We live on an Earth that from day to day seems permanent and stable. We go to work, school and perhaps the pub every day and it seems like things will never change dramatically.
Unfortunately as we are all beginning to realise things in the universe are forever changing which when looked at from the perspective of the universe as a whole is a good thing really. We are lucky though because compared to our lifespans the universe changes very slowly indeed. One of these things is the Earth’s position relative to the Sun.
If the Earth was too close to the Sun it would be hot and if it was at a great distance it would be cold. We need water for life to exist as we know it and this would not be the case if the Earth was too close to the Sun as the water would have evaporated and probably disappeared. Too far away from the Sun and we would be left with an Earth covered in ice. So you can see from these two extremes that there is a zone in which the temperature is just right. This zone has been called the Goldilocks zone (not too hot and not too cold forgetting the porridge) but inside science circles is called the habitable zone for obvious reasons.
This habitable zone was once thought to be the only place that life could survive but nowadays opinion is divided as moons like Europa around Jupiter may have an ocean beneath its icy surface which could contain some sort of life. There may also be worlds where life has evolved in ammonia and other weird and wonderful ways but what we are looking at here within the habitable zone is earthlike life which depends on water.
The habitable zone though hasn’t always been the same distance from the Sun. About 4 billion years ago, just after the solar system was formed, the Sun was 70% of its present luminosity (The luminosity is the amount of energy emitted by a star per second). As the Sun’s luminosity increased the temperature on Earth increased and the habitable zone became further and further away from the Sun. This, luckily, wasn’t that bad for a humanity as the Earth stayed in the habitable zone.
Nowadays the Earth sits almost in the middle of the habitable zone but as the sun’s luminosity increases as the sun ages the habitable zone will move further and further away from the Sun and the Earth will increase in temperature. This will as you might imagine take some time.
Mars is right on the edge of the habitable zone and is to cold for liquid water and therefore ice exists. As the Sun’s luminosity increases the temperature of Mars will increase and changes could happen. On the other side of the habitable zone Venus has no chance with high temperatures and no water.
Outside of the solar system an extrasolar planet has been found which is within the habitable zone of its star. Gliese 581 d is the outermost of four planets around the red dwarf star Gliese 581, 20 light years from Earth. This is the best example so far but many more will be found in the future I believe and as technology allows people to be able to look at the atmospheres of these planets using spectroscopy and work out whether they have an oxygen atmosphere or not. This will give us some good pointers to whether life exists elsewhere.