Colonizing Alpha-Centauri: the least and most we can do
There is at least one planet. Therefore, colonization is on the table
Yesterday it was announced that an earth-sized planet has been discovered circling the nearest star, Alpha-Centauri (around the smaller of its two main stars, actually, Alpha-Centauri B). The planet, Bb as it's currently called, has a six-day year and a surface temperature of 1500C. Not very hospitable, perhaps--but I'm about to argue that it's just fine. If we can get to this star system, we can settle it.
Let's look at two scenarios, a worst-case and a best-case, and see what's possible with each.
The Worst Case
This is a boiling hot planet. Actually, far hotter than boiling. At 1500 degrees, it's hot enough to melt rock. In a worst-case scenario, Bb has the kind of rotational resonance that Mercury does: it is not fixed with one face pointed forever at its star, like our Moon is to the Earth, but rotates so that the whole planet is regularly bathed in the blowtorch heat of the star. If there is an atmosphere, it's mostly composed of evaporated rock.
In this case, much or all of Bb's surface is a lava sea. Oh, and since this is a worst-case scenario, let's say that there are no other planets in the system, not even any asteroids. Bb is it.
If your idea of habitability is finding a more or less exact copy of the Earth and settling down on it to farm, then things are looking kinda bleak. But, if we have the technology to get to Bb, then we have the technology to live and thrive there.
Not on the surface, of course. Not even in a nearby orbit. But even if Bb is uninhabitable, it is still a great source of building material. If we have the technology to get to it, we'll have the technology to mine it, if only by dangling a skyhook down from the L2 point (or from a heliostat) to dredge the magma ocean. Haul the magma up, render it in the terrible light of the star, and ship the refined goods to a higher orbit where the temperature's a bit better. There, we can build habitats--either O'Neill colonies or, if we can harvest enough material, the coronals I describe in my novel Lady of Mazes.
With unlimited energy and (nearly) unlimited building materials, we can construct a thriving civilization around Alpha Centauri B, even if all we have to work with is this one piece of melted rock. (In terms of details, it would be a bootstrapping operation, with an initial small seed of robot miners constructing more or bigger skyhooks, more miners, etc. until exponential growth sets in, by which time it's safe for the human colonists to show up.)
The Best Case
Even for the best case scenario, I'm going to assume that Bb is the only planet in the system. It's more likely than not that Bb actually will be tidally locked to its star--i.e., it has one face permanently aimed at its sun, and the other permanently in darkness. The point that's under a permanent noon (the 'solar pole') will indeed be a lava hell. What's interesting, though, is that some simulations show that the temperature in the twilight zone around the 'equator' and further into the night side could be quite cool. Cold, even, if you go far enough. If there's an atmosphere, there might be water and a zone of permanent rain around the mid-latitudes of the dark side, in a kind of hemisphere-wide hurricane with its eye at the anti-solar pole. And there, we might settle.
I doubt there'd be any oxygen to speak of, but we can generate that ourselves. What I find interesting, though, is that this 'dark side' is not really dark at all. Because Alpha Centauri is a binary star system, Centauri A will be visible in the 'night' sky of Bb during half its year. ...Which is only three days long. So A will cross the sky in about 75 hours, and then there'll be true night for 75 hours. This has been the pattern on Bb now for more than four billion years; it's pretty stable.
Centauri A appears very dim from Bb compared to our sun, but it's still too bright to look at and has a visible disk. It's dimmer than daylight, but much, much stronger than Earthly moonlight. Granted the luminosity range at which photosynthesis happens on Earth, I'd think plant life would do quite well on Bb's 'dark' side.
If the rain's not too bad, much of the 'dark' hemisphere might be settled. Remember that Earth is mostly covered with water; if there's no significant oceans on Bb, but enough water for rivers and lakes, then the habitable land area of Bb might be greater than Earth's. Gravity is the same as Earth's, and in fact the only major difference will be atmospheric composition/density, and the length of the day. And who knows? Maybe we can game those too, by geoengineering the atmosphere, and using a combination of distant orbital sunshades and orbiting mirrors to generate a 24-hour diurnal cycle. Ultimately, Bb could be very earth-like indeed.
The Happy Medium
I expect the reality of Bb's habitability lies somewhere in between the two extremes I've just described. In all likelihood, Bb is not alone; at the very least, there should be asteroids or planetoids of Ceres-size or larger. Bb itself might have a safe spot where industrial operations can be set up, even if it's not a place where you could live. It can export vast quantities of raw materials to colonists elsewhere in the Centauris.
All of which means one thing: Alpha Centauri is now a viable destination. If we can get there, we can live there. And knowing this makes real possibilities that, until yesterday, we could only dream about.