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My titled question was, can a planet without one or more moons be habitable?

In more depth, how would that planet be affected overall, without the moon to affect the tides and without providing light during night?

How badly would oceans/seas be affected? could life be sustainable within them (fish, algae, etc.)?

Lastly, would the atmosphere be majorly affected, much stronger or weaker than would otherwise be possible on said planet?

I already know that the heat reflected by the moon is to insignificant to truly affect the heating of the planet.

From a writer's perspective, a planet like this is an interesting concept because to humans it would be a foreign idea; stars would be that much more visible at night as well as it would be very, very dark.

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    $\begingroup$ There's no reason why it wouldn't be habitable, but I've heard a theory that it might not evolve life of its own without tidal regions. No sources right now, so only a comment. $\endgroup$ – Separatrix Mar 14 '16 at 21:11
  • $\begingroup$ I don't see why not. I mean, tidepools aren't absolutely necessary for life, you could just as easily have stagnant shallow pools on the edge of oceans, instead of said pools. $\endgroup$ – Xandar The Zenon Mar 14 '16 at 22:53
  • $\begingroup$ @Split91 : just one thing without massive moon : no metal ore near surface, so no metal based tools. All tools would need to be stone based $\endgroup$ – user2284570 Aug 28 '16 at 11:25
  • $\begingroup$ @user2284570 I've never heard this before. Do you have any sources for that claim? $\endgroup$ – ThomasW Sep 16 '16 at 5:40
  • $\begingroup$ @ThomasW : I recognize I forgot the source. It was on ʀᴍᴄ24 tv channel. They explained that without plate tectonics, earth wouldn’t have usable metal ore. $\endgroup$ – user2284570 Sep 16 '16 at 20:44
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There's no reason to assume that lacking a moon would cause a planet to be less inhabitable. Animals that rely on the phases of the moon for parts of their lifecycle (i.e animals that breed only during a full moon) wouldn't do so well on such a planet, but diurnal animals probably wouldn't be bothered at all.

Oceans and seas would not be affected at all, really. Plants and animals that live in tidal zones on Earth might not be able to live easily there, but other plants and animals would be just fine. There would still be currents and tides (though maybe not as strong) due to the rotation of the planet and the gravity of its star.

The atmosphere wouldn't be affected much, either. Apparently the sun has a greater tidal force on the Earth's atmosphere than the moon does. As for oxygen production by plants, it might be beneficial for them not to have to deal with moonlight - plants avoid moonlight in order to avoid disruption of their circadian rhythms.

One thing to note is that without a large moon, there is nothing to stabilize a planet's axial tilt. Without the moon, the Earth's axial tilt could have varied by as much as 85 degrees. By itself this is not enough to make a planet uninhabitable—the tilt would only vary over the course of thousands or millions of years. This is, however, quick enough that it could cause significant issues for the development of life on a planet. Life took quite a long time to advance beyond the most basic forms which would not have been able to survive the plunge into a perpetual day/night that a significant axial tilt could have caused.

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  • $\begingroup$ While life occupying small niche environments may not adapt by virtue of lacking a continuous biome to move through, there should be no problem for most life to slowly move with the axial tilt - while significant on geological scales, it is largely irrelevant to life unless constrained. Generally speaking (excluding aforementioned niche dwellers), sea life will have no issues, land would only have issues if landmasses were small and they were unable to cross between them, and of course life around deep-sea vents don't care about the surface at all. $\endgroup$ – pluckedkiwi May 13 '16 at 13:11
  • $\begingroup$ The solar tide in the atmosphere exceed those of the moon because the diurnal heating is classed as a solar tide. The solar gravitational tide (which affects the oceans as does the moon's gravitational tide) is about half that of the moon. Thus the significant difference between spring tides and neap tides. If you just need tidal pools you can dispense with the moon. $\endgroup$ – dmckee May 14 '16 at 4:15
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    $\begingroup$ "Animals that rely on the phases of the moon for parts of their lifecycle (i.e animals that breed only during a full moon) wouldn't do so well on such a planet," More to the point, animals which depend on lunar cycles simply would not evolve in the first place. $\endgroup$ – WhatRoughBeast Jul 7 '17 at 1:19
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This is a very interesting case. A lot of factors are in play here. In the absence of a moon, all other factors would become more important than they would have, if a moon was present.

  • Tides in the oceans can still be caused due to gravity of the parent star. If the planet is situated in the inner side of the goldilock zone, the gravitational effect of the parent star would be much greater than it would be, if the planet was situated on the outer edge of the habitable zone. So yes, if the planet is located near the inner edge of the goldilock zone, the parent star can create the tidal effect.

  • However, being located near the inner edge of the habitable zone also means more radiation and much stronger magnetic storms to deal with, which would easily rip off any protective shielding around the planet (aka ozone layer) while also blowing away its atmosphere. The result would be a dead, desert planet.

  • Or you may have a planet which is located where Earth is located, and have a Jupiter-like massive planet at a distance as Mars is to Earth. I haven't researched how that would fare, but I think you could get some weak tides from the gravitational tugging between that Jupiter and the parent star. This would be better as you would not have that solar storms devastation to deal with and your planet can retain its atmosphere and an ozone-like layer.

  • Yet another possibility arises if you have rings around your planet. These rings could (at least in theory), provide you with some tidal effects.

Note that all the above possibilities are discussed for the sole sake of having tides in the oceans. There is no reason why primitive life cannot develop on a planet with no moon. However having life on a planet and having complex, intelligent life on the planet are two vastly different things.

You have to have an ozone layer around your planet if you want to shield it from the deadly high energy radiation. In the absence of this layer, you can have primitive life in the oceans but you cannot have land-based life forms.

For intelligent life, you also have to have a gravitational shield against meteors and comets. Here on Earth, that shield is provided by Jupiter. You need to have some sort of protective shielding against bombardment of massive meteorites. Having a Jupiter in the vicinity is the best way to provide that.

For intelligent life, you would also require a strong magnetic field. Otherwise the solar storms would rampage your planet and create mass extinctions over and again, effectively stopping any evolution directed towards more complex life forms.

Conclusion

Yes, you can have life on a planet without a moon. But having intelligent life on a planet without a moon is a complex matter. In theory you can have such a planet. But a lot of things would have to be just right for it, and nothing wrong. There is a reason why scientists have not found signs for any life (let alone intelligent life) anywhere other than Earth, so far.

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  • $\begingroup$ Just one thing without massive moon : no metal ore near surface, so no metal based tools. All tools would need to be stone based. $\endgroup$ – user2284570 Aug 28 '16 at 11:27
  • $\begingroup$ @user2284570: What? Why that? $\endgroup$ – Youstay Igo Aug 28 '16 at 15:27
  • $\begingroup$ Because all there wouldn’t be plate tectonics, or at least no as much as we know on earth. The only metal concentrations that would exists would be in lava : that is, too deep under surface. $\endgroup$ – user2284570 Aug 28 '16 at 15:31
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    $\begingroup$ @user2284570: Neh. The plate tectonics on Earth are a result of the primordial heat reserve of the newborn planet. It has nothing to do with our moon. $\endgroup$ – Youstay Igo Aug 28 '16 at 18:09
  • $\begingroup$ Ok I might be wrong. I recall watching a documentary explaining mankind couldn’t build metal based tools and weapons if our earth didn’t had the moon. And the reason was something like that. $\endgroup$ – user2284570 Aug 28 '16 at 19:00
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My titled question was, can a planet without one or more moons be habitable?

Habitable? Totally. Would life evolved in it? Probably. Always assuming that other conditions adjust to it…

In more depth, how would that planet be affected overall, without the moon to affect the tides

Just life marine without tides XD.

and without providing light during night?

Light is useful, but it is not necessary for life. It's highly probable that the first life on Earth emerged without photosynthesis by using chemical and thermal energy.

How badly would oceans/seas be affected? could life be sustainable within them (fish, algae, etc.)?

Not badly, probably would be different. Also, there are other phenomena that could make water currents (winds, thermal, temperature gradient, etc.).

Lastly, would the atmosphere be majorly affected, much stronger or weaker >than would otherwise be possible on said planet?

The Moon does have an affect on the atmosphere, but it is not vital for life. The magnetic field has a more important effect.

I already know that the heat reflected by the moon is to insignificant to truly affect the heating of the planet.

Yup. Probably it has a more important effect in protecting from meteoroids.

From a writer's perspective, a planet like this is an interesting concept because to humans it would be a foreign idea; stars would be that much more visible at night as well as it would be very, very dark

It would affect, not just aesthetically, but also the culture of the inhabitants. Just think:

  • How many poems and literature refers to the Moon.
  • The Moon it is a small step into space exploration and astronomy. Without it, probably those things would be more slow.
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  • $\begingroup$ Just one thing without massive moon : no metal ore near surface, so no metal based tools. All tools would need to be stone based. $\endgroup$ – user2284570 Aug 28 '16 at 11:24
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This question is similar to, but not a duplicate of, two earlier Worldbuilding Stack Exchange questions,

The title of the second question gives a very important effect that no one has mentioned yet on this question. Without a big moon days would be shorter. According to this post from ScienceBlogs, "The top 5 things we'd miss if we didn't have the moon",

A day on Earth would be much, much shorter; only about 6-to-8 hours, meaning there’d be between about 1,100-1,400 days in a year!

Our 24-hour-days may seem like they don’t change from one year to the next. In reality, the change is so tiny that it took centuries to perceive, but the Earth’s rotation slows down ever so slightly over time, thanks to the tidal friction provided by the Moon. The slow-down is very, very slow (on the order of microseconds-per-year), but over millions and even billions of years, it adds up!

[…]

But in the meanwhile, we can use what we know to extrapolate backwards in time, and we find that in order to get a 24 hour day today, the Earth had to have been spinning much faster in the past: about three-to-four times as fast more than four billion years ago! If we didn’t have a Moon — if we never had our Moon — the day would be much, much shorter than it is today, and our planet would have a larger equatorial bulge, much more flattened poles, and over 1,000 days in a year!

I think at minimum, there would be hellishly high winds.

The same post also cites as the most important effect the stabilization of the axial tilt, already mentioned in Rob Watts' answer:

Thanks to our Moon, our axis stays tilted between 23 and 26 degrees over time, even over hundreds of millions of years! But without our Moon, there would be nothing preventing catastrophic shifts in our rotational axis. It’s probable that sometimes, we’d be like the planet Mercury, orbiting in the same plane as our rotation, and having practically no seasons due to our axial tilt. At other times, we’d possibly be as extreme as Uranus, rotating on our side like a barrel, having the most extreme seasons imaginable!

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Would there be life?

Amino acids are small, highly reactive molecules composed of 20 to 30 HCNO atoms. When amino acids link together in strings they form proteins. Proteins govern chemical reaction rates and form the structural material for cell parts.

Most importantly, they can form into microspheres when heated, which serves to separate chemical reactions and processes. The problem is that with the vastness of the Earth's oceans it is statistically very improbable that these early proteins would ever link up. The solution is that the huge tides from the Moon produced inland tidal pools, which would fill and evaporate on a regular basis to produce high concentrations of amino acids, who then linked themselves into macromolecules.

As we can see from this lecture life may have evolved specifically in the tidal pools caused by having a large moon. There are other theories that state this could have happened near thermal vents, so you could have life, we don't know.

There's a secondary question here:

Would life have moved on to land without intertidal zones?

Possibly not - need sources.

Is a lifeless planet habitable?

That depends on how you're defining habitable, but in short: Not to us. A planet without plant life similar to ours is unlikely to have the right balance of oxygen/nitrogen/etc in the atmosphere for us to be able to just turn up and breathe. You're going to have to wait a few thousand (maybe million) years after seeding it with photosynthesising plants to give an atmosphere mammals could survive in.

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  • $\begingroup$ and if we exclude natural breathing as requirement , Would we be able to walk on it with just earth clothes ? The requirement would be to carry oxygen bottles. $\endgroup$ – user2284570 Aug 28 '16 at 11:32
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The importance of the moon in stabilizing the Earth's axis (obliquity) and hence its climate has been grossly overrated in the past. More recent research indicates (and calculates, models) that the stabilizing effect of the moon is actually rather small, that the stabilizing gyroscopic effect of Earth's rotation itself is much greater, and that any possible significant climate changes resulting from the absence of a large moon would rather be in the course of hundreds of millions of years, i.e. longer than present climate changes on Earth by other cause. See for instance: Obliquity variations of a moonless Earth (2011); by Barnes, Lissauer, Chambers; http://www.sciencedirect.com/science/article/pii/S0019103511004064?via%3Dihub

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