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Suppose the Earth was making one complete rotation around its polar axis every year. Will life develop, or will there be a constant flow of energy from the Sun through and around the Earth? Will a dynamical equilibrium of energy develop, with the only effect being that different parts of the Earth will have different temperatures? A dynamic balance of energy entering the Earth on one side and the same amount leaving on the other side, without using this energy for the development of life? So the only thing happening would be the passing of solar energy in a dynamical equilibrium state?

Put differently: Is the rotation of the Earth a necessary condition for the development of life?

Maybe in the depths of the ocean, there are sites wich are out of a thermodynamic equilibrium, but I think the main source of energy for developing life on the surface is the Sun.

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  • $\begingroup$ Related to may be the following: It seems that the Moon has been very important for life on Earth because it ensures the Earth's wobbling is restricted, see astrobio.net/news-exclusive/… . $\endgroup$
    – jim
    Commented Mar 26, 2016 at 21:33
  • $\begingroup$ I think that a good approach to answering this question would be to ask what the atmosphere would look like for such a planet. It's a good approach purely pragmatically, because people do run simulations of exoplanet atmospheres, and some/all of these are tidally-locked. In particular I think it's clear that the system does not equilibrate in any simple way because you get huge evaporation of water on the sunward side and corresponding condensation/freezing on the other. Whether you end up with the water frozen on the dark side I am not sure. $\endgroup$
    – user107153
    Commented Mar 26, 2016 at 21:44
  • $\begingroup$ The question is scientifically meaningless, at least for the moment. We don't have a physical model for "life", we have exactly one data point and physics can't say anything of importance about even that one data point. That's the domain of biology, which is just as scientific in its methodology, but works on a completely different level of description. $\endgroup$
    – CuriousOne
    Commented Mar 26, 2016 at 21:55
  • $\begingroup$ @CuriousOne: however we could as, for instance, whether a tidally-locked Earth would support conditions even slightly plausible for the sort of life we know about: would the daylight side boil (no), would there be liquid water (unclear) and so on. Those questions are in the domain of physics I think. $\endgroup$
    – user107153
    Commented Mar 26, 2016 at 23:27
  • $\begingroup$ @tbf: I have no idea where the "slightly plausible" criterion originates from. If you didn't know anything about biology, could you predict life from first principles? No. Can you predict all the places where you, as a human, will almost certainly die? That's basically all of the universe including the Greenland ice shelf. Is that habitable? For most of us even a managed central European forrest would be a certain death trap. There is almost nothing to eat there for humans and maybe half the mushrooms and berries that you may consider eating are poisonous. Now try that for Alpha Centauri! $\endgroup$
    – CuriousOne
    Commented Mar 27, 2016 at 0:38

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Exobiologists have suggested such scenarios. Remember, with an atmosphere there should be a region between the perpetual night side and the perpetual day side where there would be strong convection currents in the region. See for example http://astrobiology.com/2016/02/inner-edge-of-habitable-zone-for-synchronously-rotating-planets-around-low-mass-stars.html

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    $\begingroup$ This is where to look (not just this paper but ones like it). $\endgroup$
    – user107153
    Commented Mar 26, 2016 at 21:47
  • $\begingroup$ @tfb can't see any links? $\endgroup$
    – jim
    Commented Mar 26, 2016 at 22:25
  • $\begingroup$ I meant the paper you referenced as well as others like it: basically your answer is the right one! $\endgroup$
    – user107153
    Commented Mar 26, 2016 at 23:22
  • $\begingroup$ @Jim Don´t you think that in the twilight zone, as well as around the whole planet (wich you can see as a big sphere-shaped mass with an atmosphere, standing still (how to do that is another point) in equilibrium with a star, i.e. as the rotation of the planet has no angle with the plane described by the planet), the atmosphere is showing no movements (except the molecules of course, but they have no common movements, wich is wind or turbulence), no wind, turbulence. After a very long time, that is. The atmosphere is frozen, as it were. Pressure is the same allover, temperature and volume not $\endgroup$
    – Deschele Schilder
    Commented Mar 30, 2016 at 16:18
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Thermodynamically, a rotating planet and a non-rotating planet (with respect to their suns, of course) both behave the same way: they reach an equilibrium temperature and stay there, because as they heat up due to incoming radiation, their temperature causes them to radiate more energy away, until the outgoing radiation and incoming radiation balance. Of course, this is an average for the planet as a whole: for example, any given point on Mercury's surface is more or less in thermodynamic equilibrium with the half of the sky visible from that point, which will obviously be much higher temperature with the sun than without.

On any planet around a sun, rotating or not, there will be a gradient of temperature from one side to the other, and usually a polar gradient as well (that is, if the planet rotates and revolves along roughly the same plane). And within a certain range of distances from the sun, that gradient will include a zone where life as we know it is possible, given the right conditions. Those conditions include not only temperature but many others as well, primarily chemical in nature, such as the availability of water and the substances necessary for building organic molecules. Humans could live in the Twilight Zone of Mercury easily enough, for example, as long as we had access to food, water and air.

That's where you'd run into trouble. If Earth stopped spinning, for example, one aide of the Earth would heat up and the other would get very cold. Over time, the water that evaporated from the light side would snow onto the dark side and stay frozen there without any heat source to melt it. The weather between the two sides would be violent. Most life forms we are familiar with would not survive such an environment...

In the bright side, tardigrades would probably still make it.

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  • $\begingroup$ I refer to a planet with the same rotation and revolving plane. Isn´t it so that after a long time the pressure of the atmosphere evens out (no winds), but that the temperature varies over the atmosphere, and the volume density too? At the backside, where the atmosphere meets the ice, there is an equilibrium between the two states (with the liquid state maybe as an intermediary). The heat of the Sun is guided through this almost motionless atmosphere, and send into space. A condition for life is a system that is out of thermodynamical equilibrium, found on a planet with an atmosphere. $\endgroup$
    – Deschele Schilder
    Commented Mar 30, 2016 at 16:36
  • $\begingroup$ @descheleschilder when I talk about equilibrium here, I mean that the average temperature of the planet stays the same over time, with reasonably small variations. I did carelessly use "thermodynamic equilibrium" when it doesn't apply: what I meant is that as much energy leaves the planet as arrives on it and vice versa. Energy flows from hot sun->planet->cold space, and life exists as an engine extracting work from the process. But on the planetary scale the same process happens, life or no life, rotation or no rotation. $\endgroup$
    – Asher
    Commented Mar 31, 2016 at 14:08
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What matters for life depending on photosynthesis is that the entropy per unit energy in the photons that arrive from the sun is far less than the entropy per unit energy that resides in the environment. This fact would not change a lot if one side of the Earth were to always face the Sun. In that case the temperature would rise a lot, possibly making life impossible due to the molecules essential for life falling apart, but it would not get as hot as the surface of the Sun, so from only the entropic constraints, there would not be a problem. So, self replicating machines that work on solar energy could in theory still exist on such a planet.

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  • $\begingroup$ Do you say that self-replicating machines are the same as life? Is Natural life not a prerequisite for machines that simulate life? And is the fact that something is replicating a sign of life? $\endgroup$
    – Deschele Schilder
    Commented Mar 30, 2016 at 16:00

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