"It all began with a big bang... " Carl Sagan
Whether you're a Christian, or you even believe in a god at all, the place to begin in planetology is at the very beginning - at the formation of the universe. Most astrophysicists will acknowledge that within minutes (possibly), but certainly within 36 hours or less, the rules governing the universe were operating pretty much as they are today. Stars are born from hydrogen gas, live a certain length of time based on their size, and then die. When stars die, either with a bang or a whimper, they release all the complex elements (larger than hydrogen) that currently exist, from helium to the heaviest natural elements ever discovered. This is especially true of very large, hot stars that explode as supernovas.
Stars more or less clump together in galaxies and clusters. Our current knowledge of astrophysics says that planets form around stars. The Hubble Telescope and other new astronomical tools are now showing us that planets are far more prevalent than previously thought. There is some evidence, discovered in the last decade, that even multiple-star systems may have planets. That has forced a number of disciplines to begin considering some questions that only a decade ago were the sole property of mystics and science-fiction writers: can any of those planets support life? Even more to the point, can they support our kind of human life? That is forcing quite a number of scientists to re-think what for them was truth, before new information changed the equation.
We have a really GOOD example of what kind of planet would be suitable for humanity to settle in the one we currently possess. The question remains, however, how far could a possible prospect stray from the template and still be suitable for people to live on. Let's take a look at some of the factors.
The biggest factor to consider is the sun that particular hypothetical planet revolves around. Some of the factors we want to look at are size, output (actually, total solar irradiance. The Wikipedia definition is so poor I won't use it.), and variability.
Size: Stars vary from extremely stable and long-lived red dwarfs to hyper-unstable very large (and very bright, hot) "O" and "B" spectral class suns that would be very poor choices (See Hertzsprung-Russell Diagram for a discussion on stellar classes). There is a general rule of thumb that the larger the star (actually, the greater its mass), the brighter and hotter it is. White dwarfs don't fit this definition well, which is one reason they're not on the main sequence. Most stars that mankind would expect to find habitable planets around would be on the main sequence.
There are also some unusual types of stars that might be worth visiting for scientific purposes, but wouldn't be very good for colonization. This is especially true of variable stars, such as Cepheid variables and the even more exotic RR Lyrae variables. Other types of stars that might be worth examining, but would definitely not be worth living near would include neutron stars, brown dwarf stars, possibly white dwarf stars, red dwarf stars, and black holes.
Output: There are three things at work here: the overall radiative output of the star, the stability of that output, and the planetary orbit. These three characteristics define the subsequent habitability, or "Goldilocks" zone.
A star on the main sequence probably has a total solar variance similar to our sun, or about 0.1%. Earthlike life could probably survive if total irradiance varied by up to about 0.5%, but not much beyond that. Stars that are not relatively stable, where the radiative output varies by more than about 1%-10%, are not good stars to try to live around. Solar output is one of the determinative aspects that establish the Goldiliocks Zone where liquid water could be expected to be present. Solar output should be mostly in the visible and infrared (heat) portions of the spectrum.
Free water is an essential ingredient for any type of earth-like life. How much free water is available is also a limiting factor on the amount of life a given planet can support. A star that is not relatively stable would oscillate between a hothouse atmosphere such as that of Venus, or a frozen ball similar to Earth during an Ice Age. If the variability was severe enough, or frequent enough, the planet could experience both in a very short time -- as little as three or four days. Even if the period was years long, the cycle would be too rapid for evolution to allow any plant or animal life, except perhaps in deep oceans, to survive. Let me explain:
Most types of plants on Earth grow from seed, and have a life cycle that includes initial growth, maturity, and death and decay. The soil temperature must be above a certain point for seeds to sprout, within a set range for a minimum time for the plant to grow and seeds to develop and mature, and there has to be some mechanism to disperse those seeds. This is best exhibited by the concept of "treeline". Treeline is not a constant: it varies by lattitude (there are other factors, but latitude is the most predominant). At the equator, treeline may be above 16,000 feet above sea level. In Colorado, it's around 12,000 feet, and in Alaska it's at around 4500 feet. It also varies by the amount of sunlight the slope gets, but that usually accounts for less than 400 feet between east-facing, west-facing, north-facing, or south-facing slopes. The limiting factor is how much irradiance the slope receives, so that seeds can sprout and grow sufficiently to survive the coming cold.
The "treeline" for a star whose output varies by 1% or greater could change so rapidly that the seeds could not sprout, mature and disperse without some truly extraordinary differences from Earth-type plants. Animal life would require similar, rather extreme modifications. Larry Niven handled this very well in his short story, Flare Time. Niven also highlights how the significant increase (or decrease) in non-visible radiation could require some significant adaptations, as well. We on Earth are shielded from most non-visible radiation by our planet's magnetic field. We'll get into problems with that in a later paragraph, but needless to say, unless the magnetic field strength also increases with increase in radiation, more and more harmful radiation will leak through to the planet's surface. That could make life very, very difficult.
The habitability zone for a planet lies within a range from the associated star that corresponds to the ability of water to exist as a liquid, and thus for the planet to have weather driven by the water cycle. This is primarily a relationship of the planet's orbit and the sun's output. Planets would have to be very, very close to a red dwarf, farther from a sun like the Earth's, and even farther still from hotter, brighter stars. The average surface temperature of the planet would have to be above the freezing point of water (32 F/0 C), and below the boiling point (212 F/100 C). These temperatures would have to be relatively stable (trying to calculate how plants and animals would have to adapt to rapid changes from ice-age climate to hothouse climate, possibly in less than a decade would be quite a challenge) from year to year.
Orbit: Several other factors would also affect whether a particular planet would be capable of sustaining life -- any life, but especially, Earth-type life. One key factor is the planet's orbit around its star. If the orbit is too eccentric, the effect would be similar to being in a stable orbit around a modestly-variable star. The planet is too near its primary for part of its orbit, and too far away during other parts, resulting in a huge fluctuation in average planetary temperature. The Earth has an eccentricity of 0.0167, which isn't terribly bad, but which contributes to Earth having Ice Ages and Warm periods. If you're terribly interested in this, and want to learn more, check out Kepler's Laws and this.
Let's assume that we have the means to visit other star systems at will, and we've weeded out all those suns that don't meet our criteria for size, output, and stability. We've surveyed the rest of the stars, and discovered several candidates that meet our criteria, and have planets in the so-called "Goldilocks Zone". Our next task is to determine if those planets can support Terrestrial life, or have compatible life of their own. What should we be looking at?
Size: One of the major considerations is planetary size. We can ignore any Jupiter-sized planets automatically -- we couldn't tolerate the gravity. We need something about the size of Earth - about 8000 miles in diameter. That would give us approximately one earth-gravity, as well. Before we begin eliminating planets larger or smaller than this, we have to determine one other factor -- the planet's total mass. That is determined by the planet's composition.
Earth is relatively dense. In fact, it's the densest planet in our solar system at 5.51 grams/cubic centimeter. Much of that mass is located in the core, which is considered to be mostly iron (Check here for more information on the density of planets in our solar system.). Our search for an Earthlike planet, then, would be one about the same size of Earth, with a density between 4.75 and 5.8.
Planetary gravity is determined by both size and density. A planet slightly larger than Earth, but less dense, could be inhabitable. One slightly smaller than Earth, but made of denser materials, may or may not be inhabitable, but surface gravity wouldn't be one of the stumbling blocks (click here for more discussion of gravity in relation to general relativity).
Magnetic field: Another major consideration is whether the planet we're examining has a magnetic field. The Earth's magnetic field protects us on its surface from gamma rays from space, and other harmful radiation from the sun itself. It also provides a defense from the solar wind stripping away more of the Earth's atmosphere (see below). The Earth's magnetic field originates from the rapid rotation of the Earth's iron core. We'll come back to magnetic fields later.
Atmosphere: All stellar bodies have an atmosphere. Some are so dense they make life impossible (Jovian type planets), or so tenuous life can't exist (the moon, for instance). Having and keeping an atmosphere is dependent on two factors: the size of the planet/object, and its distance from its illuminating star. The larger the planetary mass, the greater amount of gas it can capture for an atmosphere. The distance from its sun will determine whether the planet in question can keep its atmosphere, as the solar wind can strip atmospheric molecules away. This (distance) is why Mars has a denser atmosphere than Mercury, even though surface gravity is slightly higher on Mercury than it is on Mars. This lecture provides some excellent information on planetary atmospheric evolution, and why some planets approximately equal in mass have different atmospheres.
Having an atmosphere is just the beginning of our examination, however. We also need to determine if it's the right kind of atmosphere to sustain Terrestrial life. Something very close to Earth's atmosphere is what we're looking for. The Earth's combination of 78% nitrogen, 21% oxygen, and 1% other gasses provides an ideal atmosphere for terrestral life, since that's where it developed. If the nitrogen were replaced with another gas, for instance florine or chlorine, terresteral life would be impossible, as both are extremely corrosive to water-based life. Also, if the concentration of what are referred to as "noble gases" was greater, it could change not only what kind of life developed, but how terresteral life would respond. Nitrogen is an essential element for plant development. If the partial pressure of nitrogen was substantially lower, there might not be enough converted to nitrites, nitrates, or nitrides to sustain terresteral plant life.
Another major goal our explorers would have to determine is what stage of atmospheric evolution our target planet was in. Some stages can be "hurried along" through terraforming; others just have to be waited out, which could take eons. Our Earth went through at least two, and perhaps more, evolutionary stages. Only the last one can support life as we know it today.
Surface: Most planets in the size and distance category we're looking for should have a combination of land and water areas. The ratio on Earth is approximately 70/30 -- 70% ocean and 30% land. While this may be ideal for Earth, significant variations could be suitable for Terrestrial colonization. Anything less than about 50% ocean would create some unusual stresses, however. Oceans provide the thermostat for planets. Heat them up, and they produce more water vapor and more clouds, increasing the planet's albedo and cooling them off. Cool them off and the relative humidity drops, more sunlight passes through the atmosphere, and it begins to warm back up. As the temperature warms, more moisture is released into the atmosphere, increasing the greenhouse effect, and further warming the planet (water vapor is the major greenhouse gas in Earth's atmosphere, contributing to 96% of our current warming). Oceans also retain and transport heat via currents from one area to another, and can have a substantial effect on the local climate. A good example is the heat transported by the Atlantic Gulf Stream from the warm waters of the Caribbean to Iceland, England, and northern Europe. Average temperatures in these areas would be as much as ten degrees F cooler without the Gulf Stream.
Land surfaces don't have to be as extensive as ocean surfaces, but colonization can't take place without them. Land surfaces provide direction for ocean currents. A mixed surface of land and water enhances the water cycle, provides additional nutrients in oceans from runoff, and provides the foundation for a greater diversity of ecologies than pure ocean environments. In fact, the oxygen/CO2 balance can't be maintained by oceans alone -- land-based plants are essential. They're also what the majority of us are used to living upon. In fact, the absence of large land areas would be sufficient in itself to affect the colonial potential of a planet. We'll go into that further down below.
Finally, there's one more thing our potential new home needs -- a sizeable moon. There may be more than one, but at least one sizeable moon is essential for the type of planet needed for terrestral plants and animals. Here's why:
Tides: Tides "stir the oceans", help form currents, act to both build and erode land areas, and do many other things. Tides are even part of the "braking mechanism" that is slowly lengthening our day. Tidal drag from the sun and the moon also play a part in plate tectonics, an essential function of the redistribution of chemical elements in a planet's surface environment.
Planetary rotation: A moon revolving around a planet can increase or decrease the planet's speed of rotation. This is thought to have happened to Earth in the past, with the tidal effects of the moon lengthened the planet's day. Tidal forces can cause a satellite to show only one surface to the planet it revolves around, and can even destroy a satellite if it approaches too close to the planet's surface. More on this later.
It may take us a long time to be able to go to other stars. What if we need more space in the meantime? Are there any possibilities in our own solar system? Yes, but...
There are two planets we can consider terraforming: Mars and Venus. Both have good potential, and both have serious drawbacks. Let's start with Mars.
Mars has both significant potential and terrible problems for terraforming. As we saw earlier, Mars has a much lower density (3.98gm/cc) than either Earth (5.51gm/cc) or Venus (5.2gm/cc). This is the first of many problems Mars has that would have to be changed to convert it into an Earthlike planet. Some of the other problems that would have to be overcome include the planet's size, atmosphere, moons, surface temperature, and a few hundred others. Yet it's not inconcievable to convert Mars into an Earthlike planet. It would be expensive, time-consuming (on the scale of 10,000 years or more), and difficult, but not impossible. Luckily for us, and for any terraforming attempt, much of the solution is close at hand in the asteroid belt.
The first consideration in terraforming Mars, not just camping on the surface (which is all living in a habitat would consist of), would be to increase both its mass and its density. This would best be done by bombarding its surface with tens of thousands of chunks of rocks from the asteroid belt. Special consideration would have to be taken to select rocks of high density, especially those consisting of nickel-iron, and some radioactive elements (radium, thorium, uranium). Terraforming would not only have to expand the general size of Mars, but also its density. It alsos need to work that nickel-iron and those heavy elements into the planet's core, where through the mechanism of gravitational compression and radioactive decay, the core will heat up and eventually again liquify (or become more liquid). This is necessary to increase the weak magnetic field of Mars to the point where it can protect unexposed human beings on its surface, trigger plate tectonics in the crust, increase volcanism to release both water vapor and other atmospheric gases, and generally begin to turn Mars into a new Terra.
In the process, the gravity of Mars would increase to approximately that of Earth, giving it the ability to maintain that earthlike atmosphere. This process would require adding as much as 1800km to the radius of Mars, and take a substantial part of the material currently in the Asteroid Belt. If sufficient material isn't available in the Asteroid Belt, or if that material isn't suitable, it would most likely have to come from the Oort Cloud, or the leading and trailing trojans from Jupiter's and Saturn's orbits, significantly increasing both the time and cost of the project.
Without increasing the diameter, mass, and chemical content of Mars, any other attempt to terraform the planet would be less than successful.
The second consideration would be to move the two small moons of Mars (perhaps adding them to the surface), and replace them with a single large moon, or two or more medium-sized moons. The two small moons of Mars, Deimos and Phobos, are part of the problem with Mars, as they orbit so close, and so fast, that they help whip atmospheric particles to escape velocity, thinning the existing atmosphere. At the same time, they're not large enough to create crustal tides, which would help develop plate techtonics and crustal evolution.
Third, it's going to take time -- tens of thousands of years. It's also going to be turbulent. There will be earthquakes as the internal structure of Mars is reconstituted. Volcanos will sprout along weak zones in the crust. It will take centuries for the heat of compression, radioactive decay, and other processes to bring the newly-added high-density materials to the core of the planet and heat that core to a significantly high temperature. Then things will have to cool down. Once liquid water begins to accumulate on the surface, the planet can be seeded with special bacteria to release free oxygen. Over the years after that, additional plants, then animals, can be added to the biosphere until Mars is a fairly close reproduction of Earth. Altogether, it would be expensive and time-consuming, but workable.
Venus presents significantly different challenges of terraforming to that of Mars. The problem with Venus is that it has too much atmosphere, of the wrong kinds of gases, and is too hot. The only mass that terraforming would have to add to the planet would probably be water and other gases, but not immediately. The first thing that's required is to provide Venus with a suitable satellite, one approximately 75% as large as the Earth's moon, but orbiting closer than our satellite. The best orbit would have to be calculated, but about half the orbit of the moon around the Earth (~230,000 miles) would probably suffice.
The addition of a large satellite around Venus would do multiple things. First, it would help strip some of the excessive atmosphere from the planet. Secondly, it would increase the rotation period of Venus, probably triggering the planet's magnetic field (which is currently extraordiarily weak). Thirdly, it would help initiate plate tectonics that would help both redistribute material in the crust, increase volcanism, and add other chemicals to the planet's atmosphere (especially water vapor).
This would not be a fast project. It would take thousands, perhaps even millions of years, to strip the excessive atmosphere of Venus, increase its rotation to something capable of supplying a significant magnetic field (even more necessary on Venus than on Earth or Mars, being closer to the sun), and reworking to crust to allow liquid water to form. Even then, the planet would be inhabitable mostly around the polar regions, with minor excursions into the lower lattitudes. There may be other problems to be overcome, but we won't know until we can actually explore the surface.
All things considered, it may be better to construct artificial habitats than to try to terraform Mars and Venus. Some of those habitats would undoubtedly be placed on the surface of those two planets for scientific exploration, but long-term living there would be problematic.