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So let's say that I have a world with a moon, you know kind of like we do on Earth. But with life on the planet, is it possible to find life on the moon? In the artifexian video on habitable moons, he suggests that they could only exist as moons of a gas giant, but gas giants are hard to justify life on.

It is possible for life to appear on a terrestrial planets moon? And if it is, is it possible for lifeforms to be biologically similar to life on the home planet?

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It is possible for life to develop on the moon of a tellurian planet, if the moon has an atmosphere, there is liquid water, and sufficient time for it to evolve.

Most persons looking at Earth's Moon will observe most of these life-supporting condition are conspicuously absent. Although sufficient time has passed, but, unfortunately, this by itself isn't enough. Therefore, it seems probable that to have a habitable moon those conditions must be present.

Such a moon must be considerably more massive than our Moon. The atmosphere needs a stronger gravity than the Moon's to prevent the losing atmospheric gases in uncomfortably short periods of time. This makes our habitable moon closer to a planet (in terms of mass, size, surface conditions, etc). So this looks more like a binary planet system than an Earth-Moon system of planet and a moon.

Short answer: to have a habitable moon of a habitable planet, the habitable must itself be more like a habitable planet.

As for biological similarity between the lifeforms on the habitable planet and its habitable moon. There will be a high probability of an exchange of meteors between the two bodies. Microbial life has an excellent chance of passing from planet to moon and from moon to planet.

Lifeforms on both worlds will most likely share a common DNA, similar biochemistry, and microbial organisms that are similar. However, evolutionary conditions will shape the majority of lifeforms on either world. While convergent evolution will undoubtedly produce equivalent morphologies or body forms, most lifeforms will be adapted to their own environments.

Expect some lifeforms in common, at least, in terms of appearance and structure, but many will be shaped by the specific conditions of their environments. Chance and natural selection will guarantee biological success. This doesn't ensure the two worlds will have common lifeforms except at the level of their biochemistry.

NOTE: This answer chose the adjective 'tellurian' for Earthlike as in Earthlike planets.

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Yes, but not (likely) as we know it. But we have to define "habitable"

If we allow things like terraforming, easy yes across the board. Life from the planet would have been sent up there

If we mean earth-like life, that would require somewhere near earth-like mass. To have this orbiting earth would make a binary planet system, not a planet and moon. You could waive this with a larger terrestrial planet, and/or shrink the moon a little bit. Theoretically yes.

However, if you settled for something a bit more bare-bones necessity, Europa and Titan could both harbour life, and are roughly comparable in size to our own Moon. Through a simple substitution, replace our boring old Moon with Europa, and you'll find it is 100% reasonable for a planet and moon to have intelligent life as advanced as octopi.

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Without doing the math or checking up on it, the answer is that it skirts the possible.

The moon has to be big enough to hold an atmosphere, have roughly 24 hour rotation, and small enough to be held by the planet

The planet need to be massive enough to hold a decent size planet, but not too thick an atmosphere, and likewise have a roughly 24 hour rotation.

The size for a terrestrial planet of this size is 10 earth masses (but you'd probably not go above 3). This means that the max a moon can be is something like .03 to .1 earth masses. The question then is, is this big enough to hold an atmosphere and generate a em field? It might be, probably small/weak one so if it is possible it really skirts it and it lives in the territory of technically possible, but likely none exist.

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    $\begingroup$ " have roughly 24 hour rotation" - why would that be a requirement? $\endgroup$ – Mołot Oct 1 '16 at 17:54
  • $\begingroup$ Cuz that spin is part of what causes the magnetic field. And there are some biological stuff that get messed up if it's too slow from what I've heard, and faster results in stronger magnetic field which is one of those situations where the just because a little is good, a more isn't better. Also there are things that it effects that affect things that reduce or improve habitability. There are other effects such as gravity, pressure, and wind that would be just impossible to live in. I forget where I saw but 24 is about the median betwee 22 and 30 hours in a day for a habitable world. $\endgroup$ – Durakken Oct 1 '16 at 18:37
  • $\begingroup$ Oh also... planets and Moons of this nature woukd be tidally locked and since the moon needs to spin faster for the same magnetic field strength, it means that the moon would get lots more radiation as time went on and likely would die first, but there is also the problem of if the magnetic fields were too strong they'd be killing creatures on the other body. $\endgroup$ – Durakken Oct 1 '16 at 18:44
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Life developed on the satellite? Low-realistic. Moon is too small, it does not have core, does not have magnetic field to protect carbon-based life from radiation, does not have gravitation strong enough to prevent hydrogen from escaping into outer space. Probably not.

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  • $\begingroup$ Earth doesn't have gravity strong enough to stop hydrogen escaping, the oxygen on Earth when it formed reacted with the hydrogen to form water, and kept it in that form $\endgroup$ – Giacomo Oct 2 '16 at 0:47
  • $\begingroup$ Moon does not have atmosphere because of low gravity and have no magnetic field. Greater gravity helps jail more hydrogen. $\endgroup$ – wandalen Oct 2 '16 at 11:07

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