Elon Musk plans to have a city on Mars by 2050. But that's a little easier said than done. Human survival on another planet, especially one as environmentally hostile as Mars, depends on finding ...

This so-called Planet City would have 10 billion people living in 1.4 million neighborhoods climbing 165 stories high in an area covering just 0.02% of the Earth’s surface.

Musk is particularly concerned with making sure the city looks cool, two other people said. ... Like Mr. Musk, many of SpaceX’s more than 12,000 employees believe in life on another planet, ...