X-Class Solar Flare. Source: NASA
In about 4 or 5 billion years, the Sun will become a red giant, its atmosphere bloated, Earth's surface rendered uninhabitable. But life will have ceased to exist on Earth a lot sooner than this. Runaway heating will cause massive climate change effects. It is possible that with this increased influx of energy, the resulting storms may make it very difficult for life to survive, with ferocious winds sandblasting the Earth, while other regions bake in unbearable heat. If human civilizations still exist on Earth, they may have migrated towards the polar regions to survive the increased temperatures.
Liquid water may be a necessity for life in purely astrobiological terms, but a storm-lashed, sunbaked world would be hellish. The presence of water is a good starting point, but the age of a planet's parent star must also be factored in when ascertaining the habitability of newly discovered worlds.
