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# Inhabitable Zone

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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? :hihi:

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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.

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Consider these lifeforms...

Pyrolobus fumarii of Volcano Island, Italy survive at 235° F.

Cryptoendoliths of Antarctica survives at 5° F.

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.

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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?

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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.

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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.

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Welcome to scienceforums, Daymare!

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.

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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/ross/phys2081/orbit/elipse.htm

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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 :friday:

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

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By the way, the earth's orbit is extremely round for an elipse.

It is 5 million kilometers closer to the Sun at perihelion than at aphelion.

The orbit of Venus is almost a perfect circle in comparison.

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OK, so let's look at that figure. The coldest part of winter comes a full month to two months after the winter solstice. This is important because by that time the sun is already beginning to warm the upper hemisphere back up, thus not allowing it to get as cold as it could should the earth orbit at a constant slant with the northern hemisphere away from the sun.

That being said, it only rarely snows south of about St. Louis, but the temperatures do get regularly down near freezing in those parts. And this is during perihelion, when the earth is closest to the sun (around the first week of january every year).

Should this phenomena occur at aphelion (around july 4 every year) the northern hemispher would get much colder, being about 5 million km further from the sun, thus letting snow occur in say San Antonio.

The difference between perihelion and aphelion being approximately double the average variance in orbit (2%) a change in the earth's average orbit of 1% would lead to regular temperature drops during winter in the northern hemisphere to the point that san antonio would begin to expect snow every year.

Compound this phenomena with less moisture in the atmosphere (total water vapor in atmosphere is directly related to the mean temperature of the earth) to equilize the heating and cooling of the earth, and we are talking about regular temps in San Antonio during the winter of about 30 degrees F.

Anyone have better stats than that? This was kind of a rough grove I tossed myself into.

http://curious.astro.cornell.edu/question.php?number=582

http://www.srh.noaa.gov/elp/climat/astro/seasons2005.html

Two interesting websites on earth's orbit and aphelion/parahelion dates.

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i have much the same question

but relative to a given galaxy. are there any factors that would make a star in a galaxy un-inhabitable such as radiation at the core of a galaxy?

its said that because of the age of stars required for larger disks (to form multiplanetary solar system) with heavier metals and radioactive isotopes, that life is most likely in roughly the same type of LHZ as a star as that of a galaxy.

but if that isn't true and heavier metals can exist closer to the galactic center are there any factors that would preclude life taking hold on rocks withing a few light years of the galactic center? or the first percent of distance in the first 1k light years?

given that the density of stars are higher there it would be ideal for civilizations to form there because it would be easier to travel between solar systems.

-----

another question is how its fine and dandy to think live exists everywhere but we're not privvy to it until because we aren't yet advanced enough to find it.

what happens when we are able to cruise the universe at 100 million LYs a second? (by then the milky way would be our playground and small pockets of humanity and seed populations of life would be totally under our control. drift would mean humanity would be speciated like animals in the galapagos. human yes, but specialized.

like water flows and matter obeys the laws of physics evolution could be a very rigid science we have yet to flesh out.

wouldn't it be sad if the universe had 1 civilization per 100 million stars, like 3 to 4 others in the milky way for instance, and each had the same basic tools to deal with to create as much diversity as possible but ended up evolving pretty much identically as portrayed in startrek?

how would we know is life we came across wasn't our own doing? and aliens developed without our own intervention?

evolution could be a static and uniform process around the universe given the starting ingredients. such that life evolves exactly the same way based on the ingredients and we could probably figure out long before we get there what aliens will be like.

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OK, so let's look at that figure. The coldest part of winter comes a full month to two months after the winter solstice. This is important because by that time the sun is already beginning to warm the upper hemisphere back up, thus not allowing it to get as cold as it could should the earth orbit at a constant slant with the northern hemisphere away from the sun.

I fail to see that there is a point here. Nothing you have written above says that the temperature of the Earth would change if the orbit was different by any percentage as far as I can see.

It really becomes futile if the argument is that the habitable zone is *exactly* that of the Earth's orbit, because we cannot test such a hypothesis unless we find life elsewhere in the solar system or beyond.

However, the initial argument was that the orbit could not be different even by one Earth radius. This is obviously false since the orbital distance from the Sun changes by more than almost 400 Earth diameters during one orbit.

There is evidence to support that life can survive outside of the Earth's atmosphere. We know that living bacteria survived a trip to the Moon, for example:

http://www.nasaexplores.com/show2_articlea.php?id=02-074

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First, I was operating under the assumption that life as we know it would not be able to live on the earth (in other words, with animals and plants inhabiting the same lat as they do now.) I also never said anything about one earth's radius in any of my posts that I remember.

If you didn't read my entire post, I was operating under the assumption that you could imagine the effects of longer winters. If the earth were further away the winters would be much longer and much more severe. When the earth is tilted away from the sun, we get winter in that hemisphere. Now imagin being tilted away from the sun and much further away, even during perihelion (as we are in perihelion when the northern hemisphere is in winter) when the earth is closest to the sun, it gets very frigid. Now calculate the amount of solar heat we would receive during a day when we are 5 million km further away during perihelion and see what effect that would have on the severity of winter. Suddenly the deepest lakes begin to freeze so that nothing aquatic (that currently overwinters say around 40 north lat.) can survive the winter and 40 north lat becomes like what 65 north lat used to be. Daytime highs only reaching -40F for 4 months out of the year.

Now once you have considered all that, imagine what effect all this extra ice will have on the rest of the earth. Sea level would drop, the overall humidity of earth's atmosphere would drop severely (and this acts just like that pesky CO2 layer everyone worries about.) More CO2 would freeze out of the atmosphere, thus reducing the effects of global warming. The earth would slowly turn into a popsicle where only a region around the equator regularly gets up to 70 F in the summer (though that region would still only see about one and a half seasons per year (summer, and chilly summer).

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it does say that the temperature variance is not that severe. though life can survive in permafrost not many biomes remain that way for the entire year. the variation could be necessary for evolution. a static orbit like that of venus might not have produced as much variation as earth.. well of course the toxic atmosphere kinda reduces the possibility for much variation.

so even though life can survive dormant or even active in space you don't see it becoming too complex.. the potential for energy conversion just isn't there.

for now planets are the only place where life can coalesce into massive biomes for life processes to continue (lasting billions of years). while in space not so long if at all. other bastians of life include sequestered zones... possibly.. like jupiters atmosphere for temperatures and chemicals and densities of the clouds, a liquid ocean under the ice cover of a volcanically active moon, aka europa.

so complex life, like civilization requires a massive biome. like a planet.

so, life can survive virtually anywhere, but civilizations require at least something the size of a planet. though easter island kinda tells you people can survive on something very tiny... and if your planet is too big people may never even meet each other (europe><america) if the cultures never get big enough to travel or greedy enough to travel. space travel though would require massive amounts of work to achieve while a boat can be built from virtually anything.

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I also never said anything about one earth's radius in any of my posts that I remember.

Like I said, this was the premise of this entire thread. Go back and read the first post by Kizzi.

If you didn't read my entire post

I did, of course.

If the earth were further away the winters would be much longer and much more severe.

This is where I fail to see your reasoning. What about the periodic ice ages that occur with about 10,000 year intervals? They happen even though the Earth is in the same orbit.

We see life survive in the heat of the desert, and in the ice in the arctic. Life is extremely adaptable. What we don't know is what are the initial conditions that will cause life to emerge in the first place. We have absolutely no evidence to back up a claim that there is a "habitable" zone, apart from studying our own planet, which is not enough to give us a conclusive theory.

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