Planets with an atmosphere suitable for humans to breathe.

Earth’s atmosphere is a mixture of gasses kept in balance by the plants and animals that have evolved here. Is it possible for a planet to have a similar atmosphere without having evolved similar flora and fauna?

This is my take on it, happy to be corrected by one of the many experts that hang around here…
Oxygen (O2) is a fairly reactive molecule. A typical fire is simply Oxygen reacting with Carbon. Oxygen also reacts with iron (rust) in certain rocks turning them red. There are many things that oxygen will react with.
The atmosphere of Earth did not have much free Oxygen until life started photosynthesizing and created it as a byproduct.
It seems unlikely that a rocky planet would have much free Oxygen in the atmosphere without some process to free it from anything it reacts with.

I have to say that it seems very unlikely that a rocky planet could develop a breathable atmosphere without some sort of photosynthetic life. As Saffer has mentioned, oxygen is a very reactive gas, so if it were present in a planet’s atmosphere it would react with the crust and disappear over the space of a few million years (or less).

One way that oxygen can be created without photosynthesis is by the effects of bright sunlight on water vapour; water is split into hydrogen and oxygen and the hydrogen is lost into space because it is so light. This process (photodissociation) occurs on Earth, but it is a very minor process compared to photosynthesis. A planet where photodissociation is occuring will soon lose the oxygen into the crustal rocks, unless there are no surface rocks exposed . On a planet with a very deep ocean the oxygen might remain in the air long enough to produce a breathable fraction, so waterworlds just might have enough oxygen to support human life. But then you have to think about the other components of the atmosphere - is there enough nitrogen or too much? Is carbon dioxide present in toxic quantities? Are there other components to the atmosphere that might be toxic - sulphur dioxide, ammonia, chlorine?

There are probably a very wide range of different types of terrestrial planet, and it semms likely that very few of them will hold a breathable atmosphere. Even those planets that do have photosynthetic lifeforms are likely to be very diverse, and few will exactly match the Earth’s atmosphere. Those that do could easily hold biological dangers of various kinds, microbes or allergens, making them uncomfortable or dangerous places to visit.

Note that several of the icy moons of our solar system have oxygen-rich atmosphers created by photodissociation; but you certainly couldn’t breathe there, as these atmospheres are very, very thin, and very very cold.

O[sub]2[/sub] is thought to only occur as a by-product of life. It’s only been recently discovered to exist in some sort of nebula, but the quantities are very very small. That’s not to say some chemical process will be found that creates our levels of O[sub]2[/sub], but then we have to figure out where all the CO[sub]2[/sub] went. Both Mars and Venus have atmospheres almost completely composed of CO[sub]2[/sub].

The Webb Telescope will only be able to read down to 600 nm (orange), It won’t be able to detect the twin 500 nm (green) lines characteristic of O[sub]2[/sub]. Perhaps the next gen telescope will.

The Webb sees in the wavelength range that it does because our studies of the Hubble data (which does cover the full range of visible light) suggest that we’ll get a lot more useful information from that wavelength band. Neither Hubble nor Webb would be suitable for looking for oxygen-atmosphere planets, even if they did have the right wavelength range, because neither one has sufficient resolution to directly pick out planets. For that, you would need a completely different design of telescope, like the (now sadly cancelled) Terrestrial Planet Finder.

Thanks for the responses.

I was thinking about this in light of the increasing number of extrasolar planets that are being identified. Ostensibly, the more earth-like planets there are, the more likely it is that humans will one day colonize the galaxy. We have been conditioned to think of earth-like planets as places where the landing party can spread out across their new homeland and start planting potatoes and pine trees. But even with suitable temperature, gravity, and liquid water, the lack of a breathable atmosphere would create a much less pleasant experience, forcing colonists to live in domes or underground habitats.

Maybe this is the answer to Fermi’s Paradox.

Just having oxygen as a constituant isn’t enough to make a planet habitable. A breathable atmosphere had to have a partial pressure of oxygen (PPO) between 0.16 and about 1 bar; too little and it won’t support respiration; too much for too long and the body suffers from oxygen toxicity. The atmosphere also has to be free of toxic chemicals; no concentrations of CO[SUB]2[/SUB] above 3% (for long durations); no significant concentrations of aromatic hydrocarbons, chlorine, or other toxic and reactive species. Our own atmosphere is largely nitrogen and trace amounts of other, mostly noble, gases. Finding this perfect (for us) mixture is highly improbable, especially as the atmosphere of modern Earth is a result of the evolution on this planet.

As far as this limiting the propagation of life across the universe, any intelligent species capable of interstellar travel would command enough energy and the ability to build large, long duration structures (or have evolved or engineered themselves not to need such) such that planets are unnecessary for habitation. Mineral, elemental, and energy resources are all more accessible and available then digging them out of the crust of deep gravity well. If this plays into a Fermi’s paradox situation, it’s that we possibly wouldn’t recognize complex, advanced life even if we saw it.


I think I read that terraforming Mars to support life would take around 500 years and even then the planet would be basically the Gobi desert.

Looking at all the problems of space travel, I think we better learn to just be happy where we are.

Not at all; the Solar System is full of the sort of resources that can support life. Carbon, hydrogen, nitrogen, oxygen, and almost exactly a billion times as much sunlight as ever falls upon our planet. We can use these resources to build habitats in space, and support trillions of people in comfortable habitats, and still have plenty of energy left over to spread to the stars.

Such a process would not be straightforward, however - these resources are thinly spread, and there is damaging radiation in space that needs to be dealt with, for example; but given the abundance of energy coming from the Sun most of these problems can be dealt with in due course. The ultimate problem in such a process is knowing when to stop.

If there’s enough UV in the atmosphere to split H2 from 0, you’d better have a pretty big bottle of SPF 1,000,000 handy.

To expand on that, such habitats, instead of being the complex assemblies of manufactured modules, could be relatively simple in construction by using fiber-reinforced ice structures spun for simulated gravity, using a liquid water “sea” for thermal mass, nutation damping, and radiation shielding, as well of course as use for hydration of the atmosphere and water for consumption, agriculture/aquaculture, and industry. Such habitats could be constructed with modest extensions of extant technology and powered by solar energy collected by a cassigrain reflector which also shields the habitat from direct solar impingement and provides sunlight directly to the interior via fiber optic conduits and diffusers. Given the available useable mass in small objects (<1 km in diameter) it is conceivable to support populations of hundreds of billions or even trillions of people in comfortable fashion comparable to modern industrial nations today.

This does happen, at very low levels, in the upper atmosphere. But no, it wouldn’t create a free O[SUB]2[/SUB] content at 21% for any significant atmospheric pressure. Highly oxidizing species can only exist when some thermodynamically regulating reaction applies enough energy to continuously separate oxygen (or chlorine, or fluorine, or another highly reactive species).

As others have noted, we won’t be able to search for diatomic oxygen spectral lines with existing telescopes, but that would certainly be a way to look for planets on which life may have developed. However, per the previous discussion, this doesn’t mean that it is suitable for human life, and in fact, the likelihood that another planet, even one containing indigenously developed life, would be habitable by humans. The human-friendly planets that are a staple of science fiction are an anthropocentric trope that is not supported by our current expectations of planetary evolution.


The sunlight doesn’t have to be that bright for photodissociation to occur; it does happen on Earth, but at an insignificant level. If it were the only oxygen-producing process on the planet, and there were nowhere for the oxygen to go, this would build up over time.

The sunlight at Venus, however, has been bright enough to split all the water on this planet’s surface, and the planet has lost nearly all its surface hydrogen. What little remains is enriched in the slightly heavier deuterium isotope, which doesn’t escape so easily. The remnant oxygen has combined with the crust, and is present in the atmosphere as carbon dioxide.