[dorgo]

I struggle to imagine what could happen to earth to make it less suitable to support human life than any other known planet.

[hasa]

There are many very likely scenarios making this planet unhabitable: asteroid, black hole coming near solar system and moving planet distances, gamma ray burst, any deviation in sun's radiation. All these are clearly possible and likely to happen in the long run.

[java-man]

An asteroid strike would make things difficult.