The Goldilocks, Or Habitable, Zone, Must Be Very Narrow...

[sandyRoe]

Is it the tilt of the Earth's axis that ensures that the poles are permanently frozen? If our planet was just a few thousand miles further away from the Sun could life have developed?

[jim360]

The Goldilocks zone is rather a lot wider than you might think. Estimates vary, partly because it depends what type of life you are looking for, and also because there's no guarantee that we've found all possible life and the only relevant conditions. But I think the margins are generally thought to be fairly generous, depending on what the model is based on.

Roughly speaking, for a star like our Sun the Habitable zone is thought to be between about the Earth's orbit, plus or minus 10% (although this is a conservative average based on a number of separate papers). Then again, different planets may after all be capable of sustaining life at far larger or smaller distances, and different star systems have different properties.

Regardless of the paper you choose, though, the tolerance of distance is measured in millions of miles, rather than thousands.

[sandyRoe]

I had thought not much further out and there'd be no liquid water, or nearer in just vapour.

[bert_h]

Agree with jim360, the evolution of water-based life seems quite plausible on a planet at the earth's orbital distance plus or minus - oh, I'd have said 25 per cent rather than Jim's 10 per cent.

The poles are frozen because the sun's rays only ever reach them at a very shallow angle. The axial tilt just means that the temperate zones have seasons (to which some plant life has adapted), while the equatorial zone has really no seasons (to which its plant life has also adapted), so it wouldn't seem that the tilt and the seasons were necessary for the development of life.

[mibn2cweus]

The Earth's distance from the Sun varies by more than three million miles (greater than 3%) throughout the coarse of its annual orbit of the Sun.

Axial tilt is obviously the reason for the seasons since winter in the northern hemisphere takes place when the Earth's orbit brings it closest to the Sun.

[jeffa]

The Northern hemisphere is at its coldest when it's nearest to the Sun? How does that work?

[mibn2cweus]

^ Three factors:

1. Less area of the northern hemisphere receives sunlight.
2. Due to the increased angle of incidence sunlight is more dispersed in the areas that do receive sunlight.
3. The Sun is above the horizon less of the time during each 24 hour period.

The combination of these effects are not fully compensated by the Sun being a little closer to the Earth in the winter.

[ToraToraTora]

the goldilocks zone is approx 20million miles wide. The tilt creates much of the climate but lack of it would not mean that all water would be ice, the equatorial region for example would be a hotter that its is now even if the Earth was further out. Life could still have evolved on Earth even with various factors being different. Life forms may have evolved differently or indeed not at all as we really don't have any sort of certainty as to the initial catalyst. It is often said that the prescence of the moon has played a huge part of the direction of evolution etc. The bottom line is that we reallt cannot say that life would definatley have evolved but we can say that the tilt and the zone variance would not in itself have prevented the conditions for life.

[atalanta]

The zone is not the only requirement for life to evolve. A planet must have water, and a moon, and therefore tides. It must also have tectonic activity. It can't have a solid core. It can't be hugely bigger or smaller than the Earth. And, of course, it must be rocky. not made of gases like the biggest planets in our solar system

[New Judge]

Of course alll of the above only applies to life as we know it. As everybody knows there is also "Life, Jim, but not as we know it" !!!!

[Old_Geezer]

Yes, they tend to post at AB.