The Long Future

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Life has existed on Earth for over 3.8 billion years because Earth's orbit around the Sun is not so distant that all the water freezes, nor so close that all the water boils off. 

Earth is said to occupy the 'Goldilocks Zone' or more scientifically, the Circumstellar Habitable Zone.  

Scientists tell us that Earth will continue remain in this zone and be habitable for life for maybe over a billion years.

Beyond this time, the Sun will expand and eventually Earth will be baked dry, taking all of life with it.  

Vita calls this potential future life-time, the Long Future.  

For argument's sake, let us assume that the Long Future is 1 billion years.

Broadly speaking Long Future is a measure for how long Earth will be habitable for living organisms. However, it remains uncertain how long Earth will remain habitable for humans.