the latter rich in the aluminum and calcium that originally had been located in the CAI. Such a layered planetesimal is said to be differentiated.

However, we have reason to believe that the vast majority of the CAI, AOA and AO did not participate in this process. They continued to orbit the protosun, along with the differentiated planetesimals. The heat generated within such small particles due to the decay of the short-lived radioactive substances could easily escape to space, and the particles remained rather cold. After 2-3 million years, the gas began to leave the inner parts of the Solar System, and the disk around the protosun became much dustier. This appear to have given rise to some form of electromagnetic phenomenon – perhaps current sheets or electric discharges – capable of flash-melting large amounts of CAI, AOA and AO, which transformed the porous clusters of dust to small, hard, compact balls of rock. These new types of particles are known as Type C CAI and chondrules, respectively. Large amounts of chondrules have been found in meteorites, and radiometric dating shows that the majority have been formed 2-4 million years after CAI. The process also seem to have given rise to a fine-grained mixture of grains consisting of olivine, pyroxene, sulfide, metal, and organic substances called matrix material. Although chondrules and matrix material individually have a chemical composition that differs from that of the Sun in terms of elements heavier than hydrogen and helium, the sum of chondrules and matrix material is very solar-like.

The chondrule formation appears to have given rise to a second, and perhaps dominating, wave of planetesimal formation. It seems like the bouncing barrier suddenly could be crossed as soon as the porous collections of grains (AOA and AO) were transformed to smaller compact spheres of rock (chondrules). The large bodies that formed at this stage did not contain very high abundances of aluminum-26 since most of the substance already had decayed. Therefore, these relatively late planetesimals were never molten, and they did not differentiate. Their interiors still contain surviving and largely unmodified grains – calcium-aluminum-rich inclusions (CAI), amoeboid olivine aggregates (AOA), agglomeratic olivines (AO) and matrix material.

Asteroids and meteorites

The asteroids we see today are a few surviving examples of these different types of planetesimals – the rest have been used to build the planets in the Solar System. Some of these asteroids are very old, and constitute the oldest differentiated type. For example, Asteroid (4) Vesta is a body that is known to contain a core of iron, nickel and sulphur, a mantle of olivine and pyroxene, and a basaltic crust – volcanic rock rich in olivine, pyroxene, silica and feldspar. We also know that many of these extremely old differentiated asteroids have been smashed to pieces in violent collisions. Such pieces of differentiated asteroids often impact Earth as meteorites – iron meteorites from the metallic core of the parent body, stony irons from the transition region between the core and mantle, and achondritic meteorites from their crusts. Vesta itself is the parent body of a large group of achondritic stony meteorites called howardites, eucrites, and diogenites, or HED with a common name. A few asteroids, apart from Vesta, also seem old enough to have differentiated. For example, so called M-asteroids may be parts of the iron core of larger smashed-up planetesimals, while A-asteroids possibly are parts of the mantle from such bodies.


The Asteroid (4) Vesta photographed by the NASA spacecraft Dawn.
Image credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA

However, the majority of main belt asteroids seem to belong to the younger undifferentiated variant of planetesimals, and their collision fragments. Pieces from these undifferentiated bodies also frequently impact Earth, and are called chondritic meteorites, since they are so rich in chondrules. The innermost parts of the main belt is rich in E-asteroids, that are believed to be related to a certain type of stony meteorite called enstatite chondrites. Besides that, the inner half of the main belt is dominated by S-asteroids, that are known to be related to another type of stony meteorite called ordinary chondrites. We know this since the Japanese spacecraft Hayabusa went to an S-type asteroid named Itokawa, and brought back small parts of its surface material to Earth. When investigated in the laboratory they turned out to be identical to ordinary chondrite meteorites. The Chelyabinsk meteorite that I have written about previously, was also an ordinary chondrite. Finally, the outer parts of the main belt is dominated by C-asteroids, that are believed to be related to yet another type of meteorite – the carbonaceous chondrites. All chondrites have one thing in common – they contain a mixture of chondrules and matrix material, in different proportions. The differences between enstatite chondrites, ordinary chondrites and carbonaceous chondrites include; the mixing ratio of chondrules and matrix material; whether iron is located in separate metallic grains (is reduced) or finely distributed within the minerals (is oxidized); whether olivine is present among the dominating pyroxene; the abundances of rare oxygen isotopes compared with the most common form, oxygen-16. These differences reflect systematic changes in temperature and pressure within the Solar Nebula, as function of time and distance from the protosun.

The asteroid belt today

There are in total 220 asteroids in the main belt with diameters D of 100 kilometers or larger. The four largest ones are called (1) Ceres (D=930 km), (2) Pallas (D=580 km), (4) Vesta (D=525 km) and (10) Hygiea (D=410 km). There are in total 680 asteroids with diameters of 50 kilometers or more, and the number of asteroids with diameters in the 10-50 kilometer interval is about 7,000. The known population is considered complete down to sizes of 10-15 kilometers, which means that we know all individual objects that are at least that big. The number of asteroids larger than a kilometer is estimated to be 1.3-1.4 million. Currently, we know about 630,000 asteroids, which means that about half of all asteroids larger than a kilometer already have been discovered.

The largest asteroids – the 220 objects larger than 100 kilometers – are most likely surviving planetesimals, i.e., they formed 4.57 billion years ago in their current form. Almost all other asteroids are considered collision fragments, i.e., they are pieces of even larger objects that have been broken up in collisions. Therefore, they have not had their current appearance since Solar System childhood, but have formed throughout the long history of the Solar System during violent collisions. There are three properties of asteroids that change systematically around a size of 100 kilometers. First, there is a clear change in the size distribution of asteroids, i.e., a list of the number of asteroids having a given size. If one consider the total number of asteroids larger than a certain size (the cumulative size distribution), it increases fast when D is reduced from 930 kilometers to 120 kilometers. But if D is reduced further, the cumulative size distribution does not change very fast at all, until a size of about 30 kilometers is reached, at which the increase is fast anew. Second, asteroids larger than 100 kilometers are almost spherical, while smaller asteroids systematically become more irregular the smaller they are. Third, there are systematic changes in the rotational periods of asteroids – in the D=100-930 kilometer interval the rotational period increases with decreasing size, which means that the smaller asteroids tend to rotate slower than the larger ones. But around D=100 kilometers this trend is reversed, so that even smaller asteroids tend to have shorter rotational periods – small asteroids spin faster the smaller they are. These three properties show that we are dealing with a population of objects with diameters larger than 100 kilometers that are primordial surviving planetesimals – their sizes, shapes and spin properties are consequences of the process or processes that formed planetesimals early in Solar System history. Instead, objects with diameters smaller than 100 kilometers are collision fragments – their sizes, shapes and spin properties are consequences of what happens when to large asteroids collide with each other at a high velocity.

Most of the mass in the main asteroid belt is locked up in the largest objects. The fact is that it is sufficient that 10-20 asteroids with sizes in the range 100-1000 kilometers collide, to explain the number of all asteroids that are smaller than this. Since we know all asteroids larger than 10-15 kilometers, and have a fairly good idea of how the size distribution looks like at even smaller sizes, we can say with certainty that the total mass in the main belt is not higher than about 0.0005 Earth masses.

However, the original amount of mass (in the form of rock, metal, sulfides and organic substances) in the 1-4 AU region must have been significantly higher, and may have been 5-8 Earth masses. This estimate is based on our observations of circumstellar disks around foreign protostars in the Milky Way, that currently are in the same stage of evolution as our Solar System was 4.57 billion years ago. In such disks, the amount of mass in the disk changes in a characteristic way with increasing distance to the parent star. If this is compared to the amount of matter that has been locked up by the planets in our own Solar System internal and external to the 1-4 AU region, we can conclude that the primordial asteroid belt must have been several thousand times more massive than today. A very dramatic event, or chain of events, must have led to this drastic mass loss in the asteroid belt.

We also know that the main belt asteroids originally must have moved on almost circular orbits, that all were located more or less in the same plane. The consequence of having such orbits is that the objects meet at low velocity when they collide – perhaps a few tens of meters per second. Such gentle collisions are needed to allow gravity to keep colliding bodies together, so that a larger body can form as a result of the collision. In order to build bodies with sizes measuring several hundreds of kilometers, it is necessary that even smaller bodies collide with each other at very low speed.

However, today the asteroid orbits have high eccentricities and inclinations. The orbits are no longer circular but are shaped as ellipses. The ellipse has a center, but the Sun is not located there but in a focus point that is displaced towards the point in the orbit where the asteroids is as closest to the Sun as possible, called the perihelion. The eccentricity is defined as the distance between the center and the focus point, divided by the distance between the center and the perihelion. The average value for the eccentricity of asteroids is 0.15, which means that the displacement of the Sun from the center of the ellipse is 15% of the distance between the center and perihelion. The average value of the orbital inclinations is 8 degrees, which is the average tilt of the asteroid orbital planes compared to the orbital plane of Earth. These high eccentricities and inclinations mean that asteroids have very high velocities when they collide – the average velocity is 5 kilometers per second. It is not possible to build anything during such powerful crashes – all collisions become destructive and only makes the population slowly grind itself to dust. Something must have happened in the asteroid belt that changed growth in dynamical cold to grinding in dynamical heat.

The first scenario – embryos in the asteroid belt

According to the first scenario the main belt originally was a few thousand times more massive than today, and all bodies moved more or less on circular orbits in a common plane. For this reason, small planetesimals could merge into larger planetesimals rather fast. Computer simulations show that it takes about one million years to form bodies as large as the Moon. The Moon has a diameter of D=3470 kilometers and a mass of about 0.01 Earth masses. It is even possible that bodies formed with masses around 0.1 Earth masses, that would be as large as the planet Mars. Bodies with masses of 0.01 Earth masses or larger are normally not referred to as planetesimals, but are called embryos.

The presence of embryos in the main belt had a large effect on the orbits of smaller planetesimals. They could no longer maintain their circular orbits in a common plane, but gradually obtained ever higher eccentricities and inclinations due to gravitational perturbations. Paradoxically, the formation of embryos – a consequence of efficient growth due to low collision velocities – therefore leads to the end of growth, and increasingly efficient fragmentation since the collision velocities are too high.

However, we know that Jupiter formed around the same time – the gas giants must have formed at most five million years after CAI since observations of gas disks around foreign protostars show that they do not live longer than that. Jupiter and Saturn must have had time to grow by consumption of such gas while it was still available. Uranus and Neptune, that mostly consist of ice, only managed to bind smaller amounts of gas since they formed at a stage when the gas disk already was dispersing. The formation of Jupiter had an extremely important effect on the asteroid belt – mean motion resonances were formed.


The asteroid mean distance to the Sun measured in Astronomical Units (AE in the figure) on the horizontal axis, and the orbit inclination on the vertical axis. Every dot in the diagram corresponds to a known asteroid. The vertical lines show the location of strong mean motion resonances. The name of the resonance as well as its distance to the Sun is marked at the top of the figure. Note that several resonances virtually lack objects (so-called Kirkwood gaps). The 3:2-resonance contains a population of objects called Hilda asteroids.

The orbital periods of the asteroids increase systematically with increasing distance to the Sun. At certain specific distances from the Sun, the orbital period will constitute a small-integer fraction of Jupiter’s orbital period. For example, in the inner parts of the asteroid belt we find the 3:1 resonance at 2.50 AU – here the asteroids revolve exactly three times around the Sun in the same time as Jupiter performs one revolution. Further out, at 2.82 AU we find the 5:2 resonance, where the asteroids complete exactly five orbits around the Sun in the same time as Jupiter makes two revolutions. There are several such resonances and they all have one thing in common – objects that end up in these resonances are subjected to very strong gravitational perturbations by Jupiter. The most common effect is a strong increase of the orbital eccentricity, which means that the perihelion point is moved closer to the Sun. It is exactly this mechanism that creates Near Earth Asteroids. The asteroid is normally destroyed by colliding with the Sun, or more rarely, by colliding with one of the terrestrial planets.

Embryos and mean motion resonances is a deadly combination for asteroids – it took about one million years for the embryos to shuffle about 99% of the asteroids into the resonances, so that these objects left the main belt. The embryos also perturbed each others orbits, and disappeared one by one via the resonances. When the last embryo disappeared from the main belt, only traces remained of the once so massive population, and the remaining objects had received a high degree of dynamical heating – a memory of the ravaging of the embryos. If this scenario is correct, the current asteroid belt is therefore the remains of a region where the planet formation process had time to progress rather far, before Jupiter cleaned away all large bodies and only left a few smaller asteroids behind. The E-, S- and C-asteroids, the parent bodies of enstatite meteorites, ordinary chondrites and carbonaceous chondrites, thus formed very close to each other which means that the physical and chemical properties of the Solar Nebula changed very fast with distance to the protosun. The embryos that redecorated the main belt did not manage to fully erase this strong chemical gradient within the asteroid belt.

The second scenario – Jupiter visits the asteroid belt personally

However, it is possible that the clean-up and dynamical heating of the asteroid belt happened in a completely different manner. In another scenario, the entire 0.5-2.0 AU region initially consists of E-asteroids while the 2.0-5.0 AU region is completely dominated by S-asteroids. The C-asteroids are formed among and beyond the giant planets, which are jostled in a region around 5-15 AU from the Sun. If this is correct, the difference between the various chondritic meteorites is caused by a change in the chemical and physical properties of the Solar Nebula that happened very gradually across a region that covered a vast range of distances from the Sun.

In this scenario Jupiter is formed somewhere beyond 5 AU, but does not stay there since it starts to drift towards the protosun. This is called migration. As is evident from my previous post about the Kepler exoplanets, such migration is rather common for gas giants around other stars in the Milky Way. It has been investigated what would happen if Jupiter was allowed to drift all the way to 1.5 AU – were we find the planet Mars today – before Saturn catches up with Jupiter, and both gas giants drift back outwards in formation. As it turns out, a smaller external gas giant can reverse the migration of an internal larger gas giant.

If Jupiter plowes through the main belt twice – once on its way in and a second time on its way out – the asteroid belt will be heavily depleted, and subjected to dynamical heating. On the way in, Jupiter will force many S-asteroids to relocate to larger distances where they mix with C-asteroids. When Jupiter travels back, such S- and C-asteroids are replanted into the asteroid belt, which could explain why we observe such a variety of bodies within a fairly narrow region from the Sun. This scenario is called The Grand Tack.

The difference between scientific hypotheses and unfounded opinions

The two scenarios above have one thing in common – they are both solutions to the so-called N-body problem. This problem can be formulated in the following way: if there are N bodies with known masses, that are located in specific positions and are having specific velocities at a given moment, where will the bodies be at a later moment of time, and what will the velocities be, if every body feels the gravitational pull of all the other bodies? In brief, the N-body problem is about tracking the motion of a swarm of bodies as function of time, where the movements of any given body is the result of the gravitational pull from the other bodies in the swarm.

This problem was formulated very early, e.g. by Robert Hooke in 1674 and by Isaac Newton in 1687. The latter found an analytical solution to the problem for N=2, which is called the two-body problem. It is this solution that states that a single planet that orbits a star will move in an elliptic, parabolic or hyperbolic orbit, depending on its distance to the star and its velocity at the beginning of the calculation. The equation that describes how the acceleration of the body depends on the forces that acts on it – the so-called equation of motion – can be formulated rather easily when N bodies are involved. However, it is not easy to solve the equation, which means that one get access to the positions and velocities of the bodies at a given moment of time. Massive efforts were made in the 19th century to find such solutions, but it was only possible to find exact analytical solutions for N=3 under very special conditions – this is the so-called restricted three-body problem. However, methods were developed that yielded approximate solutions, that turned out to be extremely useful.

For example, it was possible to show that Uranus did not move as expected, when taking into account the gravitational force from the Sun, Jupiter and Saturn (a four-body problem). Independently of each other, the Englishman John Couch Adams and the Frenchman Urbain Jean Joseph LeVerrier used these discrepancies between the theoretical and observed orbit of Uranus to calculate the position of an hitherto unknown planet, assumed to be responsible for these discrepancies. By observing the sky in the vicinity of the calculated position of the unknown planet, the German astronomer Johann Gottfried Galle could located the object in 1846 – the planet that is now known as Neptune. While Uranus was found by accident in 1781, Neptune was found since the mid-19th century scientists had perfected the art of measuring and calculating the positions of planets in the sky to a level where they almost had complete control over the mechanical properties of the Solar System.

In a similar manner, LeVerrier could later show that the innermost planet, Mercury, did not move exactly as is was supposed to, even if the gravity of the other planets were taken into consideration. This discrepancy was later to become one of the most important proofs that the general theory of relativity by Albert Einstein was correct, since it led to an adjustment of Newtonian mechanics that exactly matched the “error” in Mercury’s movements.

Astronomers and physicists have therefore studied the problem of calculating the motion of a body affected by the gravitational forces from several other moving bodes for a long time, and fantastic progress in this field was made already 150 years ago. Nowadays, when we have access to computers with enormous capacity, it is no longer a problem to solve the N-body problem with extreme accuracy, even when N is a rather large number. For example, a spacecraft that travels through interplanetary space will be subjected to significant gravitational forces from the Sun and all planets. Yet there are no problems for ESA, NASA or any other organization to navigate in space – this sort of calculations is routine.

It is the same type of calculations that are made in order to understand how the asteroids in the main belt will react when they are exposed to the gravitational force from embryos and Jupiter. De scenarios described above are therefore based on very accurate and detailed calculations, and are extremely realistic. The starting point is some basic assumption (i.e., whether Jupiter is migrating or not), and then well-known physics and established methods are used to calculate the consequences of this assumption. Details on how these calculations are made, and the results of the calculations, are published in scientific journals, that only can be read by experts since they are filled with mathematics, physics and technical terminology. Such documentation describes a scientific hypothesis. It is a hypothesis in the sense of being a proposal, but this proposal is scientific since it is substantiated and supported by arguments and calculations that are based on centuries of research, and methods that have a demonstrated correctness.

Popularization of science aims at describing the essence of a scientific hypothesis, using few and simple words, which e.g. is made in this blog. This is an important task, because it allows large numbers of people to take part of scientific discoveries, which enriches the mind of every human, increases his or her understanding of the world and the capability to interpret what happens around us. However, it is impossible for a popularized text to give justice to the richness, complexity and depth of the investigations on which the hypothesis is based – if one wishes to understand the hypothesis at depth one must consult the original texts. The simplicity and brevity that characterizes a popularization of science can therefore be treacherous – it is difficult or impossible to imagine the complexity of the underlying machinery. It is easy to get the impression that a hypothesis is nothing but an opinion – that some scientist “believes something”. It is easy to confuse this with opinions and beliefs that are not based on any form of deeper knowledge, or investigation of the actual properties of Nature. This is what we may called unfounded opinions. In an increasingly media-based world, where the internet and social media have given people the possibility to express their opinions at an unprecedented extent, and were the flow of information is accelerating, it can be difficult to tell the difference between scientific hypotheses and unfounded opinions – at a first glance they may look similar. This becomes even clearer when unfounded opinions are presented by using a language and a “technical terminology” that gives the impression of being scientific – that is to say, pseudo science.

To claim that the Earth is a few thousand years old is an unfounded opinion, and if one tries to sell this opinion by using various technical arguments it is pseudo science. When a scientist claims that Earth was formed about 50-150 million years after CAI, that are 4.57 billion years old themselves, it is not an opinion. It is not even a scientific hypothesis, since this specific example is not a proposal – it is a fact.

Astronomers and physicists therefore do not sit and do “philosophy” in some general manner. The idea that embryos or a migrating Jupiter are responsible for the current properties of the asteroid belt are not just fantasy or speculation, it is not a matter of opinion or believing – they are scientific hypotheses since they have been formulated by using scientific methods. But if so, how come there can be two or more hypotheses claiming to explain the same thing – is not natural science about “knowing” and telling right from wrong? Why do we speak of facts and “to know” when the age of Earth is concerned, while the properties of the asteroid belt is the subject of hypotheses? The answer to that question gives further insight into the workings of the scientific process.

When a scientific hypothesis becomes fact

A scientific hypothesis is built on a number of basic circumstances, established and well-known physics, and mathematical calculations performed with the goal to explore the consequences of the basic circumstances. In order for a hypothesis to dominate over other hypotheses, and finally be referred to as a fact, comprehensive comparisons with Nature itself are needed. Such empirical data must lend support to the basic circumstances, as well as to the consequences of the hypothesis. If measurements or observations of Nature do not agree with the hypothesis, the hypothesis must be rejected or reformulated so that it no longer contradicts reality. The problem is that such empirical data often are not known, why two or more hypotheses may co-exist for a long time, until observations, measurements or laboratory experiments can shed light over the unknown circumstances needed to prove one hypothesis, and disprove all others.

Scientists are working hard to figure out how grains, not larger than a thousandth of a millimeter, merge to build boulders that are a few decimeters or meters across. There are several hypotheses on how such boulders grow to planetesimals with sizes in the 100-1000 kilometer range. The following process – the formation of embryos – is better known. In spite of this, it is difficult to know exactly how long it took for embryos to form, what size distribution planetesimals and embryos had at that time, and exactly how large the eccentricities and inclinations had grown. Another source of uncertainty is exactly when Jupiter had grown big enough that its mean motion resonances in the main belt were strong enough to effect the evolution there – did this happen only once the embryos had existed for a few hundred thousand years, or were they present even before the embryos formed? Yet another source of uncertainty is whether Jupiter migrated substantially or not. Scientists send spacecraft to asteroids and comets, make advanced computer calculations, perform experiments in laboratories and measure the properties of meteorites to answer such questions as far as possible, or at least limit the range of possible answers.

In such an uncertain situation, there is room to formulate various realistic but hypothetic basic assumption. For example, one can assume that embryos have formed, that these and planetesimals have a certain size distribution, that they have dynamically cold orbits, that Jupiter forms at a time when embryos already exist, and that Jupiter does not move substantially – one then ask, what happens then? The answer can be found in the detailed computer calculations that tracks the motion of thousands of planetesimals and embryos. Once the calculations are finished, one can explore what has happened – how many asteroids are left, what distances to the Sun do they have, what are their eccentricities and inclinations? If E-, S-, and C-asteroids initially are placed within well-defined regions at different distances from the Sun, to what extent have these populations mixed due to the ravaging of the embryos? It is exactly this type of detailed information from the computer models that are compared with observations of the Solar System today – accounting, as far as possible, for processes that may have changed various properties during the long history of the Solar System.


The Asteroid (253) Mathilde photographed by the NASA spacecraft NEAR. Mathilde is a C-asteroid, that are parent bodies of the meteorites called carbonaceous chondrites.
Image Credit: NASA
Credit: NSSDC Photo Gallery
Original image:

As it turns out, the eccentricities and inclinations of the surviving asteroids in computer models that include embryos are very similar to those seen in Nature, which is encouraging. However, there are some difficulties – the innermost parts of the main belt become too sparsely populated, since embryos in those regions turn out to have a long lifetime before they are removed, and in the meantime they manage to clear the region completely. Furthermore, the region around the orbit of Mars is not cleared enough – models of this kind lead to a mass of Mars that is many times larger than that of the real planet.

The Grand Tack scenario seems to solve these problems. If Jupiter is given the opportunity to drift into the asteroid belt, and then head back out again, the amount of matter around 1.5 AU from the Sun is reduced to a level that explains why Mars only carries a tenth of an Earth mass. The distribution of matter within the modeled asteroid belt corresponds fairly well to the observed one. The computer model makes a very good reproduction of the distribution of S- and C-asteroids at different distances from the Sun, if they originally formed within, and far outside, today’s main belt, respectively. The asteroids in the computer model have inclinations very similar to those in the asteroid belt today. But also this hypothesis has problems – the eccentricities of the modeled asteroids do not resemble the ones found in the main belt today. This may be explained by the Late Heavy Bombardment, a powerful disturbance to the main belt caused by the giant planets, that is believed to have happened when the Solar System reached an age of about 700-800 million years.

Both these scientific hypotheses constitute advanced working models, that will co-exist until the amount of data gathered about the Solar System becomes large enough to prove one of the hypotheses and disregard the other – unless it turns out that a hybrid of the hypotheses is more suitable, or a third yet unexplored scenario turns out to be the right one.

This shows that Solar System science is alive and is constantly evolving. It shows that the asteroid belt contains traces of large-scale dramatic events that took place early in the history of the Solar System. The exploration of the main belt is therefore extremely important, since it helps us to understand what happened near the region where our own planet formed. Assume, for example, that Jupiter actually penetrated deep into the inner parts of the Solar System. An interesting consequence of the Grand Tack scenario is that the inner Solar System receives large amounts of mass from farther regions. If this was the case, could it be that our planet would not have formed, or that it would have looked very different, had Jupiter stayed away? Does our planet, and thereby humanity itself, exist as a result of Jupiter’s migration? It is questions like this that starts to erase the difference between Solar System history and human history – we cannot understand our own past without describing that of the Solar System.


Petit, J.-M., Morbidelli, A., Chambers, J. (2001). The primordial excitation and clearing of the asteroid belt. Icarus 153, 338-347.

Walsh, K. J., Morbidelli, A., Raymond, S. N., O’Brien, D. P., Mandell, A. M. (2011). A low mass for Mars from Jupiter’s early gas-driven migration. Nature 475, 206-209.

Walsh, K. J., Morbidelli, A., Raymond, S. N., O’Brien, D. P., Mandell, A. M. (2012). Populating the asteroid belt from two parent source regions due to the migration of giant planets – ”The Grand Tack”. Meteoritics & Planetary Science 47(12), 1941-1947.