There’s a scene in the 2nd or 3rd Star Trek movie, I forget which one, where some sort of high-tech projectile is launched at a barren planet. In a burst of light, the planetary surface springs to life. A fully functioning biosphere arises in the twinkling of an eye.
The technical term for this transformation is “terraforming.” A planet is made to resemble the Earth (Latin terra). The advantages of terraforming for human interstellar travelers are obvious: If you can turn any old rocky planet into a Garden of Eden, the galaxy is your oyster. (And we’re going to sidle away from that image without examining it too closely.)
For that reason, terraforming is a popular topic in science fiction novels. Kim Stanley Robinson had, I believe, a success with his trilogy on the terraforming of Mars. I dropped out halfway through the first book when I hit a scene where it became painfully obvious that Robinson didn’t understand the mechanics of lighter-than-air travel using a balloon or dirigible. If he didn’t know how balloons work, I figured, he wasn’t going to be much use on the science of terraforming Mars. But that’s beside the point. The point is, the subject of terraforming is a rich source of story ideas.
Unfortunately for authors, terraforming is about three orders of magnitude more difficult than you think it is. And that’s probably true no matter how difficult you think it is.
Finding a planet of suitable size whose orbit is at the right distance from its primary to provide a surface between the freezing and boiling points of water isn’t even the start of the difficulties. The planet is going to need an iron core that’s rotating in relation to the surface. Why? Because the magnetic field generated by the core is what keeps life on the surface from being toasted by cosmic rays. If the planet doesn’t already have a rotating iron core, there is no conceivable technology that could create one, so your terraformers may have to hunt for a while to find a planet that’s a good candidate.
Nor are cosmic rays the only source of toasting. Your planet is going to need an ozone layer high in the atmosphere to screen out the ultraviolet light.
But let’s not worry about that yet. Ozone is made of oxygen, and you haven’t got any oxygen. Oxygen is produced by photosynthesis. In order to put free oxygen in the atmosphere, you’re going to need plants — or, at the very least, cyanobacteria. But let’s not worry about that yet. First you need water. Billions of tons of water.
Our best theory about where the water on Earth came from is that there’s water in comets, and early in Earth’s history (like, four billion years ago), the surface was being bombarded by comets. This is a very reasonable theory. Science fiction writers may therefore want to imagine that terraforming a world (either Mars or one in another solar system) will involve steering a whole bunch of comets in from the outer edges of the solar system and crash-landing them on the planet. Computing the proper trajectory so as to get a comet to hit a planet is fairly trivial, even for a 21st century computer. But first you have to find the comets, and then you have to propel them.
The amount of rocket propellant required would be non-trivial. If your shipload of intrepid explorers is in some other solar system, they definitely won’t have brought along the amount of rocket propellant required. Whether it’s even possible to travel by rocket to another solar system is very doubtful, so your novel is going to have to propose some form of magic physics, both to move your explorers’ ship and to then move the comets.
The comets, even after you find them and aim them in toward the inner part of the solar system, won’t arrive for some years. Until they arrive, the terraforming can’t even begin.
Once you have a planetary surface swimming in fresh ocean, what are you going to do? Our current technology is not able to build even a single living cell from scratch. To design an entire ecosystem, which will of necessity contain millions upon millions of species (many of them microbes), is not something that contemporary science can even imagine. And to drop the entire ecosystem down on the planet at once, trillions of tons of living organisms — living earthworms, living insects to pollinate the living plants, living bacteria to fix the nitrogen in the plants’ roots — oh, wait. We forgot the part about the oxygen. Your plants and earthworms are about to be toasted by the ultraviolet radiation. You have to make gazillions of tons of oxygen before you initiate the cycle of seeding living organisms. How are you going to do that?
If you can wait a couple of billion years, this stuff gets a lot easier. Life on Earth has been around for at least 3.5 billion years. But for the first 2 billion years and more, it was all single-celled life. There weren’t even any jellyfish yet. Multicelled life appeared on Earth only around 500 million years ago. And even then, the evolution of vertebrates who could live on land proceeded very, very slowly.
I’m not saying that terraforming is impossible. I’m saying merely that it would require Godlike powers. If your interstellar travelers bear even the faintest resemblance culturally or technologically to the familiar humans you meet on the street, forget it. There is no conceivable technology with which any alien species, much less humans, could produce a livable planet in less than a thousand years or so, and even that vastly accelerated process would be so filled with pitfalls that your Godlike aliens would surely have to work the kinks out by trying and failing multiple times.
This is why I write fantasy rather than science fiction. Science fiction is too hard.