I'm writing a story which is about humans colonizing a planet, possibly in the alpha centauri system.

The planet needs to sustain human life but does not appear to sustain any life of it's own (maybe some bacteria) so I'm thinking the one thing this planet has going for it is the right gravity and temperature.

The atmosphere of this plannet is possibly CO2 so it cannot sustain human life. However, the colonies are filled with a genetically modified plant that can convert the CO2 into O2 and somehow create a sustained breathable atmosphere that surrounds the open colony (no domes etc)

Water is extracted from the ice below the planets surface.

I would really appreciate it if anyone could shed some science theory behind this as I'd like it to be as realistic as possible. Also a description as to what this planet could look like (eg. surface and sky colour, sun etc) would be really helpfull too

Somebody else will have to give a better answer, because I'm no expert, but I wonder if CO2 can be created by a non-life process. It may be that it requires life.

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Venus is covered in a thick blanket of CO2. It didn't require life.

Volcanism makes lots of CO2 on Earth--probably what happened with Venus, but much worse.

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Plants require oxygen. How did the process get started?

US Department of Energy: Ask a Scientist: Oxygen vs Carbon Dioxide use in Plants

And over winter. Anyone know of plants that function without any oxygen?

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I think the planet itself is perfectly plausible, but your idea of sustaining an atmosphere of oxygen with nothing holding it back doesn't seem possible. The O2 would just get blown away by the wind and greatly diluted, unless there was something to prevent it drifting off. A dome would of course solve that problem, but you say you don't want that, so I don't know. Maybe the oxygen could be kept around by some kind of electromagnetic field, although that would require the O2 to be charged in some way, and I don't know how to do that. If you wanted to go Star Trekish, then you could just say it is being held there by a "Force Field", or whatever, but you said you wanted it to be realistic.

I don't know what colour the atmosphere would be on a CO2 planet, but I can offer some links to stuff that you might find useful in designing the planet and star system (although in this case the star system is Alpha Centauri so you'd have to use the real known make up of that system, fortunately there's plenty of info on that... http://en.wikipedia.org/wiki/Alpha_Centauri).

*This is a good website with some useful calculators and info: http://www.geocities.com/albmont/hard-sf.htm
*This calculator can let you work out the details of a planet around a star: http://www.geocities.com/albmont/mseqstar.htm
*This one just calculates planetary parameters: http://www.stardestroyer.net/Empire/...alculator.html
*This site allows you to work out a realistic surface temperature for a planet (I've tested it out for the planets in our solar system and its pretty accurate): http://www.astro.indiana.edu/~gsimon...perature1.html
*This calculator allows you to do Kepler’s laws, fast. To calculate the orbital period (length of year) of your planet, etc: http://www.ajdesigner.com/phpgravity...ion_radius.php

To start off with, first you'd have to decide which star of the Alpha Centauri system your planet orbits, then you can begin working out how hot or cold you want it to be (by how far away it is from its star, etc), the atmosphere would influence this a lot if it is CO2 (i.e. probably big natural greenhouse effect).

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I think you could make that scenario work. I suggest you look up the pre-Cambrian Earth which was in a fairly similar state. There is however one slight issue there in that this is what our planet was like early in its history & Alpha Centauri is a system that's older than our Sun so I'd suggest either using another younger star or if you are going to use Alpha Cen. making some plausible reason why this world has only now developed life. Eg. its been on the icy fringe of A.Cen's habitable zone and has only now begun to become habitable as the star gets hotter and brighter with age.

If you go for that latter option I'd guess a distance of roughly a bit beyond Mars which orbits at a distance of 1.5 AU / 13 minutes light time / 228 million km from our Sun. Your fictional planet could be a little further off given a Co2 rich greenhouse atmosphere. But don't put it too much further out as I think there's a limit on how distant an orbit from either star can be before the planets orbit would be made unstable by the other stars gravity.

Gravity and temp. are not too hard to imagine being similar if you have a world like Earth in a reasonably similar orbit. A more eccentric orbit will give you more exaggerated seasons - Brian Aldiss'es 'Helliconia' trilogy may give you some idea of how that can work & I'd recommend reading that.

There's stacks of info. on Alpha Centauri and its been used in SF before - Isaac Asimov has written a whole book on it* and James Kaler has a good mentions of it on his stars website :

http://stars.astro.illinois.edu/sow/rigil-kent.html

& in his book ’The Hundred Greatest Stars’, (Copernicus books, 2002.) which, again, I'd recomend as a handy and fascinating reference source.

Incidentally, there was, I think, a BA blog thread dealing with simulations of the formation of planets around Alpha Centauri B (the cooler K type component) which strongly suggests that terrrestrial exoplanets do indeed exist there. It may not be too long before planet hunting astronomers actually detect a world or two around that star which could be inconvenient or convenient for you depending on :

a) What sort of planet(s) they find exist(s) there & what its/their orbit(s) are like

&

b) The timing -whether its before you write (potentially helpful!) or during (may mean a few revisons) or after! (Too late to change if you need to & dating thebook unless by fluke the world you hypothesise matches the /a real one!)

One thing somewhat puzzles me here though :

"Water is extracted from the ice below the planets surface."

Now why is that necessary? Are you imagining a planet that's extremely water poor? That's perhaps a reasonable assumption given some ideas mentioned in Lynette Cook & Ray Villard's excellent exoplanet tome, ‘Infinite Worlds : An illustrated voyage to planets beyond our Sun’, (University of California Press, 2005.) - & another one I'd highly recomend reading if you can find a copy - suggesting the protoplanetary disk around Alpha Cen was water poor because of instability with having two stars interfering with the distant parts of the disk beyond the "snowline" (ie. the region where water can exist & where Jupiter and the other gas giants probably formed.)

You could imagine a sort of very dry polar desert type planet. But why ice underground then? It also makes it much harder for life to get started there and also perhaps a lot more difficult for the atmosphere to be a Carbon dioxide rich one.

I think you'd be better going with a world that's water-rich with large oceans thus easier for life and probably also forminga CO2 rich atmosphere. You could even have a super-earth type planet with heavy gravity, stacks of water and perhaps a layer of "hot ice" (high-pressure, high temperature forms of ice) although this may limit your surface land options.

A medium version planet with a "Cambrian Earth" type feel with bacteria and maybe sea life, cold and wild with oceans, lakes and glaciers makes some sense to me and would remove the need to mine ice from underground but then that's all very much up to you and what you've envisioned for your story.

One final piece of advice - I'd suggest you do these two things if you haven't been doing them already. Read a wide raneg of SF yourself and see how other authors have come up with fictional worlds - so you have some idea of common strategies and don't repeat stuff that's too much like what others have already done. Also join a writers group either online or face to face & get some feedback on what you've written and where you can send it to be published. These are both things I do and would highly recomend - and btw. these are good fun too!

Oh & finally START WRITING & see how you go!

Its too easy to procrastinate & not get on with putting the words onto paper. (Hey that's what I do! A lot! ;-) ) Once you've got a story going in terms of plot & characters you can flesh out the details and if need be come up with some plausible science scenarios.

Let me know how you go - I'm happy to help out more if I can. Good luck!
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* Asimov, Isaac, 'Alpha Centauri, The nearest star, Lothrop, Lee & Shepard Company, 1976

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wow! I didn't expect this much help so soon! appreciate it guys

Jens & SkepticJ: The CO2 to O2 plant theory was one suggested to me by Katja Riedel (atmospheric chemist):

So the plant would be engineered in a way to deal with this process. The Volcanism is an interesting idea.

01101001: Thanks for the link, will read up on that. Maybe the plants could be initially given oxygen then re-use some of the oxygen they create from the carbon dioxide?

Murphy: Yeah the whole floating atmosphere was something I was never sure of. I wanted the city to be open air so you could see the sky. I guess this is where it gets complicated. Cool! Thanks for those links. will play around with these calculators tonight. I remember watching a program on a group of people that create planets this way via software. it actually predicts what it will look like and you can explore the surface etc. crazy stuff!
Will need to read up again on Alpha Centauri. I think I determined which star was the most suitable from watching alot of documentarys, but I guess reading about these stars is probably best.

StevoR: Yes! pre-Cambrian Earth or a pre-stage of the earth was something this planet was supposed to reflect. I guess that was the whole point as to why they travelled such a great distance to colonize it as it had a simular atmosphere to earths. This is also why I wanted to avoid the city in a dome thing as that can be done on planets in our own solar system. The new planet was supposed to be a new earth, though I didn't want to have to deal with vast species of aliens or any aliens, hence not having any oceans or seas of any sort as that would most certainly spawn life along with an O2 rich atmosphere.
I think this is my main concern is the whole alien life thing. I'm trying to pin down a planet as close to Earth as possible but limited of vast amounts of alien species.

As for Alpha Centauri it was the only other solar system other than our own that I knew about. It's also the closest. Though if you can suggest any other more plausable solar systems that would be younger than our own and would more likely house a younger Earth planet, that would be ideal
I am hoping to turn this into a graphic novel. Something I've had in my head for 15 years but never got around to writing it. Now that I've started to write it I found that I couldn't really proceed until I had all of this stuff figured out as the planet is a potential key role in the plot.
Basically humans can live on this planet just like earth, though they still need something essential for their survival (hence the artificial plant that makes the atmosphere breathable) the idea is that terrorists are trying to destroy this essential survival thing thus wiping out all human life. These terrorists aren't entirely human (engineered) so they do not need the essential survival thing the humans do.

Maybe it's just easier to have a planet rich in water with an O2 atmosphere and lifeforms. I would just need to re-think the plot a little.
Yeah I definately need to read more Sci-Fi, Asmov is a great start. Do you have any other book recomendations? I think once I get this whole planet thing sorted then I can move on with writing this thing. The story I had in mind 15 years ago left me with these fascinating characters who I've grown up with but no real plot. Are you currently writing your own novels StevoR? I could certainly do with more help on this as you seem to know your stuff and I am a total science noob when it comes to planets. I've also started looking into interstellar travel. heh, never thought writing science fiction would be this tough :P

ok, I think I'm gonna have to read over this thread again. lots of info to take in. Please keep the suggestions coming

?? Where at any point did the OP say that there was "nothing to hold it back"?

EDIT: D'oh, never mind, I see it.

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Best to set the ice story on a cold moon of 47 Ursae Majoris instead of Alpha Centauri.
Great links Murphy Is it possible for a planet like this to exist? Thanx

ok folks, change of plan.

I've decided to go with an earth-like planet, so that means oceans, ozone. Victoria meadows says that larger earth-like planets (50% earths mass) that are in the habitable zone of a star would be unlikely to house life (for some reason??). If this is the case could they still sustain life in an open environment?the sence of human migration in an open environment?

The reason, IIRC, is that 50% probably isn't big enough to have enough tectonic activity and recycle the elements in the atmosphere absorbed by the crust (though that also depends on the amount of radioactives in the interior, a hot-cored small planet might work); so no, not livable in the long run. Better to have an Earth-sized world with oceanic algae-equivalent (to give it oxygenated air) and have one major continent that has only recently come above water; that way you get the best of both worlds, no land life to compete with the newbies but a life-supporting planet nonetheless.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

As CO2 is toxic in very low concentrations, and no plant, even hundreds of miles of bufferzone full of them, has the capability of transforming even a partial CO2 atmosphere to a low enough CO2 level for human respiration, you would have to use domes.

On the other hand, if this is science fiction and you have a super-plant that can accomplish this, there's no logical reason you can't seed the planet with it and terraform the entire planet in a couple of years.

So, if the plant's that good, just terraform the planet. If it's not, use domes.

Those are your two choices if you want to maintain logical consistancy.

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If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

you could make the panet young enough to be in a precambrian state. it really takes a loong time for life to come up with anything really complex. it's most advanced life would be stromatolites and perhaps sponges and primitive yellyfish if it's in the late precambrian state.

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Life forms should be CBL forms and I would imagine the planet would have to have similar parameters to earth

Thanks for the feedback guys. So the planet should be the same size as earth.

Noclevername: The single continent idea is an interesting take, though that might make room for lots of sea life and I kinda like the idea of a rocky planet surface which brings me to the terraforming.

mugaliens: Is it possible for a CO2 based planet to sustain seas of water? It needs something to offer in order for the colonists to travel all that way for it, otherwise they could just terraform a planet in our solar system.

Antice: This sounds cool. My previouse idea was pretty much this, kinda like discovering an early earth
My question is would human beings be able to survive a precambrian state earth? what are the major environmental differences between earth no and earth then?

Also would the star system need to be simular to ours? ie. would the planet need a good jupiter a moon etc?

cheers guys

Just a thought, but if you need a reason to travel to another star system rather than terraforming, say, Mars, you could set the story far in the future with our sun becoming a red giant.

This adds the factor of not having help available from home, which could be interesting.

Of course, you need to account for the life cycle of your destination star. You don't want to pick one that will also be dead by then.

I don't know if any of that fits into your story, but thought I'd throw it out there.

precambrian earth would not be able to suport us without breathing gear. the atmosphere would ether be too oxygenated or too CO2 saturated. or even both at once.

one has to remember that a Precambrian stage planet likely do have oxygen releasing cyanobacteria and stromatolites. it's a very hardy colonial structure and so do lend itself to evolving in extreme environments. actually. you are likely to have oxygen caused mass die offs happening regularly for a long while until you have some kind of animal life to help counter the oxygen producers.
on Earth this is theorized to have happened after mitochondria evolved into a symbiont with Eukaryotes.

A planet that is both over oxygenated and with a high percentage of CO2 would be interesting. but i suspect that would usually be a very short lived and or localized phenomena. A planet in this stage would have a lot of free Oxygen sinks like Iron and sulfur as well. the oceans would likely be saturated with free iron, and electrolyzing the water might be a cheap way to get this metal. as far as survival gear for humans. well.. the temperature would likely not be too bad. but the air is either going to have too much oxygen or none at all. depending on the availability of atmospheric oxygen sinks.
either way a simple breathing mask might be enough for survival.

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it sounds a lot like an early version of Venus (or perhaps a Venus further away from the sun thus having a longer process time), a Venus that basically hasn't reached a critical CO2 level to the point of high atmospheric pressures & gas-house affect to the point where water can't exist as ice anymore...

it is "possible", but to be honest i don't think any scientists at this point in time can answer the question of whether its plausible. one thing to importantly note is that "the one solar system none of of our models of how solar systems form seems to explain is our own", that is the chemical ratio's in our own ancient solar disc make absolutely no sense, and IMO the only thing maintaining our current theories of solar formations is the lack of capacity to find smaller planetary bodies...

so right now, i say the scientific knowledge is agnostic about your planet. the only models we have to determine the chances of a planet's chemical composition in a certain distance from a certain sun have being disproven...

whether your ok with using an agnostic level of plausibility as the core of your story is up for you. if you go forth with it, i would say its somewhere on the thin line between "hard sci-fi" & "soft sci-fi" but at this point doesn't go one way or another.

...or you could wait for the planetary finder, but with this economical state you need to take the chances of the project reaching completion with a grain of salt.

Solar system formation models are not how we determine how the earth formed.
Geologists are the ones who helps solve that mystery. as well as one piece of rock brought back from the moon. the genesis rock.
it has been well established that the proto earth had an impact event causing at the least a partial melt of the earth. an a new moon that was entirely molten at formation. the moon's surface solidified pretty fast and the oldest rock from our closest neighbor has been brought home and analyzed. they give the date for this event.
the oldest rocks found on earth are no more than 500 million years younger than the genesis rock. and those are indicative of not only a wet earth. but one where life were already doing it's thing and altering the environment trough their activities.

There are many competing theories for just exactly how and what happened. but the chronology is in the rocks. So the fact is. we do know how earth avoided becoming another venus. Life happened to it.

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Sorry, but that's beyond the scope of my knowledge.

I can conjecture that if you had enough hydrogen and oxygen for oceans, most of the hydrogen would either be chemically bound or blown away, and you'd have an oxygen-rich atmosphere.

Immediately after Earth's early impact, it lacked an atmosphere, but it's surface was molton. Replenishment of that atmosphere came from steam escaping from the crust and volcanic gases. Bolide collisions provided Earth's water.

It's early atmosphere contained almost no oxygen, but plentifu CO2 and water, combined with early forms of photosynthesis and a couple billion years of biological activity, such as what still occurs in stromatolites, resulted in the oxygen concentrations we enjoy today.

Sorry I couldn't be more detailed.

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If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

A water-rich world wouldn't develop an abiotic oxygen rich atmosphere , because the rate of oxygen formation by photolysis is quite slow, slower than the rate at which the oxygen produced in this way would be absorbed by the crust.
However a planet completely covered in a very thick layer of water (a waterworld) could develop an oxygen-rich atmosphere in this way, because the crust would not be able to absorb the oxygen through all that water.

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wouldn't it take an awful lot of time for all the iron disolved in the waterworlds world spanning ocean to be oxidated away? or do the iron only disolve in running water like rivers?

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Would the oxygen tend to float upwards in an CO2 atmosphere, like how large releases of CO2 tend to displace O2 along the ground here on earth?

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the tendency for O2 to float upwards and CO2 to migrate downwards is real even in macro scale systems. however. in a dynamic environment like a planetary atmosphere it tends to become remixed continually. so while CO2 levels are higher at ground level th O2 is still mixed in with it.. otherwise we would be in trouble here on earth as well.

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