For the sake of this guide the planet we will create will be called my_cool_planet for a mod called my_cool_mod.

Note, that in order for your planet to spawn ingame, you will have to explore new areas of the universe, or, even better, create a new one.

This guide will not tell you how to create a biome. If you want to learn how to do that, you can read this guide:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2128333589
This guide will not tell you how to create or add weather to the planet. If you want to learn how to do that, you can read this guide:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2608845860
This guide assumes you already know how to unpack Starbound game files (images, textures, icons and scripts). If you don't, here are helpful links:
https://starbounder.org/Modding:Basics#Step_1_-_Unpacking_Assets
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1161535874
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2145917164

First of all, we need to have three things:

1. Planet base colours and dynamics;
   
2. Planet horizon textures and masks;
   
3. Planet icons.

Of course, if you don't want to draw these things, you can use stuff from already existing planets. If you deside to do that, you will have to reference them accordingly in other steps.

Here we design how your planet will look on the celestial map.
These are just circles filled with a certain colour, that are applied to different layers on the planet.

If you want to create your own base colours, you should create them on a specific path:
"my_cool_mod\celestial\system\terrestrial\biomes\my_cool_planet"
And call them "maskie1.png", "maskie2.png", etc.

Note: you can use different paths and names, but for the sake of this guide I will use names and paths similar to the ones used by original Starbound resources.

Example for the forest planet:
As you can see, all colours of the forest planet except water are here.
Similarly, when creating base colours for your planet, you should exclude any liquids. We will add them separately.


These are basically all possible masks, to which our base colours will be applied.

Of course you can create your own, but I would highly recommend just going with already existing ones, for example here are temperate dynamics:
If you want to use them, you don't need to copy them anywhere, just skip this step.

But, if you want to create your own dynamics, you can create them on a specific path:
"my_cool_mod\celestial\system\terrestrial\
dynamics\my_dynamics"
And call them "0.png", "1.png", etc.
Again, you can use your custom path, but I highly recommend this one.

Here we design how your planet will look up close while you're on your ship.
It's hard to explain how these should look, so let's just look at an example for forest planets:
If you want to create your own horizons, you should create them on a specific path:
"my_cool_mod\celestial\system\terrestrial\horizon\textures"
And call them "my_cool_planet_l.png" and "my_cool_planet_r.png".

Again, if you want to use already existing ones, you don't have to copy them.

These are all possible masks, to which our horizon textures will be applied.

You can create your own, but I would highly recommend just going with already existing ones, for example here are temperate dynamics:
If you want to use them, you don't need to copy them anywhere, just skip this step.

But, if you want to create your own dynamics, you can create them on a specific path:
"my_cool_mod\celestial\system\terrestrial\
dynamics\my_masks"
And call them "0_l.png", "0_r.png", "1_l.png", "1_r.png", etc.
Again, you can use your custom path, but I highly recommend this one.

These are two almost similar icons that will represent your planet in user interface.
If you need examples, they are located on these paths:
"packed\interface\bookmarks\icons", "packed\interface\cockpit\planets"
There they are:
In this case it is absolutely important that you place them on their respective paths:
"my_cool_mod\interface\bookmarks\icons", "my_cool_mod\interface\cockpit\planets"
Don't forget to call them after your planet, like "my_cool_planet.png".

Let's put our graphics to use.
Create a file called celestial.config.patch in the root folder of your mod, like this:
"my_cool_mod\celestial.config.patch"

This is a json-patch file. If you're not sure how they work, you can checkout this guide:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2124444255

Put the following code in this file:

Everything following two dashes (//) are comments and can be omitted.

Here's what we would get if we were to add another forest planet:

Same as with celestial.config.patch, you will need to create terrestrial_worlds.config.patch in the root folder of your mod.
Put the following code there:
Again, if you want to make your own biome for this planet, here's a helpful guide.

Here we will set your planet's description and other stuff for the celestial map.

Create a cockpit.config.patch file on this path:
"my_cool_mod\interface\cockpit\cockpit.config.patch"

Put this code in the file:
That's it!

Sometimes when testing, your planets may appear oversized on the celestial map. If this happens to you, check the game log.

That is all I can share with you on this topic. I will expand it if I find/learn more about it.
If you need full planet examples, you will need to unpack game files. There's a number of Steam guides that tell how to do that.

I know that there are guides on this out there, but information in those that I found turned out to be somewhat outdated, which is why I made this guide.

Please let me know if I forgot or mixed up anything.
Thank you for reading!