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 trough 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 ?