# Is life required for a world to be habitable for humans?

Well, the strongest barrier that might prevent life from appearing might be described by that analogy :
it’s like hoping to cook an apple pie by mixing the ingredients with random quantities and at a random orders. That is even with the right conditions and with the correct amino acids, no self replicant form of living would ever emerge (due to the complexity of the required molecule mixture).

By habitable I mean being able to :

• Walk on it’s surface with normal earth clothing.
• Drink surface liquid water without treatment.
• Create soil by mixing local pounded rocks and earth bacteria.
• Breath dioxygen through bottles filled by electrolysis of water (the electricity could be produced through solar panel). You just wear heavy bottles and pipe in noses which allow you to drink and eat normally.
Dioxygen being an oxidizer, it can’t exist at breathable quantities over long period.
• Being able to grow edible plants with artificial atmosphere (this would be done by filling transparent box with the correct gas ratios). The vegetables would be brought in the form of seeds.

Those conditions basically correspond to what can be brought through the first space probe landing on such planet (provided it also carry humans). The idea is once landed they could supply their own needs.
Of course, this implies the following conditions :

• Habitable atmospheric pressure.
• Must have ground which is not covered by water (this exclude complete ocean planets).
• The star should be stable (typically red dwarf tends to multiply by twice the radiated power from time to times).
• Have liquid water on it’s surface.
• The gravity shouldn’t be too high or too much low.
• The atmosphere should be non toxic (no cyanogen please).
• There shouldn’t be massive radiation like X-ray or microwaves (don’t know if uvc is a strong barrier if you wear sunglasses).
• The ground should have the following atomic elements in small quantities on it’s surface because they are required components of humans or edible earth plants :
nitrogen carbon chromium copper molybdenum cobalt selenium nickel manganese zinc iron magnesium phosphorus sodium chlorine boron lithium potassium vanadium silicon fluorine arsenic iodine calcium.
• Have enough light so edible earth plant can grow.
• Not be struck with large asteroids too much often.

By comparison Earth without life would have accumulated more methane gas (because of volcanism) than the atmospheric dioxygen can convert to carbon dioxide. The resulting greenhouse effect would had been strong enough to start boiling oceans away. Water vapour being a powerful greenhouse gas itself, this would lead to a runaway greenhouse effect (sounds something that could explain how Venus is now). The average temperature would be around 900℃.

So could a lifeless planet or moon or asteroid or comet be habitable for actual humans if we exclude natural breathing from habitability criterias ?

• there are billion of millions of possible combinations to allow for life not only that one on earth...The right combinations are incredibly more common than you can even imagine. Aug 28, 2016 at 15:31
• @渡し守シャロン : by living conditions, I mean correct elements with correct temperatures pressure and light (no X ray). There are several reasons to believe getting such apple pie cooked at random is unlikely. Also, I’m talking about habitable in human terms. Aug 28, 2016 at 15:45
• Your planet with water will be in for a very fast and rocky replay of the earlier stages of evolution on earth, because you will bring all the modern genes with you, allowing life to adopt very quickly to e.g. the increasing oxygen level that the cyanobacteria you introduced will bring. After a few thousand years, this could be come very interesting.
– Karl
Aug 28, 2016 at 21:48
• @Karl : nope, the primary thing introduced that would really develop isn’t cynobacterias with oxygen but methane (whatever the target living anaerobic conditions are). So methane would quickly become the main gas of the atmosphere if it wasn’t already during thousands years, that is, until high level of dioxygen kills them underground then in water and then on the ground. Each step would takes several thousands years. if such settable world would exists in our solar system, it would need to already orbit after Saturn in order to keep liquid water. Aug 28, 2016 at 22:04
• @user2284570 Where is the energy source for your methanogens? And once there is methane, there are also methanotrophs. The cyanobacteria are the only ones with an ubiquitous energy source. Anyway, i wouldn't bet on a specific sequence of events.
– Karl
Aug 28, 2016 at 22:56

So the main issue as you've stated is the runaway greenhouse effect.

The best I can come up with is to consider that the planet is covered almost entirely by ice. Think somewhere between ice-age Earth and Enceladus. The low albedo of the planet (near 0.9 for snow compared with 0.06 for water 1) should compensate for the fact the high levels of methane keep temperatures warm enough for liquid water to exist (at the equator?). I haven't done the maths on this, but it seems feasible that the would be possible- it is, after all, the same process that makes ice-ages on Earth self-sustaining, at least for a time.

If it helps, a slower rate of rotation of the planet would create a larger temperature differential, ensuring more liquid water on the daylit side, which may then freeze on the dark side. This would also allow for evaporation, and hence snow, and snow is a much higher albedo than ice.

• The best I can come up with is to consider that the planet is covered almost entirely by ice. but as explained in my question the planet should be initially habitable for edible plants. A planet without liquid water on surface (which what earth would be without greenhouse gas effect) excludes this. Oct 3, 2017 at 18:20
• I don't think we are assured of a runaway greenhouse effect on the planet automatically (sciencedaily.com/releases/2016/10/161007090659.htm) and since we are going to be growing our plants in sealed containers anyway could we just avoid upsetting the planets ecological balance by not releasing any life into the wild. Oct 4, 2017 at 17:17
• I take your point, but I'm not suggesting there is no water, just that most of the surface is ice, like an extreme version of an ice-age. The remaining equatorial regions would have liquid water and available rocks as required for plants. Oct 19, 2017 at 18:13
• @PChapman Interesting article! That would make the setup of the planet I descibed a lot more difficult! I guess I'd probably turn to heavy volcanism replenishing the methane, and/or an unusually radioactive core for heating. On a different note, not releasing life is hard- theres the famous story about the bacteria in the Surveyor 3 probe camera Oct 19, 2017 at 18:27
• @precipitation exact early conditions on earth are far from known. If I were setting up a lifeless planet I would simply say 'These are the inital conditions' and not worry about it after that. Oct 19, 2017 at 19:15

If the planet is in the goldilocks zone of a stable star and had an atmosphere thick enough to stop most meteorites from making it to the ground, habitability without life would not be inconceivable. Oceans and lakes of (sterile) water is of course necessary. As well as sufficient light from the star.

The main determinant is the atmosphere. If you wish to breathe through a nasal prong, the atmosphere can't be outright poisonous (e.g. partially carbon monoxide), and it will have to contain a certain degree of oxygen from the start since a nasal prong only can deliver a portion of your total consumption. One alternative would be a scuba-like regulator connected to a tracheostomy, which could allow you to draw breath from your oxygen tanks and exhale through your vocal cords. (if you wish to keep the mouth free)

Another obstacle to overcome, I think, is the hardness of the soil. Assuming this is, like earth, a volcanic planet which over time has developed an atmosphere, the landmass would consist mostly of rock. And since you can't plant seeds in rock, that would be a problem. That is why I think rain is the most crucial phenomenon on such a planet. Rain erodes the rock to sand, and downfall could also create ice-ages that would be crucial in creating the soil and form of the landscape. You could of course also bring soil with you, provided you had enough room and fuel in the shuttle, but I assume you wish to expand your food production as well.

In order to grow plants, many microorganisms are needed because "large" plants such as grains, grasses, or trees can't produce some of the biological biochemicals needed (in other words fertilization). This is what bacteria, moss, and the like are good at. Therefore I would suggest that the settlers bring not only seeds but a bibliotek of different microorganism colonies as well.

• One alternative would be a scuba-like regulator connected to a tracheostomy, wich could alow you to draw breath from your oxygen tanks and exhale through your vocal cords. (if you wish to keep the mouth free). Not needed. The model used by our army sends pure oxygen but in small quantities. This allow the rebreather to have a very small mass. Rocks can be converted into sand manually though an hammer. I agree for bacterias, so I added it to my question. Aug 28, 2016 at 22:06

You have asked a very difficult question. I would wager that life itself is not a requirement for all the things humans would want with a planet, but having already-existent life on a planet makes things far better and flexible. Here is some explanation.

Life on Earth has formed cycles of elements and substances. For example, carbon cycle, water cycle and nitrogen cycle. This cycling of elements (a process where atoms of an element absorb or release energy and form different chemical compounds, so that they end up in being the same compound which they started with) enables the elements to be used and reused over and over again forever.

In the absence of life on a planet, the planet might be able to sustain human life for some time (ranging from thousands to millions of years, depending on the number of humans and the size of the planet), but this will gradually create dumps of compounds which would not cycle back into their original formulas. This will (in a long time) result in humans depleting all the natural resources of the planet.

• Or perhaps very quickly, maybe centuries. Without any prior existing life and with perfect living conditions, methane would be the primary gas in the atmosphere even if it wasn’t present before. Because the only thing that can prevent methanogen from being the primary form of life on earth is dioxygen which is toxic for them. Or maybe it’s because there’s already a runaway greenhouse effect that liquid water at earth pressures exists on destination. In that case, yes, it would takes more time Aug 28, 2016 at 19:07

You underestimate the power of organics when it comes to growing crops. The elements you require are often present as a direct result of life. For these molecules to be bonded/present without life seems unlikely to happen (not completely impossible. Soil is extremely complex and teeming with life. And that life needs sh*t and/or decaying matter. I'm talking literal waste product here. There's a whole ecosystem in our soil. Now, you might be able to do it temporarily with our own waste products, but it would be difficult for it to be self sustaining over a decade.

Think too, about the dustbowl created by NO VEGETATION covering the soil in the US during the depression. BY saying there's no life, at all, if there is particulate matter made from hard stone, and no roots or anything to keep this soil there. These are dustbowl conditions. So, in this case, you would have to contrive something in order for that not to be so. And if it isn't sand, it would be rock, and getting something to grow is rock is difficult. Difficult, but not impossible. Expect big dust storms.

Honestly, I think no life whatsoever with these conditions is difficult--and I am very very glad the breathability thing got crossed off, because that was going to be a big issue.

The conditions you are talking about are perfect for life, and so rarely happen. Some of the conditions actually aren't likely to happen without it (like the perfect combination of organic compounds in the soil without anything living around).

Now, I can see some of these conditions on a world in the Devonian Era type world, but that would require life.

Great question BTW! I know my answer might not be quite what you are looking for (it's a very detailed question that might better be broken up in some way?) but I hope it's a start!

• You underestimate the power of organics when it comes to growing crops. The elements you require are often present as a direct result of life. Then we disagree. They are molecules needed in all form of life replication that are very tricky to get build. No to mention not any self replicable form was build from raw amino acids through a laboratory experience. I also see dust bowl difficult in a new mountain area or a small island. Rocks can be converted into sands with a small steam hammer. Aug 28, 2016 at 17:09
• However earth life is pretty efficient at converting raw materials like metal ore into usable things. Aug 28, 2016 at 17:14
• I'm talking mainly about life at this level of complexity, from earth--it's evolved for certain conditions. I can see it being sustainable over a couple of years, however, it's a complex web that might collapse in 5-20 years, because the rest of the system isn't there. On a temporary basis, absolutely, you could do it. Just think it's going to take a lot of imported stuff, including on a micro-level. Aug 28, 2016 at 17:52
• it's a complex web that might collapse in 5-20 years, because the rest of the system isn't there. The changes introduced by humans would lead to this. But if there’s less than 100 humans, I think it would probably take centuries in a known way : without oxygen, the primary form of earth life that develop the most quickly is methanogen instead of cyanobacteria or algue that produce dioxygen. This would lead to a runaway greenhouse effect because the main gas produced by life would be methane Aug 28, 2016 at 18:05
• @user2284570 why would you try to build a self replicating organisms out of amino acids?, amino acids are not self replicating, RNA is self replicating AND the thing that makes stuff out of amino acids.
– John
Dec 28, 2020 at 16:15