Earth might need a backup plan. Elon Musk certainly thinks so, and SpaceX relentlessly pursues its goal of colonizing Mars...The good: Mars is the obvious choice. It is close enough for humans to travel to with existing propulsion technologies within about six to nine months. The planet also has soil, surface ice at the poles, and liquid water below ground at lower latitudes. There is enough sunshine to provide solar power. The thin Martian atmosphere provides a modicum of protection from cosmic and solar radiation. All of the above factors suggest that humans could build a somewhat sustainable colony over time on Mars, although it would largely be an indoors existence.

The bad: There is so little atmosphere that it might as well not exist. About 19km above the Earth, pilots reach the Armstrong limit, where water boils at the temperature of the human body, and the atmosphere is about 6 percent that of Earth's surface. The Martian atmosphere is about one-tenth the value of the Armstrong limit. An exposed human on the surface would die within a few dozen seconds. The recent discovery that solar wind has stripped the Martian atmosphere away over billions of years has also dealt a blow to hopes that the world might be terraformed by releasing carbon dioxide trapped in Martian rocks.

The Moon

The good: Proximity. Humans can get to the Moon within a few days, making travel and resupply by far the easiest of any off-world location. Scientists also believe that ample amounts of water ice exist at the lunar poles, providing a hefty source of drinking water, radiation shielding, and rocket propellant. The moon's surface contains valuable minerals, such as silicon, which could be used to make solar cells, and Helium-3, which might be a good source of energy. It is also plausible that large lava tubes exist just beneath the Moon's surface, which could be large enough for human cities.

The bad: The Moon has no atmosphere, of course, making surface activities hazardous and leaving astronauts vulnerable to radiation. The Apollo astronauts, too, found the Moon's dusty surface was difficult to contend with when they brought the dust back into their spacecraft.

Titan

The good: Energy, and lots of it. The great lakes of Saturn's moon Titan are filled mostly with pure methane, providing a near limitless supply of power for human activities. The surface pressure is about 1.4 times that of Earth, too, which means it is one of the few places in the Solar System where humans wouldn't need a pressure suit (aside from an air mask) to walk outside. The nitrogen, methane, and ammonia in Titan's atmosphere could be used as fertilizer to cultivate crops in greenhouses, and there may also be subsurface water. The thick atmosphere would provide good radiation shielding.

The bad: Temperature and distance. The surface of Titan is about -180 degrees Celsius (so no pressure suit, but LOTS of layers of warm clothing).