Habitable or Goldilocks zone

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.

Habitable Zone

Habitable Zone

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.

Possible life on Europa

Possible life on Europa

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.

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  1. [...] Staying with the subject of extraterrestrial life, Chris Dann, who writes the Weird Warp blog, has a very concise and informative beginners guide to just where habitable planets might be found around alien stars – i.e., the famous “Goldilocks Zone“. [...]

  2. Gale Martha says:

    I linked your article “Habitable or Goldilocks Zone” to my science blog article Goldilocks Zone. Love your blog!

  3. Phillip Mullis says:

    hmmm. interesting the concept of a habitable moon would make, it easier for humans to make a new colony on… Well… that is if we ever get there -.-’

  4. Deepanjan says:

    NASA scientists has already found that there is a huge salted ocean (more than 100km deep) beneath the top icy layer of Europa-the 2nd moon of Jupiter.It also has many organic chemicals. The atmosphere is very-very thin comprising mainly of O2.

    Europa and the rest of the moons doesnot get much enegry from Sun. Rather it draws tidal energy from Jupiter. Just image the way our moon creates high & low tides in our ocean. But for Europa, Jupiter is a damn huge planet and its gravity is very-very high.It is so high that it can create tides even on solid surface. Tides generate friction from the continious movement of the solid surface and which in turn generates heat within itself.
    Europa has a thick ice-layer(10-15km) to take care of radiations from Sun and Jupiter.

    Under such circumstances, Europa qualifies to be a habitable moon. Possibility of life beneath quite high.

    With advanced technology human can survive on the surface of Europa on selective places covered with antiradiation glass and insulate heat also which can withstand pressure diffenrentiation as human’s would require 1 Atmospheric pressure to survive.

    They need to draw energy from beneath the surface to keep their civilisation bit warm(-10C to 5C). At present the surface tempearute is very very low (less than – 200C).

    With advanced technology human can survive on the surface of Europa(selective places covered with antiradiation glass and insulate heat). They need to draw energy from beneath the surface to keep their civilisation bit warm(-10C to 5C). At present the surface tempearute is very very low (less than – 200C).

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