Terraforming

Note to stability:

Whenever the requirement of stability is mentioned, you should also consider the proper timescale, as compared to the human lifespan. Geological and astronomical phenomena can take LONG. If a planet becomes uninhabitable after ten thousands years, it is perfectly fine for settling down now! Even centuries may be okay.

Orbit and rotation

Changing the kinetic properties of a planet is not an easy task, though more a question of raw power than of intricate knowledge. A typical method would be bombarding it with directed comets and asteroids, giving it little pushes at the right moments (and handily depositing organic substances, incidentally heating up the atmosphere a little).

But this is not only a matter of how long you prefer your day, or year to be! Distant planets receive less sunlight, closer ones more; a too distorted elliptic orbit may cause extreme shifts in temperatures over the year, and require intervention. Day and night mean variations as well, so they need to be balanced similarly.

Atmosphere

For planet-based life, an atmosphere is a most handy place to deposit the waste, and find basic nourishment. As everything diffuses in it fast, basically any lifeform from the surface can be expected to interact with the atmosphere to some degree. It is therefore of key importance to have this part covered.

Small planets may have none to speak of, so a weak adaptation would be to make an atmosphere that is sufficient to allow people to carry masks instead of vacuum suit. A breathable air, though, is something different. If lacking the gravity to hold its air, some parts or all parts may over time 'evaporate' into space. Especially hydrogen is susceptible here, this is suspected to what happened to Mars - water would decompose in the upper layers of atmosphere, the hydrogen leaving, oxygen staying to be eventually bound to other chemicals.

Still, small planets may be designed with this risk taken into account, renewing the composition of gases regularly. (Consider stability.)

Temperature

To a large part, the temperature of astronomical bodies considered here will be determined by the activity of their sun - and what happens with the radiation on/around the planet itself.

An important term is Albedo, the reflective ratio of electromagnetic radiation power; simply put, white surface reflects light (high albedo), while dark surface absorbs it (low). For our purposes, we can extend this beyond the visible parts of the spectrum.

The albedo of a planet can be influenced by the atmosphere itself (and the clouds). And these three in turn can be influenced by the biosphere once it has sufficient impact - so select the initial flora carefully, to avoid complications.

But let's reconsider this from the main point of view: Life, as we are used to it, requires a thin range of temperatures to prosper in. Life in general can be expected to be much more robust.

Surface

For a comfortable living, the surface should be mostly stable, as intense tectonic activity can spoil anyone's day. From the initial and unstable periods of a planet's existence, this activity can be expected to lessen over time, as a homogeneous body forms and the internal heat with radiation cool down. (Moons) and other interstellar bodies disrupt this tendency towards piece.

According to some theories, tectonic plates we know on Earth are caused by life as well. The slow, but relentless processing and transporting of some materials, and the frugal deposition of others (a classical example are corals), keep exerting pressure on parts of the Earth's crust. Given enough time on the (geological scale), this may drive the tectonics of a whole world.

This doesn't even start on the topic of surface itself, ie geography. There can be 'young' worlds with many extremes, and strongly eroded 'old' worlds. There can be worlds mostly covered by oceans, or purely desert worlds. Anything is possible.

Moon(s)

While all of this is hypothetical, the role of our own moon is to many scientists vital, if not critical for our environment. On the one side, it probably helps keeping the core of Earth in rotation, thus enabling a strong magnetic field which is also deemed important. On the other hand, there is the immeasurable impact on our oceans.

Note that our moon is relatively large when compared to our planet. The effect of a smaller moon or several is likely to be different.

The Gaia factor

Life, when in sufficient amount, will influence or dominate a planet's weather, and determine things like temperature, humidity and surface composition. Where such a complex system will evolve may be hard to tell - it may very well grow beyond the plans of its creators, and beyond their control, though not necessarily to the detriment of life in general.

The same is true for the composition of the atmosphere, so care should be taken to pick the right fauna and flora, and if needed change it to achieve the necessary dynamic stability.

The proper science for the study of these effects may be geophysiology, more popularly known as the Gaia theory. Life, it claims, has a tendency to support itself.

Methods of terraforming:

- life - where applicable, organisms, probably genetically engineered or altered, will tackle on the task of converting the geosphere into a biosphere, see also Ecopoiesis.

Although effective, and good at creating an initial store of organic compounds for the later, more advanced biosphere, this can take VERY LONG, picture hundreds or thousands of years. Races with a sufficient lifespan or foresight may seed this way numerous planets for later use - and be quite annoyed when they come to their new home only to see it was 'discovered' by somebody else!

Even if other methods are used, it is useful to start planting useful plants as a planet becomes more and more habitable. Making the planet alive will take time.

- chemistry - making the problem one of the right catalysts, all is needed is a massive reactor to supply energy, and the time for the brute-force approach to get the job done.

- nanotechnologies - a potentially very viable option, to let tiny machines do the work for you. This may be easy, but watch for the chance of their 'mutation', or getting out of control like with the lifeforms. (Most likely they will have to multiply on their own to cover a whole planet, so tiny flaws could be replicated...) It is suggested to make them of limited duration to naturally pass away once their purpose is fulfilled.

Many worlds

The Firefly setting, as cool as it is, rests on one very unrealistic assumption: dozens of planets and moons fitting for life exist in a single solar system. But for the sake of storytelling, we can't let ourselves limit with something that is merely 'improbable', can we?

But there can (and should) be even stranger planets out there.

One source is the Worlds and Planets scroll, some examples will follow.