So, you want to create a planet? Whether it is for a game or just for geofiction, as part of a larger Conworld or alone, a planet is a good place to start building your setting. Starting with the planet gives you the ability to set up your world from the big to the small, from the large-scale setting to the tiny details. You can build the environment, then the various lands and places, the climate, weather, and down to the cultures, languages and characters of the world.
In this Wikibook, we'll get started on the construction of a conplanet. We'll be working from the general rules and guidelines of building a planet down to the details of climate and geography.You can skip steps if they don't seem to fit your particular world, or focus on one area more than another. Do whatever works best for the particular world you are creating.
Planets are complicated things. To build them, you must understand them. So, to begin: Some planet basics. All that follows assumes laws of physics that do not differ from those of our universe. The farther you get into the realm of fantasy, the less you need stick to these guidelines.
Planets are round. This is the simple bit. Any fictional planet that you construct will be round (roughly spherical) for the simple reason that all planets are, by definition, round.
All planets are round because, as Wikipedia says, a planet is "massive enough to be rounded by its own gravity." So, any constructed planet must be massive enough to become rounded by its own gravity. You can play around with non-planet celestial bodies (e.g. a giant oblong object floating in space) as the basis of your setting, but if you do you will not be dealing with an actual planet.
A planet must be massive enough to be round, but it cannot be too massive. If, for example, you create a planet that is as big as Earth's Sun, you will have a big problem. Past a certain point, the planet becomes so massive that it begins to cause thermonuclear fusion. Then your planet becomes a sun.
A planet must be massive, but not too massive; round; and also orbiting. The word "planet" comes from a Greek word (gr) which means "wanderer." Wikipedia puts it very nicely; a planet must be "a celestial body that is or was orbiting a star or stellar remnant."
So, any planet that you will invent must be massive enough to be round, not massive enough to cause thermonuclear fusion, and orbiting a star. Any setting that you create that does not fit this basic description is not a planet. Which is OK. Sometimes breaking the rules a little, exploring the edges of the "normal," is exactly what you need to do to give your constructed world a little flare.
Things to Consider
A great number of events had to occur in order for Earth to become habitable to complex, multi-celled life, including humanity. While science-fiction for the past 60 years has been filled with a seemingly endless line of aliens from all sorts of strange and amazing worlds, our current understanding of planets and what requires from them in order to exist means that life is likely to be fairly similar in chemical composition and biology. We know that life is possible in galaxy because it's happened once before: on Earth. What scientists don't know for certain is just what are the minimum requirements life has. If you are planning on creating a truly imaginary alien race and are not that concerned with scientific accuracy, then you don’t have too much to worry about. You can skip this part and get stuck into your writing. For everybody else, it's probably best for us to begin by discussing our own planet's history and what allows us - humanity - to exist.
As a bare minimum, for a planet to be naturally habitable by any organism more complex than a single-cell, we first need a star of the right age (probably quite similar to our own sun, or Sol) so it has enough of the heavy elements needed for planet formation. Then those elements need to be transferred to the rocks and debris circling the star as those rocks begin to form a planet. The planet has to form within the so-called Goldilocks' (or habitable - HZ) zone where liquid water can form and for that HZ to remain constant over millions of years. The star's luminosity is responsible for the HZ position and if it varies, even by a tiny amount, life may not develop. Once the planet forms, it will require tectonic plates to maintain the planet's temperature and volcanism to allow new crust to be created. It should be noted here that water boils at 100c but no life form, single-celled or otherwise has ever been discovered on Earth that can survive anything above 50c. Therefore, the planet must cool down even if it is in the HZ. The planet also requires a moon, to induce axial tilt. This creates seasons and stimulates biological growth. However, the axial tilt cannot be so severe that the temperature difference between summer and winter kills life. Likewise, the planet will also probably require a day-night cycle but not one overly long so life is also wiped out by the cold of the night and heat of the day. Assuming all this occurs, the next hurdle for life to overcome is the one of actually developing. As yet, science is still unsure as to what caused life to first appear in Earth's oceans all those millions of years ago and even if life does appear, there are no guarantees it will be intelligent. Evolution doesn’t have a "goal" as such. There are countless millions of animal species on this planet and only one has intelligence.
Once an intelligent species appears, it will then have to create civilization which can both survive the natural world and develop technology to grow, while all the time hoping that it doesn’t fall victim to any of the deases and natural disasters a planet can throw at it.
There are things we can say for certain about an alien life form. Firstly, it will be organic, meaning it will have emotions and a survival instinct. If an organism is individualistic in nature, like humanity, it will require resources to survive (heat, food, water for example) and will take those items by force, if necessary. Therefore, said species are likely to have wars throughout their history. Some form of bartering system, giving a relative value to a resource is also likely. Unless the organism possesses some form of empathy or "hive-mind" linking each individual unit, separate groups will form communiaties and may eventually become nations.