How Do Planets Form?

by Carson
protoplanetary disk

Have you wondered how planets like our Earth formed? They form from a disk that surrounds a star at the time of the star’s formation. Let’s learn more about the planet formation process in this article.

The Making of a Star

To learn how planets are made, we first need to understand how stars form. In the universe, there are many clouds of gas and dust that are loosely held by their gravity. These clouds are nebulae and are the regions where most stars form. When there is an abrupt disruption, like with a nearby supernova, some of that gas could get compressed, and gravity pulls them in even further. A runaway round of accretion begins, where the cloud starts collecting more and more matter until it uses up all nearby material.

At this point, a central object forms, which holds most of the mass. But not all of the system is within that object, be it a brown dwarf, a rogue planet, or a star, as residuals are always present. The leftover material makes up the protoplanetary disk, which then forms, you guessed it, planets.

Formation of Gas Giants

The process of making gas giants is easy to explain. It’s simply an extension of the star formation mechanism. Again, even though the disk might appear uniform, it’s not. Some regions carry more mass than others, given the complex way that particles interact, passing stars and galactic perturbations. Clumps of gas and dust having more mass start attracting other particles, eventually forming a central object within that disk.

The central object, which orbits the star, is usually a gas giant if it’s no more massive than about 13 times the mass of Jupiter. However, it could be a brown dwarf, or even a stellar companion, if it collects more material. There are leftovers when these large objects form, which become regular moons. For example, the famous Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto) are likely formed with Jupiter in its own little protoplanetary disk.

Formation of Rocky Planets and Other Objects

The formation of rocky planets is also similar, except that the main drivers are now dust particles. Remember that most of the material out there is gas, which means that dust accumulation could only occur in dust-rich regions, like the area inside the frost line. As the dust particles develop uneven mass distribution, clumps accrete material due to their gravitational pull. They become numerous small pebbles, as they further accumulate into planetesimals. Some of them are so massive that they collapse into a sphere, and turn into the terrestrial planets that we commonly see today.

The remainder of this material remains detached from any massive body and becomes the asteroids and comets we know today. Because of the small surface area, most of them haven’t been involved in major, composition-changing collisions, making them one of the most pristine populations in the Solar System. However, they could be affected by thermal evolution and space weathering over billions of years, which must be accounted for when studying them.

But this property generally only applies to larger asteroids, which are those over 10 kilometers wide. Most smaller asteroids are reaccumulated groups of fragments from major asteroid collisions. Therefore, they are just gravitationally bound packs of particles, known as rubble-pile asteroids. While they might be more collisionally evolved than the preserved planetesimals, they are still some of the most original samples of Solar System formation that we can get.

Why Is It Still An Active Area of Research?

Now we’ve explained the way that planets form. So why is it still an active area of research? That’s because this process is less predictable than you think. It’s highly chaotic. Consider how many particles there are and how much galactic tides and passing-star perturbations affect their motion. And think about exactly how these particles interact with each other to form clumps and accrete into planets. You’ll quickly realize that this is so chaotic and probabilistic in nature, that two models, even with just a slight difference in parameters, will evolve into radically different planetary systems.

This means that we cannot trace back the state of the Solar System (or any other planetary system) to its formation to look at what’s happening. Instead, we must assess numerous scenarios and find which one fits our observations best. That’s the challenging part of understanding planet formation. Even with decades of research, we still have divergent models about how the Solar System formed. They are continuously improving as more data, particularly those on asteroids, comets, and exoplanets, is discovered over the years.


In this article, we’ve discussed how planets form. As a star is taking shape, the giant planets form quickly, grabbing most of the remaining material. Then, the rocky planets form more slowly, taking up the dust particles inside the frost line. Finally, the remainder of the matter, the asteroids, undergo more collisional evolution before they settle down into the places they’re in today.

If you would like more details from this article, please leave your suggestions in the comments below.

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