Planetesimal Theory of Planet Formation

Planetesimal Theory of Planet Formation
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What are Planetesimals?

Planetesimals are small objects that can range from millimeters (mm) to the size of several hundred kilometers (km), formed out of grains of dust that collide and get stuck together to form larger bodies such as planets. The planetesimal theory was first proposed by a Soviet astronomer by the name of Viktor Safronov. His theory is generally accepted by astronomers worldwide, however alternative theories also exist. We will look at what the theory states and how planets in our solar system are thought to have been formed.

The Planetesimal Theory

In general, the planetesimal theory states that during the formation of our planetary system, a protoplanetary disk was formed from the materials of the solar nebula. The solar nebula is thought to have been a disc-shaped cloud of gas and dust that remained after the formation of the Sun. As a result of accretion, the smaller sized dust grains began to clump together as they collided, forming larger bodies of planetesimals. When the planetesimals reached a size of about 1 km, they began to attract other smaller bodies due to their gravity.

Eventually the planetesimals got large enough to attract bigger bodies that in turn formed protoplanets and eventually the planets. Many collisions between the planetesimals resulted in breaking up of the bodies into smaller and smaller bodies, however, some did end up clumping together rather than breaking apart. According to the theory, asteroids and Kuiper belt objects (KBOs) are thought to be planetesimals. Many of the moons of the planets are also considered to be planetesimals.

We will now look at the details of the theory as it pertains to the formation of the terrestrial planets (or inner planets), the gas giants (or the outer planets), and objects that exist in the Oort cloud and the Kuiper belt.

Formation of the Terrestrial Planets

The Terrestrial Planets

The formation of the terrestrial planets, which took place billions of years ago, is a complex one. The terrestrial planets are thought to have been formed by the accumulation and collision of larger bodies, the planetesimals. The solar nebula accumulated a certain percentage of the mass in the form of micron-sized dust grains that formed in the turbulent atmospheres of exploding stars. Space density within the solar nebula allowed planetesimals to stick together from collisions.

Brownian motion as well as turbulence promoted the further collisions of the dust grains. Particles started to settle towards the midplane, based upon turbulent eddies within the nebula. Close to the midplane, spatial density further propagated the coagulation of the dust particles. This process eventually led to the formation of approximately 10 km sized planetesimals.

Orbital eccentricities of the resulting planetary embryos increased, due to perturbations in their gravity resulting in orbits that crossed each other’s path. This meant more collisions and with time, roughly planetesimals the size of planets began colliding into one another at speeds exceeding 10 km/sec. The result was protoplanets that were molten. Nearly 100 million years (Myr) after the process of planetesimal accumulation began, Earth and Venus had formed with several smaller planets.

One thing to note is that the processes of collision and accumulation of the planetesimals were stochastic or random and various models have to be taken into account. Another thing of interest is that the orbital eccentricities of the inner planets were not affected much by further collisions, resulting in near circular orbits. At the same time, Jupiter-mass planets are thought to have acted as giant perturbers of gravity, influencing the formation of the terrestrial planets.

Formation of the Outer Planets

Icy Planetesimals in the Solar Nebula

The current understanding of the formation of the outer planets is relatively murky. There are two theories that are generally regarded for the formation of the outer planets. One is the core accretion theory, while the other is the disk instability. Theoretically, both can form the outer planets but with difficulties. For simplicity sake, we will examine only the core accretion theory.

Planetesimals, in the form of icy dust grains, began accumulating by collisions. The process is similar to the formation of the terrestrial planets. Planetesimals eventually formed planetary embryos, with the exception of more mass available for the planetary embryos. The accretion of the planetesimals leads to an accretion of a core that is 10 Earth-mass cores. The core grows in size acquiring nebular gas, which continues to thicken with an increase in the size of the core. The rate of the accumulation of the gases, at a certain point, increases and is more than the rate of accumulation of solids. This results in the collapse of the nebular gas onto the protoplanet, eventually depleting the surrounding disk of gas. What forms is an atmosphere rich in hydrogen and helium that envelopes a solid core, which is the classical structure of the gas giants. The planets have near circular orbits.

The biggest problem with this model of planetesimal accretion into a solid core is the time scale involved. Observations of younger stars have revealed that the disk of gas surrounding the star disappears within a few Myr. The process of core accretion takes longer than this. The alternative is the disk instability model which also has its problems.

Turbulence as a Factor in Planetesimal Formation

The conventional theory of planetesimal formation involves dust grains sticking together within the disk. This can be offset or disturbed due to turbulence within the disk. In order for clumps to form they need to be in regions of low vorticity. This is required to avoid rotational breakup of the clumps. Further, the clumps need to have a sufficient amount of self-gravity so as to avoid breakup due to the ram pressure generated by the surrounding gas.

The rate of formation of planetesimals are reliant upon the rates of rotation of the largest eddies within the disk. The rate of formation of planetesimals increases if the largest eddies rotate slower in comparison to the disk. The rates of formation increase by an order-of-magnitude or possibly even more as one moves outward across the snow line. The snow line is the distance from the protosun in the solar nebula, where the temperature is cool enough for water, methane and other hydrogen compounds to condense into solid grains of ice. This means that the solid surface density increases by a factor of two. Modal size of the planetesimals increases by approximately the square root of the distance from the star.

Orbital Migrations of Planets

Various N-body simulations suggest that for planets with masses smaller than M (Earth solid core mass), in a planetesimal disk of minimal solar mass, orbital migrations are typically of type I. Type I migrations commonly occur when the planet loses angular momentum and migrates inwards within the disk. Type II migrations are common for planets with masses greater than M. Type II migrations of planets means that the planet has cleared the distance of Type I migrations, thus being able to form in the outer parts of the disk. These results were based on simulations involving migrations between a 3D Keplerian disk of planetesimals and a massive planet.

Binary Planetesimals

Binary planetesimals are planetesimals that orbit one another and influence the development of planetary systems. The role they play in the development is yet not very clear. However, it is theorized that binary planetesimals exist around distant stars. Some of the planetesimals in our own Solar System are thought to be remnants of such binary systems.

Asteroids and Kuiper Belt Objects

Rendering of a Kuiper Belt Object

Asteroids, such as Vesta. are thought to be the result of two larger planetesimals colliding with one another and breaking apart. Many such asteroids are thought to be formed from planetesimals. The Kuiper Belt Objects are also thought to be formed according to the planetesimal theory. It is widely believed by astronomers that around 3.8 billion years ago, most of the planetesimals from the Solar System were ejected and sent far out to the distant reaches of the system, where today we find the Oort cloud and the Kuiper belt.

What Can We Learn From Planetesimals

The planetesimals that still survive today in our Solar System are like a time capsule, as they provide a glimpse into the past. Many things about the formation of the Solar System and the initial solar nebula can be deciphered from studying them. Meteorites are of particular interest to astronomers because of this reason. Even though due to solar radiation their chemistry can be altered, the contents within the planetesimal remain intact from the time of the formation of the Solar System.


1. Safronov, V.S., Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets, 1969, Moscow: Nauka

2. Marcy, G.W. and Butler, R.P., Ann. Rev. Astr. Astrophys., 36, 37, 1998


Safronov, V. S. 1969, Evolution of the protoplanetary cloud and formation of

the Earth and the planets (Moscow:



Image Credits: Wikimedia Commons/NASA/Jet Propulsion Laboratory at the Califoria Institute of Technology/Jack/Qbek/NASA Image of the Day