Which planets are habitable?
In the wider Universe there are likely to be many planets that humans could survive on, but we really don't know for sure yet. Our understanding of planets outside our Solar System is very limited, and in most cases all we have is educated guesses.
Here in our own Solar System, the only planet that is habitable for humans is Earth. That is, unless you have a spacesuit and other life support systems. If we think about places where humans could live inside sealed habitat areas, then there are a few possibilities. Let's start in the inner Solar System and work outwards...
Mercury has no atmosphere and is the closest planet to the Sun's harsh radiation. It's not a good option.
Venus is very inhospitable. The high surface temperature and air pressure make it practically impossible to build any kind of habitat. However, the upper atmosphere offers an intriguing opportunity. An airtight aircraft filled with Earth-like air (at Earth's atmospheric pressure) would be buoyant enough to float at a high altitude. It would be difficult and there are problems such as sulphuric acid in the atmosphere, but it is theoretically achievable. There is no ready supply of water or other supplies, so this would all have to be transported there.
We already know that people can survive on the Moon, although we have yet to test how well that would work long-term. Of course the Moon isn't actually a planet (it's a moon).
The most human-friendly planet outside Earth is Mars. People could theoretically live there long-term as long as they spend most of their time in areas protected from radiation. As radiation is a big problem all over Mars, and spacesuits don't offer much protection, people would not want to spend too much time outside their homes. It would also be a big challenge growing food and taking care of other critical life tasks. Water should be available via mining underwater ice.
In the asteroid belt, there is one dwarf planet (Ceres) and a few large asteroids. These don't offer much for humans, but there is at least the possibility of finding water there.
Farther out, the gas giant planets are not places you could set up a home. They don't even have a solid surface to land on, and their atmospheres are violently wild. The best option would be to set up bases on their moons, but they are very cold and survival would be difficult.
Pluto and other more distant objects are extremely cold, get very little usable energy from the Sun, and wouldn't be any fun to live on.
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Author: Dave Owen