We’ve got a lot of work to do here! All we have is dust and a very hot oven. Let’s hurry up, your oven, the Star, is a pretty violent thing. It would continue to accrete matter onto itself and also heat up the materials around which can cause photoevaporation. The disk loses mass very fast, you have only around 10 Myrs to build your planets! So let’s go one step at a time…

You need your dust particles to grow, but not all the materials you have around the star are solids. The temperature varies with the distance from the star, and so do the states of different materials. This leads to specified regions in the protoplanetary disk where materials can stay as solids and thus “grow”. But how exactly?

A very special ingredient would help you know - Van der Waal Forces. Guided by these attractive forces along with a pinch of electromagnetic forces, the particles collide and stick to each other. As a result, your micrometer-sized dust particles will grow into millimeter-centimeter-sized pebbles. If you can reach this stage, then you are on the right track. If not, you may want to start over again!

The next step is going to be tricky, you have a lot of “barriers” in your way. Once you have your mm-cm-sized particles, they are not as friendly as the micrometer-sized particles. They collide against each other, but most of them do not stick together but rather bounce off - The Bouncing Barrier.

Life’s difficult for these poor pebbles. Even if the lighter ones manage to stick to some of the heavier ones, they face a “drag” from the gases around them. The gas slows them down, and they can spiral all the way down into your star - which of course you don’t want. This is the Drift-Fragmentation Barrier. Crossing these barriers would be difficult, but don’t worry kids, you still have the magic element of Gravity, which can help cross this barrier. With various things going on, if a region in the protoplanetary disk happens to reach a higher local density, then gravity takes over and results in a local gravitational collapse. Once the local gravitational collapse has started, the smaller mm or cm-sized particles around the area of high local mass density are also attracted towards the center, and finally what you get is a planetesimal. But but but, these processes are very fine-tuned, one slight change here and there and you may never have a region with high enough local density to start the gravitational collapse. So don’t be disheartened if your disk does not form any planetesimals, it's not an easy job, after all, keep trying! But if you are to succeed, you would be able to see about 1-100km sized planetesimals.

These are the important building blocks for the core of the planet you want to form. It's a delicate process as they slowly fuse together over a period of MILLIONS of years, slowly depleting the 'feeding zone' around them. This is what we call the embryo of the planet. It's like a tiny sphere, floating in space, with a half-empty feeding zone around it.

Now comes the interesting part, for you must choose! Depending on what kind of planets you want to make, you may hurry up or take it slow. Let us discuss the different possibilities one by one.

First comes the Gas Giants, with their Huge size and Gaseous bodies. In order to form these Giants, you need to hurry up. You must have enough gasses around your planetesimals to accrete, and your furnace, the star, is always trying to push away these gases. So let's hurry up and cook these Giants!

But kids, tread with caution because giant planets are made of huge reserves of highly inflammable hydrogen. So get your safety goggles on and let's get going!

With lots and lots of Hydrogen and Helium filling their bodies, giant planets can be larger than 7 times the size of earth! So, how do we continue our recipe? By adding lots of gas obviously. The newly formed embryo is pretty hungry and starts collecting the H and He from the protoplanetary disc around it (core accretion). This gas piles on top of the embryo, gradually increasing the density and temperature, while the building pressure opposes this. Again, it's a constant tug of war between outward pressure and inward gravitational pull. Pressure gets stronger with time and voila! We have a stable atmosphere. So the question is, how much gas should you feed the core? Well, that's a simple answer! You can add as much mass of the gas as the mass of the core itself.

In Fact, sometimes the strength of the gravitational torque is so high, that it gobbles up huge amounts of gas nearby in a very short time, leading to the formation of 'gaps' in the protoplanetary disc around it. The most fascinating part is that deep gaps are the origin of rings around a planet! Once the gaps are formed, the gas outside starts streaming in, bringing solid particles along with it. These solids it turns out, are the birthplace of satellites! Not the Death Star one's mind you, the natural ones.

We are nearly there! Now the furnace comes into play and removes the remaining bit of gaseous material away from the newly formed planet using a thermal wind.

Moving on to the next type of planets, the terrestrials… You can slow down a bit for them, you don’t need to do much, they’re pretty easy to make. Unlike the core accretion in Giants, the primary process which leads to the terrestrial planets is pebble accretion. These mm cm-sized pebbles fall down into the hungry planetary embryo, making them as big as about 100 times the individual planetesimals. The furnace which was initially a bane for the Giants, prove to be a boon for the terrestrials. The star dissipates away the gaseous protoplanetary disk, removing the drag mechanism. This allows for the embryos to stay in a stable orbit and accrete for a very long period of time - the “late-stage accretion”. The huge embryos clear out the materials around them either by accretion or by disrupting their orbit with their huge gravitational pull and ultimately throwing them off the system. And now you can rest… let the system evolve for some million years and you would end up with the sweet little terrestrials you wanted.

Hurrah! Good Job everyone. I hope all of your planets turned out the way you wanted.

A few words of caution from me and then Grandma and we'll leave you to your delicious feast.

Remember to not keep those planets out for too long, or soon your precious planet will be crawling with annoying pests. It happened to Grandma once, I left a cooked a planet, it was my special recipe called “Earth”, but then I got a call and forgot about it. After the call, when I came back it was ruined, infested by HUMANS! Trust me, kids, you do not want that happening to your planet too. They destroyed my beautiful planet and turned it into a planet full of dust, dirt and what not!

That’s all from Grandma’s side. Now run along and enjoy your tasty planet meal.