What is a Protoplanetary Disk?

by Carson
Artist's impression on a protoplanetary disk

A protoplanetary disk is a disk of gas and dust surrounding a newly-formed star that appears during planetary formation. How do they form? What are they made of? And how do they ultimately complete planetary formation? We’ll answer these questions in this article.

How Do Protoplanetary Disks Form?

First, let’s explain how they form in the first place. They all start as materials randomly floating through the star-forming regions. Those are dense regions for gas and dust, such as nebulae. When an instability occurs, such as when a high-mass star explodes in a supernova, the material gets redistributed around. Sometimes, it disrupts the symmetry so much that the more massive regions become better at attracting nearby matter. This thus forms a chain reaction, where they absorb more matter and become even more massive. This kickstarts planetary formation.

Soon after that, most of the material gathers inward and collapses to form the star. However, it’s leftovers that we care about, as this article is about the protoplanetary disks. As the matter comes in at different orientations, they enter a closed orbit around the star and thus becomes part of the planetary system.

If matter can come from all directions, why does the disk look so flat? It’s because of a process called collisional damping. There are so many particles out there that one can easily collide with another in a reasonable amount of time. As the particles crash into each other, they tend toward the state most of the particles are in. This means the majority of them are within a few degrees of inclination (thus making the disk flat), and their eccentricities are also within a few percent of each other. This is why the orbits of objects coming out of the disk are commonly assumed to be circular and coplanar.

What Are Protoplanetary Disks Made Of?

As the materials in the disk originate from the star-forming regions, it is reasonable that they have about the same composition as the nearby stars. That means they are mostly hydrogen and helium. But apart from the gases, there are also some heavier dust particles. They are usually made of heavier elements or molecules, such as carbon, metals, or water. These particles coalesce together instead of floating free, but the pieces are still extremely small.

But what is the percentage of heavier elements in the protoplanetary disk? The truth is that it depends. Specifically, it depends on the metallicity of the star, which is exactly that ratio of heavier elements to the hydrogen and helium. The metallicity of a star depends on its age as well as the region where it formed. When the universe first began, it wasn’t rich in the heavier elements. Thus, the older generation of stars have low metallicities as well. But as high-energy events like supernovae release lots of different elements through nuclear fusion, they enrich the nearby space with these materials. That’s why the later generations of stars, such as our Sun, have higher metallicities. And this affects what makes up the protoplanetary disk as well.

How Do Protoplanetary Disks Form Planets?

After reading all this, you might ask, “How do these disks build the planets we see today?” Well, they do so via accretion. The particles crash into each other, but the forces holding them together, like electrostatic forces, are larger than the forces trying to pull them apart, like those resulting from a large coefficient of restitution and a high impact speed. If two particles stick together, this means they form a larger object.

An illustration of the forces applied on two particles after a collision.

Over time, random fluctuations and external disturbances mean that asymmetries arise in the mass distribution of the disk. This means that some regions become better at sucking more objects in. This process continues until almost all material is concentrated in massive bodies, not swirling around in the disk. At this stage, the massive body becomes fully-formed planets, while the lucky leftovers are the asteroids and comets.

Of course, the story doesn’t end there, as many dynamical instabilities might still occur after this stage. For example, there’s the famous Moon-forming impact, which is hypothesized to have happened in order to form our Moon. And there’s also evidence that a dynamical instability occurred because of the dispersal of the protoplanetary disk. But that’s a topic for another article.


In this article, we described what a protoplanetary disk is, how it forms, and what they’re made of. Remember that they are the disks that surround young stars during planetary formation, and the materials ultimately accrete together to form planets, asteroids, comets, and moons. You can go into a lot of complicated detail that is still an active area of research today, but that would be the topic for a more advanced article. If you think there there is something important missing, please tell us in the comment section.


  1. Morgan, M., Seligman, D., Batygin, K. (2021, July 22). “Collisional Growth Within the Solar System’s Primordial Planetesimal Disk and the Timing of the Giant Planet Instability”. Retrieved May 14, 2023, from https://arxiv.org/abs/2107.10403
  2. Liu, B., Raymond, S. N., Jacobson, S. A. (2022, May 4). “Early Solar System instability triggered by dispersal of the gaseous disk”. Retrieved May 14, 2023, from https://arxiv.org/abs/2205.02026?

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