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View Full Version : Is the habitible zone a myth?



Thomas_Anderson
06-25-2009, 11:09 AM
For example, a planet would have to be the exact distance from a sun in order to be habitible? Terraforming is only theoretical, but if it were possible, then couldn't a planet much further away from a star be just as habitible as Earth if it had enough greenhouse gases? Or if it was the same distance as Venus, and simply had a lot less greenhouse gases?

Pthom
06-25-2009, 01:01 PM
I am fairly certain that a planet's habitability has to do with a lot more than the makeup of its atmosphere.

I'm too tired now to do the research, but my guess is that there is a much vaster complex of factors that combine to make a place livable by humans. That is your point, yes? Habitable by humans?

However, I suppose that there is some leeway: a denser planet might hold more atmosphere, allowing it to be farther from its star. The star could be hotter. The planet could have more moons, or rings, or more tectonic activity, a larger magnetic field. I don't know which of the myriad features of Earth we could get by without and still survive but my guess is very few.

Vincent
06-25-2009, 01:32 PM
Life can exist outside the 'Goldilocks Zone', sure. There is speculation that there might be life in liquid sea oceans under the ice of several of Jupiter's and Saturn's moons. Life might even exist suspended in the atmospheres of gas giants themselves. But our best bet for ecosystems as we know them, with photosynthesizing plants, warm seas and abundant land animals, exists in this zone. But that's not the full story.

Some consider Mars and Earth both as existing in this zone, with Mars being just a bit too small to retain its early, thicker atmosphere. And Venus is so inhospitable precisely because it is just a bit too close to the sun. If it wasn't for the heat, life would probably have sorted out those greenhouse gasses in the first couple billion years. You could probably terraform both... for a time.

Thomas_Anderson
06-25-2009, 01:33 PM
Well, I'm sure it has to do with more than greenhouse gases, and oxygen is obviously a big requirement. Basically, what I was trying to get at is, is it written in proverbial stone that a rock must be three spaces away from a yellow star in order to be habitible by humans, or human-like beings?

Also, you brought up rings. Can a terrestrial planet have stable rings? So far, every planet we've seen with rings are gas giants.

Liosse de Velishaf
06-25-2009, 05:37 PM
I think you'd be better off not thinking in terms of "spaces". The habitable zone of a star has to do with having the right amount of heat to have liquid water. Here's the general equation. (Yes, I jacked this equation from wikipedia. My memory ain't what it used to be.)

http://upload.wikimedia.org/math/6/0/4/60475c653ce5afe54a1f99a126e51c21.png

DAU=radius of the zone
Lstar=luminosity of star
Lsun="ditto" for the sun

The habitability of a planet has somewhat less to do with greenhouse gases. Earth had high levels of carbon dioxide before life emerged. The presence of a high oxygen atmosphere is a diagnostic sign of life, because oxygen as a free element does not exist on its own. This is because it is a highly reactive and forms compounds, (especially oxides) readily with most other elements. It can appear in small quantities on planets without oxygen producing life when compounds like carbon dioxide are split by ultra-violet radiation.

Even though brighter stars could theoretically have larger habitable zones, the short amount of time in which they proceed through their main sequence lifetime means that life would not be able to get very far on planets orbiting them.

More later.

efkelley
06-25-2009, 11:38 PM
For a ring, I think it would depend on the composition of the ring and how it formed.

Most gas giant rings are from broken up moons or ejecta from massive impacts on the gas giant itself.

The ring would have to be made up of materials too small to damage the planet as their orbits decay. The ring could not have been formed from a planetary impact (http://en.wikipedia.org/wiki/Giant_impact_hypothesis). Such an impact would preclude our type of life on the surface of the primary for many millions of years.

There may be other 'gotchas' I'm missing.

Pthom
06-26-2009, 12:19 AM
The ability of a place to support "life" doesn't necessarily mean it is "habitable." At least not when you define habitability in terms of human beings.

Marcus
06-26-2009, 01:54 AM
As i understand it, the Habitable zone is a specific reference to where and how far from a given star that liquid water is able to form. Life as we know it is completely Dependant on water, though we are finding out now that there is life deep in the earth that exists differently than all the other forms of life as we know them. So our definition of life is also undergoing changes as we find new forms of life that break the acceptable rules.

Methane plumes on mars point to the same kind of deep crust life that i just mentioned.

As mentioned by others, the moons of jupiter and saturn are good candidates for life if the moons have liquid water under the surface... and they are Well outside the "habitable zone"

Habitable zone is just an easy rule of thumb that tells us where the liquid water would be able to form for a given solar mass in a given star system. :)

Liosse de Velishaf
06-26-2009, 06:58 AM
The moons are heated by tidal forces.

blacbird
06-26-2009, 11:17 AM
Well, I'm sure it has to do with more than greenhouse gases, and oxygen is obviously a big requirement.

Not so obviously. A big biota of anaerobic microorganisms exists on earth which not only do not require oxygen, but find it deadly toxic. That's why hyperbaric oxygen treatment in hospitals works for treating certain bad infections. Many of these organisms metabolize sulfur from hydrogen sulfide, and they are the basis of the food chains in deep ocean volcanic vents. A lot of biologists and paleontologists believe these kinds of things were the very first life forms on Earth.

The free oxygen in the Earth's atmosphere is totally of biotic origin, generated through the action of photosynthesis of plants. You don't get life because you have free oxygen, you have free oxygen only through having an organic biota capable of continuously replenishing it. In fact, the very existence of free oxygen in a planetary atmosphere would be a lead-pipe cinch for the existence of living things on that planet.

caw

Smiling Ted
06-29-2009, 11:33 PM
http://upload.wikimedia.org/math/6/0/4/60475c653ce5afe54a1f99a126e51c21.png

DAU=radius of the zone
Lstar=luminosity of star
Lsun="ditto" for the sun


I think that formula yields a result that is a fraction of the radius of Sol's habitable zone: in other words, Habitable Zone of Other Star/Habitable Zone of the Sun.

Smiling Ted
06-29-2009, 11:34 PM
For a ring, I think it would depend on the composition of the ring and how it formed.


Earth already has a ring.
It's just not very glamorous - it's the ring of space junk we've lofted into geosynch orbit in the last fifty years.

efkelley
06-30-2009, 01:04 PM
Earth already has a ring.
It's just not very glamorous - it's the ring of space junk we've lofted into geosynch orbit in the last fifty years.

In that particular case, it's more like a protective shell. Like tartar or a scab.

....

Yep. Definitely not glamorous.

Pthom
06-30-2009, 11:06 PM
A lot of Earth's ring (composed of the sort of junk Ted suggests) isn't limited to geo-sychronous orbit. Quite a lot of it is in LEO (low earth orbit). But a planetary ring orbits (generally) in the equatorial plane. As efkelly suggests, our space junk is more like a shell.

Lhun
07-01-2009, 01:37 AM
I think that formula yields a result that is a fraction of the radius of Sol's habitable zone: in other words, Habitable Zone of Other Star/Habitable Zone of the Sun.No, what it actually produces is the distance at which the light from the star has the same intensity as the light from the sun has here on earth. (since 1AU is the distance from earth to sun)
Habitable "zone" is nearly impossible to describe with any accuracy, too many factors involved.

small axe
07-04-2009, 06:02 AM
There was an interesting show on TV some months ago, which suggested that not only did Earth's position from its Sun contribute to Life beginning and surviving ... but that also vital was our rather unique relationship with our Moon.

I can't recall all the ways they gave that the Moon protected Life on Earth. Certainly it was a shield against murderous meteor and comet hits (and how many still got through to almost end Life?) and made our planet's spin slow and steady and Life-affirming. Something about the oceans too, I recall. Seems there were about five ways that Life (as we know it, certainly; Life may have begun but vastly differently, sans Luna) needed the Moon.

Long lazy walks along seductive moonlit beaches ... they inspire Life and procreation. The moon sends us pearl-radiant dreams.

'habitable' still wants something dreamy to live FOR.

blacbird
07-04-2009, 08:35 AM
The moon also seems to have been created via a major planetesimal collision early in the Earth's formative period, which stripped off a big proportion of the light, siliceous crustal material, resulting in the anomalously thin, fragile, mobile and tectonically plastic crust we have today. Which is hugely important in keeping the planet renewed with atmosphere and resources vital to the biota. Neither Venus nor Mars has a big moon, or that kind of active thin crustal layer.

caw

efkelley
07-06-2009, 12:20 AM
The moon also gives us tides, assists the oceanic currents, and contributes to our seasons and weather patterns.

Small Axe, you're thinking of Earth: The Biography, I do believe. A good show.

The moon wasn't the largest contributing factor to our protection from meteors (though it does help). That was Jupiter.

Justin K
09-27-2009, 03:19 PM
Transporting life to another celestial body is probably one of the most feasible ideas in existence. We should load up a spaceship with the earths most resilient forms of life (we can get them from the depths of the ocean or maybe the polar ice caps) and send them to a place in our soar system where they have a chance. I don't see how it's any different from transplanting species of animals around the world to places where they can survive but could never have made the trip there. But then again, we'd have to account for anomalies in space that some scientists say make it impossible for life to exist at certain distances away from earth. On an unrelated topic, I'm really on the fence as to whether or not we really had people travel to the moon; I would be equally as proud to live in a country responsible for the successful hoax of it. But yeah, the habitable zone arguably has no boundaries; it's a function of what life as we know it can inhabit.

Miguelito
09-27-2009, 06:56 PM
Not so obviously. A big biota of anaerobic microorganisms exists on earth which not only do not require oxygen, but find it deadly toxic. That's why hyperbaric oxygen treatment in hospitals works for treating certain bad infections. Many of these organisms metabolize sulfur from hydrogen sulfide, and they are the basis of the food chains in deep ocean volcanic vents. A lot of biologists and paleontologists believe these kinds of things were the very first life forms on Earth.

The free oxygen in the Earth's atmosphere is totally of biotic origin, generated through the action of photosynthesis of plants. You don't get life because you have free oxygen, you have free oxygen only through having an organic biota capable of continuously replenishing it. In fact, the very existence of free oxygen in a planetary atmosphere would be a lead-pipe cinch for the existence of living things on that planet.

caw

Indeed. It's called chemosynthesis (instead of photosynthesis).

MargueriteMing
11-11-2009, 12:53 AM
Life can exist outside the 'Goldilocks Zone', sure. There is speculation that there might be life in liquid sea oceans under the ice of several of Jupiter's and Saturn's moons. Life might even exist suspended in the atmospheres of gas giants themselves. But our best bet for ecosystems as we know them, with photosynthesizing plants, warm seas and abundant land animals, exists in this zone. But that's not the full story.

Some consider Mars and Earth both as existing in this zone, with Mars being just a bit too small to retain its early, thicker atmosphere. And Venus is so inhospitable precisely because it is just a bit too close to the sun. If it wasn't for the heat, life would probably have sorted out those greenhouse gasses in the first couple billion years. You could probably terraform both... for a time.

When you consider that 500 million years ago the earth was frozen solid for millions of years, scraping away up to 1.2 billion years of the geologic record in some places, and over the last million years we've spent less than 20% in warm interglacials, the idea that Mars could have been warm enough to sustain complex life seems very unlikely.

MargueriteMing
11-11-2009, 01:02 AM
I can't recall all the ways they gave that the Moon protected Life on Earth. Certainly it was a shield against murderous meteor and comet hits (and how many still got through to almost end Life?) and made our planet's spin slow and steady and Life-affirming. Something about the oceans too, I recall. Seems there were about five ways that Life (as we know it, certainly; Life may have begun but vastly differently, sans Luna) needed the Moon.



The moon is so small compared to the arc of sky that we can be hit from that it provides no meaningful protection from asteroids or comets.

The action of tides created a biosphere in tidal pools, where part of the time life forms where exposed to air. To survive there, they had to adapt to be able to handle it. Most likely it was a stepping stone to life forms to spend more and more time out of the water, until some evolved that were truly independent.

It should be noted that 500 million years ago the earth was completely frozen over, for millions of years. All life on land crawled out of the oceans and evolved since then. It is entirely possible there was a complete biosphere before the earth froze, and it was all wiped out. There may even have been intelligent life. We'll never know, the ice scraped away the geological record, between 200 million years in some places, to 1.2 billion years in others.

For more on this, research the pre-cambrian discontinuity.

small axe
11-11-2009, 01:38 AM
The moon is so small compared to the arc of sky that we can be hit from that it provides no meaningful protection from asteroids or comets.


Better to keep our shield close to us to cover us from arrows, than to hold it far away, yes indeed.

I still recall them suggesting that was one of the Life-aiding aspects of the Moon in the TV show though. Just passing on the info.

It may be that they meant it in some way other than merely filling a large "arc of the sky" too? Something gravitational, or something to do with sweeping up debris near Earth (we can see by the craters how often the moon has gotten clobbered too)?

I've heard it say that having a giant gas planet like Jupiter sweeps up debris in early solar systems, sparing inner planets, and that's puny as far as filling an arc of the sky goes. (I was reading about TAU CETI for a thing I'm doing that suggests the same):



Although no planets have been detected orbiting Tau Ceti as yet, it is likely that any planet found to orbit within the star's dust disk would experience relatively frequent bombardment from asteroids and comets of the size that is believed to have wiped out the dinosaurs and other types of multi-cellular life on Earth. As a result, some astronomers have speculated that it is likely that with so many large impacts, large and complex forms of Earth-type multi-cellular life may not have had the opportunity to evolve and persist on inner terrestrial planets orbiting this star. Others (such as Glenn Schneider (http://nicmosis.as.arizona.edu:8000/) of the University of Arizona and Scott Kenyon (http://cfa-www.harvard.edu/hco/astro/people/homepages/kenyon.html) of the Smithsonian Astrophysical Observatory), however, argue that a giant planet in the system could gravitationally deflect comets and asteroids away from inner planets that may support life in the liquid water zone, in the same way that Jupiter protects Earth in the Solar System.

http://www.solstation.com/stars/tau-ceti.htm


I've also read that Jupiter is a main culprit in disturbing and throwing debris at us from the asteroid belt.

It's a complex and dangerous Universe.

But I recall the main thrust of the show was:
Without the Moon, our earth would have been much less favourable to Life as we know it.

Without re-reading the thread, I think I saw and I'll risk repeating that the gravitational effects of giant outer planets can keep their tiny frozen moons hot and bothered enough to allow for liquid water, and heat the moons via gravitational stress.

blacbird
11-11-2009, 12:25 PM
The moon is so small compared to the arc of sky that we can be hit from that it provides no meaningful protection from asteroids or comets.

Except that its orbital motion sweeps out a much larger arc of space that does that of the Earth. But it's a tough and theoretical question to answer, at best. All we really (seem) to know is that the last really big smack to hit Earth was that 65 million-year-ago event that whacked the dinosaurs. the last really big moon event we've discerned is the prominent Tycho Crater, estimated at something like 150 million years, by an article I read recently.

caw

efkelley
11-11-2009, 11:45 PM
The moon is so small compared to the arc of sky that we can be hit from that it provides no meaningful protection from asteroids or comets.

It's a gravity thing. The moon almost never acts as a physical barrier.

Julie Worth
11-12-2009, 12:30 AM
When you consider that 500 million years ago the earth was frozen solid for millions of years, scraping away up to 1.2 billion years of the geologic record in some places, and over the last million years we've spent less than 20% in warm interglacials, the idea that Mars could have been warm enough to sustain complex life seems very unlikely.

Yes, but before that it wasn't frozen. And if Mars had a substantially thicker and CO2 (or methane) rich atmosphere in the past--as did Earth--liquid water would be everywhere. And today we would see dried up ocean basins and super sized grand canyons--just as we do.

Julie Worth
11-12-2009, 12:34 AM
Except that its orbital motion sweeps out a much larger arc of space that does that of the Earth.


No, it's all a matter of cross-section and gravity. The Earth took far more hits than the moon (including the giant impact that created the moon to begin with), but almost all of it was erased by tectonics and erosion.

small axe
11-12-2009, 02:27 AM
I came across an article that was also suggesting that besides just the habitible zone of a single sun, scientists need to consider the sun's location in the galaxy.

It seems like the general aprox issues were: If your sun is too far out towards the sparse-populated galactic rim (I think it was the outer 20%), it may be too young or metal-lacking to form our sort of rocky Earths.

If your sun is in too close to the densely populated (by stars) center of the galaxy, then supernova, supergiants, and othe kinds of bad radiation events may fry life, every time a star explodes (and at the dense center, any one sun may have many more closer neighbors waiting to kill off its life-bearing planets)

We seem to be in a milder area of the galaxy. Though I did see a TV show that I think said that a gamma burst can fry us from a startling far distance. From thousands of light years away.

Another idea is I read was that we're on a regular 62 million year cycle, where when the sun wobbles up above the plane of the galaxy we get hit by massive radiation does that account for mass-extinctions.

Sorry for the generalities ...

But the point was that "habitible" zone maybe has several considerations beyond a single sun.

lpetrich
01-28-2010, 08:58 AM
For example, a planet would have to be the exact distance from a sun in order to be habitible? Terraforming is only theoretical, but if it were possible, then couldn't a planet much further away from a star be just as habitible as Earth if it had enough greenhouse gases? Or if it was the same distance as Venus, and simply had a lot less greenhouse gases?

Increased amounts of greenhouse gases can easily solve the faint young Sun paradox (http://en.wikipedia.org/wiki/Faint_young_Sun_paradox). The Sun started out at about 70% its present brightness, which would make the present-day Earth freeze over. However, there is abundant sedimentary-rock evidence for liquid water, which makes the Earth hotter than expected.

The main one is carbon dioxide, which is emitted by volcanoes and removed from the atmosphere by rock weathering and the like. When the weather gets too cold, weathering slows down and CO2 accumulates, heating up the Earth. But when it gets too hot, weathering speeds up and CO2 gets removed, cooling down the Earth.

In the early Earth, there were other possible ones, like methane and carbonyl sulfide. Methane would be able to accumulate because there was no free oxygen back then.

Ambri
02-19-2010, 10:24 PM
One factor I don't think I've seen specifically mentioned in this thread is the energy from the sun/ the system's star. In addition to being in a zone where liquid water exists, Earth also receives a much greater percentage of the sun's energy than Pluto does. I don't know the exact formulas or percentages off the top of my head, but I remember learning about this factor way back in Astronomy class. The energy from the sun allows photosynthesis to take place, which makes an oxygen atmosphere and animal life as we know it possible.

lpetrich
02-20-2010, 04:17 PM
The Earth has liquid water because it is close enough to the Sun to be heated up to liquid-water temperatures. Pluto is too far away for that.

Satori1977
02-22-2010, 08:13 PM
A few months ago I saw an interesting documentary about terraforming Mars on the National Geographic channel. How to go about it, how long it would take, is it really possible.

http://channel.nationalgeographic.com/episode/mars-making-the-new-earth-4588/Overview#tab-Overview

Pretty cool, not sure if they are still showing it.

Xelebes
02-23-2010, 12:26 AM
The Earth has liquid water because it is close enough to the Sun to be heated up to liquid-water temperatures. Pluto is too far away for that.

At least on the surface. Pluto is showing to have at least enough movement in the atmosphere to suggest that there is enough tidal movement for there to be sub-crustal liquid something there. Could be water, methane or both.

blacbird
02-23-2010, 12:36 AM
A few months ago I saw an interesting documentary about terraforming Mars on the National Geographic channel. How to go about it, how long it would take, is it really possible.

http://channel.nationalgeographic.com/episode/mars-making-the-new-earth-4588/Overview#tab-Overview

Pretty cool, not sure if they are still showing it.

I haven't seen it, but I'm curious.: How do they get around the low Mars gravity (too low to hold either nitrogen or oxygen in the atmosphere), and the lack of significant magnetosphere (which allows solar particles to blast away much of the atmosphere that remains).

Mars is barely large enough to have sufficient gravity to hold a carbon dioxide atmosphere (CO2 is considerably heavier than either N2 or O2), and even that is tenuous, having at the planetary surface about 1/100 of the atmospheric pressure of Earth. At that low pressure, liquid water evaporates rapidly, and water vapor, lighter than N2 and O2, will simply escape into space. It's quite likely that this process happened in the distant past on Mars, and accounts for the relative paucity of H20 there today.

The absence of O2, which wouldn't be present without biogenic production anyway, also means there's no O3 (ozone) in the atmosphere to protect against deadly concentrations of solar UV radiation.

caw