Breatheable atmosphere =/= Earth-like planet?

As far as I know (not an expert!) I'd say yes, this is entirely possible. Humans would need the right composition of gases in the air on the planet to be able to breathe, and as long as those humans have their own supply of food and water, that should be fine.

However, putting humans on such a planet will slowly change the air over time, as we breathe in oxygen and expel carbon dioxide. Without the right sorts of life to reverse the changes we'd make, the humans may not be able to live there indefinitely without engaging in terraforming of some kind.

But just because it may be possible doesn't mean it's probable. Having the right conditions for humans to survive is highly unlikely on any planet, and that doesn't apply just for the composition of gases in the air (which would be difficult enough). The temperature might be too hot or cold. Gravity may be unbearable. The atmosphere might be too thin or too thick. If it's too thin and a star is close, too much ultraviolet will get through, which will do damage to human DNA, eyesight, skin, and probably more. There are lots of variables.

But details like that rarely stop a good story. ;) And besides, we don't need perfect conditions to live - plenty of people on Earth live in sub-optimal environments - they just risk an earlier death.

I'm thinking about this one. My inclination is to say no, it's not possible. Or maybe probable is a better way to say that. In some respects, you're saying that this planet has no water. Or very little. But water and atmosphere are developed in much the same way, and I find it difficult to imagine that the planet's history allowed for the creation of an atmosphere rich in oxygen but without water.

It may be that I'm not being creative enough in my twisting, getting around it. Others might come up with some way to do it.

After all, Earth itself's atmosphere/hydrosphere is extremely peculiar, and scientists are still not entirely agreed on how it came about. E.g., thinking that Earth must have been hit by a massive asteroid early in it's history (same one that created the moon?). What kind of weird chance event is that?

So if weird events can happen to make Earth, why not your planet? I'd be inclined to say go for it, and let the scientists figure out why the planet is so godawful weird. :)

Climate and ecology are somewhat separate issues, by the way. It is entirely possible to have a savannah type planet, where lichens, algae, and primitive plants make up the bulk of the biomass. Also entirely possible to have water not moving around much. It's the lack of oceans that is making me balk somewhat.

Another thing (Christ, don't I ever shut up?). Our planet configuration, the topography of having so much deep ocean and so much high land, and not a whole lot of in between, is due to the mechanism of Earth-style plate tectonics. This is an issue separate from water in those oceans. If all of your land is essentially the same topography, or even if it is sort of evenly distributed, with lows, low middles, high middles and highs essentially falling on a smooth curve, then your planet probably has no plate tectonics as we know it. Not sure that makes sense. Easier to show with a graph and arm wavy gestures towards a map of the globe.

Wow - more than half way through and this question pops up. I wouldn't think too much on it, unless you get into explanations for it in your novel. Planetary scientists didn't think it was possible for massive unstable planets to exist so close to their stars either, but they're finding new ones practically everyday. My personal belief is that the reality of the universe(s) is far more vast and varied than we could ever possibly imagine. Your planet could have a breathable atmosphere because of micro-organisms that exist somewhere in the atmosphere, or on the surface. They use one part of the planet's atmosphere to survive, and their waste is oxygen. Voila! The possibilities are endless - type on...

adding on to my earlier comment -- gave thought, and putting the two bits together, topography and no oceans, and I realized that the planet could have lots of water, but stored as deep groundwater, where there were no low ocean basins for it to collect in. This has it's own problems (how is there pore space that deep if you don't have lots of erosion and sediment creation, and how do you get that with little tectonics) but frankly, those are easier to get around and fudge with.

So yeah, go with it. And as someone else pointed out, it's not as if you have to explain all of this in your story (in fact, it'd probably be boring to do so). Though knowing you have deep groundwater might give other interesting aspects to the whole story.

And incidentally, having deep groundwater makes it easier for lifeforms to evolve -- even primitive ones.

I seem to recall reading somewhere that modern scientists consider any rocky ball about the size of Earth "Earth-like," so a rocky ball with a breathable atmosphere (in this definition) would actually be a sub-type. As to why the atmosphere is breathable, well, it could be anything from lightning that split the water into hydrogen and oxygen to a fully-developed biosphere. Or something created it that way...

I would say, though, that any planet with a biological cause for the atmosphere's makeup would have another reason for that gas to be converted into something else, preferably a gas used by the first biological thing in the first place.

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Wow, thanks everyone!
I think I will just go with some sort of vague mention of groundwater, somewhere, and let anyone who really cares come to their own conclusions. I was just mostly interested to know if this was totally impossible.
:3

....and the plot thinnens.

A breathable atmosphere is much easier than you think as all you need is enough oxygen at the right pressure and nothing nasty. So if you have a light planet, with a large amount of rock bound oxygen and its close enough to the local star for heating, then the oxygen will boil and your astronuts can catch a breath. The problem comes from the temperature and making sure there isn't anything nasty in the air otherwise.

Interestingly, if you were going for a chemically produced atmosphere, the high levels of oxygen would likely prevent the evolution of life due to the high levels of free radicals, that is, until humans with their oxygen adapted prokaryotes come along.

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It's easy for a big rock to have an atmosphere at the right pressure, but oxygen is the problem. Oxygen is a very reactive gas and would really rather be bonded with stuff rather than floating around in the atmosphere. (hence things such as fires) The only reason that Earth has oxygen in its atmosphere rather than all bonded with the rocks as oxides is that oxygen is a byproduct of photosynthesis.

In fact, oxygen is so reactive that when the first explosion of photosynthesizing life came about, it essentially poisoned the world until aerobic critters evolved to make use of oxygen's reactivity to get lots more energy. For more info, read up on the Great Oxygenation Event: http://en.wikipedia.org/wiki/Great_Oxygenation_Event

Bottom line, you need some explanation for having a non-equilibrium gas like oxygen in the atmosphere of your planet. You mention savannah, which implies plants, so in that case sure. But you can't just have a dead ball of rock with an oxygen atmosphere.

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