Red giants are big stars that have left main sequence. They change fast and will, at some point, go supernova or will decay into White dwarfs. However, because human lifetime is so short compared to a star, they can be a good destination for settlers. They have a certain Habitable Zone.
The star Edit
First of all, there is not one single type of red giants. There are sub-giants, giants, supergiants and super-supergiants. Second, they don't all have the same color. This is the fate that our sun, Sol, will face one day.
As a star finishes its hydrogen supply, it starts to fuse helium into heavier elements. Then, when helium is exhausted, it will start using whatever it finds. Depending on star mass, there is an upper limit. Anyway, since no human has lived enough to witness the whole transformation and we only have seen stars in different moments, we cannot know exactly which star transforms into what. The transformation is not very fast. B - type stars, for example, follow the next pattern: First, when the star starts fusing helium, it turns into a larger but colder star, blowing more heat (white, but not red, similar to what the star was first). Then, when helium is exhausted, it starts to visibly increase in size and to become orange, then red. The process is very slow first, but much faster later.
Red giants have terrible solar winds. They lose matter in a very fast way. Such a strong wind is like a scourge to the poor nearby planets, so strong that it can blow a significant part of a gas giant's atmosphere away. Well, in the first stages, while the star is a yellow or orange giant, still a planet cannot be endangered that much.
Not all red giants have the same luminosity corresponding to the same size (this is why there are subgiants, giants, supergiants and super-supergiants). So, each one is different and there is no general pattern.
Hosted planet Edit
There is a research about star Canopus and a possible hosted planet (since Frank Herbert's Dune is supposed to be there). For the moment, Canopus is an orange giant, so it is still safe. Other models are available from ISDB:
- Planet around Canopus: comfort distance = 310 AU, orbital period = 263 years
- Planet around Betelgeuse: comfort distance = 67 AU, orbital period = 123 years
Planets are confirmed to exist  around red giants.
How would look a world with a red giant? Well, not in the last phases, when it is losing mass, creating so violent solar winds. Let's imagine Solar System with a huge red sun. The Earth will be heated to 1000 K, low chances to find life. Mars will also be overheated and will lose all gasses. The asteroids will do so. Many of them contain water, so they will most likely become comets. So will do the satellites of Jupiter. The four large moons will have liquid water, so that they would be a good place for humans (?) still wondering around. The other smaller ones will become comets, spinning around Jupiter, who will look completely different. Its ammonia clouds will be replaced by water. Around Saturn, in that phase, ice will not start to melt, but it will sublimate, so that the icy moons will have an atmosphere. The rings will sublimate, creating giant clouds around the planet. This is what the world should look around an yellow, orange or red giant, a world with many comets and bodies losing their volatiles.
Extended habitable zone includes all planets that could be terraformed with some use of technology. By adding greenhouse gasses, as long as plants have enough light, new planets can be suitable for life. However, red giants, like any red star (including M - type stars, generate most of their light in infrared, while the visible light is mostly in red. Plants can survive with a light of 0.1% Earth intensity, in both red and blue spectra. Orange giants have more blue, but still red is the dominant color, while yellow giants have the same kind of light our sun has. In the case of Solar System, plants can survive 32 times further away from comfort zone (Earth), so they can live as far as the orbit of Neptune, if they have correct temperatures. If solar light were more red, the outer distance would be the orbit of Uranus or even Saturn. Also, remember that sub-giants and stars that are just leaving main sequence have a light thousands of times dimmer then a supergiant. The following list shows some stars and their habitable zones:
- Pollux (orange subgiant)
- Comfort zone: 5.6 AU
- Outer limit for Earth-like plants: 120 AU
- Canopus (white-yellow giant)
- Comfort zone: 310 AU
- Outer limit for Earth-like plants: 8000 AU
Because distance is so big, orbiting period is long. A planet in comfort zone around Pollux will have an year of 8 Earth years. This means that, if that planet is tilted, seasons will last two years each. Will plants survive such long winters? Will humans wait so long or will prefer to migrate? It will be very hard for many species to adapt. This is similar to what we will find on a planet orbiting A - type stars.
And about Canopus? Since the year will be so long, a season will take a lifetime to complete. No plant will survive such a long time. Ice will gradually cover one hemisphere, while the other will experience a long summer. Some people will live all their life in moderate climate (spring or autumn). Still, seasons are shorter then what we see on planets orbiting B - type stars. Ice will not have time to grow so thick, so the oceans will not decrease dramatically. In summer, endorheic lakes will not dry completely, so the climate will not be so dangerously affected. Equatorial land would be less affected (while in a planet orbiting a B - type star, it will be affected, because a lot of ocean water is moving into polar ice.
Given the huge extended habitable zone, many planets can be terraformed. For Solar System, for the moment when Jovian moons will have liquid water, the outer border of habitable zone will extend to 150 AU. This means that all large Kuiper Belt Objects could be terraformed and could sustain Earth-like plants. At 150 AU, we are in the interstellar environment. As for large giants like Canopus, outer range is 8000 AU or 0.13 LY. That might be considered out of a stellar system's border. So, a free-floating planet that came too close to a planet (but is moving too fast to become a satellite) can be terraformed.