[Kizzi]

Does anyone know how wide the Inhabital Zone is for our Solar System?

Considering it freezes at the poles, and gets fairly hot at the equator, the inhabital zone must be fairly fine tuned, because doesn't this suggest that if earth was 'the radius of earth' further away there would be ice around the equator?

[CraigD]

Originally Posted by Kizzi

Does anyone know how wide the Inhabital Zone is for our Solar System?

I can’t say, without consulting some reference source, but I can make a guess using some very simple Physics – see below

Considering it freezes at the poles, and gets fairly hot at the equator, the inhabital zone must be fairly fine tuned, because doesn't this suggest that if earth was 'the radius of earth' further away there would be ice around the equator?

I don’t think so.

The reason the poles are so cold, and the equator so hot, is that sunlight strikes the latter’s surface at a more nearly 90 degree angle, vs. the nearly 0 degree angle of at the poles. With an axial tilt of about 23.5 degrees, this angle changes a lot during the year, resulting in about a 30% change (1-cos(2*23.5 degrees)) in the amount of sunlight the surface gets. It’s the primary cause of the seasons.

Changes in the distance from Earth to Sol, which varies between 147 and 152 million km, resulting in about a 7% (1 – (152/147)^-2) change in sunlight intensity, is less significant that latitude and seasonal inclination. A change of one earth diameter, a mere 12,7500 km, would result in less than a 0.02% (1- (150012750/150000000)^-2) change in sunlight intensity.

Ignoring the effects of atmosphere, a place at latitude 65 degrees (eg: northern Alaska) gets about 40% the annual sunlight of a place at the equator, so, very roughly, again ignoring the atmosphere, for the equator to get the same amount of annual solar energy that northern Alaska presently gets, the earth would have to be about 60% further (1 - 0.4^-2) from Sol than it presently is. At 1.52 AUs, Mars is just inside this orbital radius.

All this is very approximate, but it shows that the solar “life belt”, while narrow compared to the total diameter of the solar system, is not razor-thin.

[C1ay]

Consider these lifeforms...

Pyrolobus fumarii of Volcano Island, Italy survive at 235° F.
Cryptoendoliths of Antarctica survives at 5° F.
Deinococcus radiodurans survives 5 mrad of radiation, 5000x enough to kill humans.
Life has been found more than 3km below ground.
Life has been found to survive at a ph of 0 and a ph of 13.
Bacillus subtilis (NASA satellite) has survived 6 years in space.
Life has been found at 1200 times atmospheric pressure.
Haloarcula lives with 30% saltiness or 9 times human blood saltiness.
Carnobacterium pleistocenium survived being frozen for 32,000 years.
Cyanidium organisms were found living in a ph of 1 at Yellowstone.

These are forms of life as we know it. Who could guess what the limits of life could be that we are not aware of. There could be long frozen life on Mars or life in the hot, acidic clouds of Venus. Me thinks it will be a while before we can posit an answer to your question.

[Daymare17]

One thing I don't understand about the inhabitable zone is: The availability of liquid water depends not only upon the amount of solar "heat" received but upon the atmospheric density. For instance Venus is hotter than Mercury despite being about twice as far from the Sun, because it has an atmosphere that is 92 x Earth. Our solar system's "inhabitable zone" borders just inside Mars' orbit and just outside Venus' orbit. But if Mars was big enough to retain an atmosphere, it would be hot enough to retain liquid water. Mars would have oceans even if it occupied Jupiter's orbit, if it just had a dense enough atmosphere.

So isn't the whole "inhabitable" model totally flawed from the outset?

[Tormod]

Both Mars and Venus are inside the habitable zone. That the three planets have different atmospheres today may be down to many reasons.

As for the Earth being colder if it was a radius further out - it is easily proven wrong. The Earth's orbit around the sun is not circular but eliptic, and the difference between when we are clostest to the sun and farthest from it is much more than an Earth diameter.

BTW the average temperature of the Earth is currently 15 degrees Celsius. The average temperature on the Moon is about -23 degrees Celsius, even without any atmosphere.

[Tormod]

Originally Posted by Daymare17

if Mars was big enough to retain an atmosphere, it would be hot enough to retain liquid water

The surface temperature on Mars would allow liquid water today, and might do so in certain periods of the year (at least subsurface under the ice). It is the density of the atmosphere that causes water to evaporate.

The evidence from current Mars probes clearly show that there have been long periods of liquid water flowing on the Martian surface. We don't know why the atmosphere there is so thin today, but it certainly hasn't been like this forever. Mars was most likely a warm place until some time ago.

Size has little to do with atmosphere - Titan is small but has a dense atmosphere.

[CraigD]

Welcome to scienceforums, Daymare!

Originally Posted by Daymare17

One thing I don't understand about the inhabitable zone is: The availability of liquid water depends not only upon the amount of solar "heat" received but upon the atmospheric density.

Simply put, the immediate availability of liquid water depends on 2 things – temperature, and atmospheric pressure. Temperature must be above 0 C. Pressure must be great enough that the boiling point of it exceeds the vapor pressure of water for the temperature, or the water will exist only as vapor.

Temperature depends on the amount of solar energy the planet absorbs, the amount it radiates into space (both in the infrared, and other frequencies), and the amount it gets from other sources, such as its own residual heat, the decay or radioactive elements, or gravitational kneading.

This produces interesting situations where an earth-like planet just outside of the “life ring” may have liquid water for a time, then become perpetually frozen, while a smaller moon of a giant planet may experience enough gravitational kneading to have liquid water, as is thought to be the case with Jupiter’s moon Europa.

But if Mars was big enough to retain an atmosphere, it would be hot enough to retain liquid water.

I think so.

Mars would have oceans even if it occupied Jupiter's orbit, if it just had a dense enough atmosphere.

I think not. At that distance, Mars would receive less solar radiation ( (1.5 AU / 5 AU)^2 = ~ 9% its current) than it radiated into space in the infrared, until it cooled to a point that its infrared radiation power roughly matched its solar input. Mars appears to lack sufficient radioactive elements to heat itself much, and is too small to retain much primordial heat. So, in Jupiter’s orbit, even with a very massive atmosphere, Mars would freeze.

So isn't the whole "inhabitable" model totally flawed from the outset?

The model is approximate. It doesn’t rule out the existence of planets and moons with conditions hospitable for earth-type life due to unusual circumstances (eg: gravitational kneading), it just provides a reasonable guess for the distance that hospitable earthlike planets would be found from a particular star.

Of course, even bodies not even vaguely friendly to earth-type life might contain interesting extremophile life. Our tendency to search first for life like our own could be very biased.

[cwes99_03]

Having an atmosphere also requires availability of gases.
Considering their availability to be plenty, then atmosphere depends upon ambient temperature and mass of the planet.

Ambient temperature determines the velocity of the gas molecules. Mass determines the escape velocity of the planet. If a significant amount of molecules travel faster than the escape velocity, a very thin atmosphere will remain.

Of course, different gas molecules have different masses.

If we are then talking about habitable by oxygen-nitrogen breathing life, then it is simply a matter of calculating a ratio for ambient temp and mass of planet.

The habitable zone for the earth is fairly narrow. I believe that if the orbit of the earth were to take it more than 1% further in orbit than it currently does, there would only be a habitable zone on the earth of about +/- 22 degrees north and south of the equator (rough estimates from memory, suggest checking websites.) By the way, the earth's orbit is extremely round for an elipse.

Check out this site for earth's orbit.
http://www.physics.uq.edu.au/people/...bit/elipse.htm

[CraigD]

Originally Posted by cwes99_03

I believe that if the orbit of the earth were to take it more than 1% further in orbit than it currently does, there would only be a habitable zone on the earth of about +/- 22 degrees north and south of the equator (rough estimates from memory, suggest checking websites.)

A planet’s solar input varies as the inverse square of its distance from the sun, so a 1% increase results in only about a 2% (1.01^-2) decrease. This is a pretty big decrease, compared to, for example, the sun’s 11-year periodic variance (about 0.1%), or historical minima such as the Dalton mimimum, which is thought to have contributed to the 1816 “year without a summer”, so the extreme shrinking in Earths habitable latitudes (from the current values of about +-65 to +-22) seems with the realm of the possible.

Still, I’m curious how you arrived at this specific prediction

[cwes99_03]

Something I read once, been looking for a cite that might refer to it. Don't remember where i read it.