An investigation into the fate of the Earth and of the life upon it has found that we are all doomed. According to a paper that will soon be published in MNRAS, by Klaus-Peter Schroder of the University of Guanajuato, Mexico and Robert C. Smith of the University of Sussex, England, the Earth will no longer be habitable in around a billion years and will be enveloped by the Sun in little over 7.5 billion.
As the Sun gets older, it will become 10% more luminous. This results in the habitable zone (the distance from the Sun within which water can remain liquid) moving outwards, away from the Sun. The Earth will still be at roughly the same distance away from the Sun as it is now and will become too hot to sustain life.
As the Sun gets even older, it will expand and become a red giant. At its largest, the radius of the Sun will be 256 times that of the present day value. It will also virtually cease to spin. This will cause an area of the Sun to always face the Earth and have a strong gravitational pull on it.
This pull on the Earth will eventually prove too much and, having already swallowed both Mercury and Venus 3.8 and 1.0 million years earlier, the Earth too will succumb to the gravitational pull of the Sun 7.59 billion years from now.
At an age of 12.5 billion years or so the Sun will lose its outer layers and only the core will remain, leaving what is known as a white dwarf. If the Earth was currently 15% further away from the Sun, it would not be destined to be enveloped. Instead it would continue to orbit the white dwarf.
Is there no hope? Several suggestions have been made on how life could be sustained. A scheme where life rafts are sent out from the Earth away from the Sun is a possible solution. Although this raises the difficult question of who deserves a place on the raft. An alternate method involves using passing asteroids to slingshot us further out. Although a slight miscalculation could hasten our demise. Fortunately we have several hundred million years till our descendants have to seriously worry about this problem.