3.2 Nebular Theory: Building Planets from the Protoplanetary Disk

In Chapter 2, we left off at the protoplanetary disk stage of the nebular theory (which we are calling Step 3). In this stage, the original nebula has collapsed to form a flat, spinning disk of gas and dust. At the center of the disk lies a hot protostar that contains the majority of the mass of the entire system. At this point, the material in the disk itself is spread out over a large region. To produce a planetary system resembling our own, this dispersed material has to coalesce into several discrete planetary bodies, including asteroids, comets, moons, and planets (Figure 3.5). This is the final stage—Step 4—of the nebular theory.

The fact that protoplanetary disks are a common occurrence around very young stars suggests that disks and stars form together. Astronomers can use theoretical calculations to see how solid bodies might form from the gas and micron-sized dust particles in these disks as they cool. These models show that, in general, progressively larger and larger bodies are built through the collision and clustering of matter in response to gravitational attraction or electrostatic forces in the protoplanetary disk. This oftentimes (but not always) violent and chaotic process is called accretion. The scientific community’s model of how dust grows into planets that are several thousands of kilometers or greater in diameter in size is constantly evolving, but we will discuss the basics of what we know about planetary accretion in our solar system here.

Around 4.6 billion years ago, gravitational collapse of the protosun’s core increased its temperature and pressure enough to ignite fusion, officially turning the protosun into the Sun we know today. The intense heat released by the newly formed Sun likely turned most of the solid particles that already existed in the protoplanetary disk into a vapor.  However, planetary accretion requires solid material to occur (at least until an object grows large enough in size to gravitationally attract and capture gases in the protoplanetary disk) which means new solids had to condense as the protoplanetary disk cooled.

In the early days of our solar system, solid material existed as tiny dust particles sub-microns in size. The composition of this dust was determined largely by temperature, because different compounds condense at different temperatures. Refractory compounds condense at high temperatures while volatile compounds can only condense at lower temperatures. As our solar system formed, first the nebular cloud and then the protoplanetary disk developed distinct temperature zones. Temperatures were very high close to the center, only allowing condensation of refractory materials like metals (e.g., Fe and Ni) and silicate minerals (e.g., olivine and pyroxene). (Note that silicate minerals are often referred to as “rocky” materials or compounds.) Farther from the Sun, the temperatures were lower, allowing the condensation of lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water. The boundary within the protoplanetary disk past which temperatures are cool enough for solid volatiles to exist is known as the ice line (or frost or snow line) (Figure 3.6).

Early in the accretion process, dust particles collected into fluffy clumps because of static electricity, similar to dust bunnies in your home. A flash heating event may have melted many of these dust clumps which then resolidified into chondrules—submillimeter- to centimeter-sized  spheres of silicate minerals like olivine and pyroxene. The source (and timing) of this intense heating is still debated and an active area of research. Note that any aggregate that is millimeters to meters in size in the protoplanetary disk, whether it is a chondrule or not, is often referred to as  a “pebble.” As clumps of dust, chondrules, and pebbles grew larger, gravity eventually took over as the primary “sticking” force, allowing for the formation of larger solid masses called planetesimals with diameters hundreds of meters to hundreds of kilometers. With their larger mass, planetesimals are can more easily gravitationally pull more material towards them, and some of these planetesimals accreted additional chondrules, clumps, or even other planetesimals in the disk, eventually growing into protoplanets (hundreds to thousands of kilometers in diameter) and, if lucky enough, planets (several thousands of kilometers in diameter).

If you were one of these planetesimals in the early solar system, and participating in the accretion game with the goal of becoming a planet, you would have to follow some key rules:

• Keep your velocity just right. If you move too fast and collide with another body, you both smash up and have to start again. If you move slowly enough, gravity will keep you from bouncing off each other and you can grow larger.
• Your distance from the Sun will determine how big you can get. If you are closer, there is less material for you to collect than if you are farther away.
• To begin with, you can only collect mineral and rock particles. You have to grow above a certain mass before your gravity is strong enough to hang onto gas molecules, because gas molecules are very light.
• As your mass increases, your gravity becomes stronger and you can grab material from farther away. The bigger you are, the faster you grow.

You would also have to watch out for some dangers:

• In the early stages of the game, the protoplanetary disk is turbulent, and you and other objects can get thrown into different orbits or at each other. This might be a good thing, or it might not, depending on how the rules above apply to you.
• If the game progresses to the point where there is no more material within your reach and you are not yet a planet, then it’s game over.
• If you slow down too much (e.g., from bumping into other objects), you could spiral into the Sun (game over).
• If another planet gets big enough, it can:
  • Rip you apart and then swing the pieces around so fast that for the rest of the game you collide too hard with other pieces to grow any bigger (game over)
  • Fling you out of the solar system (game over)
  • Grab you for itself (game over)
  • Trap you in an orbit around it, turning you into a moon (game over, and incredibly humiliating)

The outcome of the accretion game is evident in Figure 3.8. Today eight official winners are recognized, with Jupiter taking the grand prize, followed closely by Saturn. Both planets have trophy cases with more than 60 moons each, and each has a moon that is larger than Mercury. In general, the eight planets can be classified into three categories based on what they are made of (Figure 3.9). Terrestrial planets are those planets like Earth, Mercury, Venus, and Mars that have a core of metal surrounded by rock. Jovian planets (also called gas giants are those planets like Jupiter and Saturn that consist predominantly of hydrogen and helium. Ice giants are planets such as Uranus and Neptune that consist largely of water ice, methane (CH₄) ice, and ammonia (NH₃) ice, and have rocky cores. Often, the ice giant planets Uranus and Neptune are grouped with Jupiter and Saturn as gas giants; however, Uranus and Neptune are very different from Jupiter and Saturn.

These three types of planets are not mixed together randomly within our solar system. Instead they occur in a systematic way, with terrestrial planets closest to the sun, followed by the Jovian planets and then the ice giants (Figure 3.8). Smaller solar system objects follow this arrangement as well. The asteroid belt contains bodies of rock and metal. Bodies ranging from meters to hundreds of meters in diameter are classified as asteroids, and smaller bodies are referred to as meteoroids. In contrast, the Kuiper belt (Kuiper rhymes with piper), and the Oort cloud (Oort rhymes with sort), which are at the outer edge of the solar system, contain bodies composed of large amounts of ice in addition to rocky fragments and dust. (We will talk more about smaller solar system objects in a later chapter.)

The rules and dangers of the planet-forming game help to explain many features of our solar system today.

• Proximity to the Sun explains why the terrestrial planets are so much smaller than the gas giant and ice giant planets.
• Mars is smaller than it should be, given the rule that distance from the Sun determines how much material a body can accumulate, and this can be explained by its proximity to Jupiter. Jupiter’s immense gravity interfered with Mars’ ability to accrete. Further evidence of Jupiter’s interference is the debris field that forms the asteroid belt. From time to time, Jupiter still flings objects from the asteroid belt out into other parts of the solar system, some of which have collided with Earth to catastrophic effect.
• The Kuiper belt is an icy version of the asteroid belt, consisting of fragments left over from the early solar system. The material in the Kuiper belt is scattered because of Neptune’s gravity. From time to time, Jupiter interferes here as well, flinging Kuiper belt objects toward the Sun and into orbit. As these objects approach the Sun, the Sun causes dust and gas to be blasted from their surface, forming tails. We know these objects as comets.
• Comets may also come from the Oort cloud where gravitational forces from outside of the solar system can hurl objects from the Oort cloud toward the Sun.

The whole process of turning a nebula into the solar system we know today took only a few hundred million years; this is a pretty short time period considering the solar system has been around for 4.6 billion years! Today, the solar system is a much less violent place, thank goodness. However, some planetesimals have continued to interact and collide, and their fragments move about the Solar System as roving “transients” that can make trouble for the established members of the Sun’s family, such as our own Earth.

What this video for a summary of this section.

Text Attributions

This text of this chapter is adapted from:

• Chapter 8 of An Introduction to Geology (2017) by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, and Cam Mosher. Licensed under CC BY-NC-SA 4.0.
• Section 22.3 by Karla Panchuk in Physical Geology – 2nd Edition (2019) by Steven Earle. Licensed under CC BY 4.0, except where otherwise noted.
• Chapter 17 of Introduction to Earth Science, Second Edition (2025) by Laura Neser. Licensed under CC BY-NC-SA 4.0.

Media Attributions and References

1. “The Building Blocks of a Planet | How the Universe Works | Science Channel.” YouTube, uploaded by Science Channel, 27 Jun 2025, https://www.youtube.com/watch?v=V9X71slGO0I&t=242s.
2. Widdowson, Mike. “Chapter 2: The Internal Structure of the Terrestrial Planets.” An Introduction to the Solar System, Third ed., Cambridge University Press, 2018, pp. 52–55.