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I've been thinking what is the theoretical size limit of a living being? I asked myself this question:

Could a living being grow to the size of a planet?

This super massive organism would have its own gravity, but would gravity be dangerous for this organism? It would have to have a molten/solid iron core to produce a magnetosphere, but wouldn't that be deadly for this organism?

How would he obtain enough energy to support such a large body? What would your metabolism be? Which element would be your ideal block to build this body, silicon, carbon? Any other element capable of making long, complex chains of molecules?

How would it reproduce and could other organisms form on its surface?

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    $\begingroup$ sure, ask your mum. (sorry, you walked right into that one) $\endgroup$ Commented Jun 24 at 9:00
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    $\begingroup$ Planets are alive -- in several SciFi books already out there. $\endgroup$ Commented Jun 24 at 13:33
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    $\begingroup$ A planet IS a living being. $\endgroup$
    – MonkeyZeus
    Commented Jun 24 at 14:16

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A planet-spanning organism, or a planet-size one?

Definitions of "life" tend to include not just the ability to react to stimuli but also a metabolism and reproduction. A planet-sized lifeform would likely be excluded from that.

But the organism surrounding a planetary core, which you mention, could metabolize parts of the planet. Think of large fungus and have a lifeform like that in a symbiosis with some other lifeforms, infested by yet others as parasite. Basically a thin coating of this lifeform on the entire surface of the planets, with other life above and below.

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    $\begingroup$ but then again Fungi is more like a colony, I feel like OP asks more like a human/mammal/plant like organism, unless of course the fungus' colony evolves to be more like one whole organism, with each colony acts like certain organs/function or something like that. $\endgroup$ Commented Jun 24 at 3:58
  • $\begingroup$ The organism could move nutrients from one area to another, possibly over long distances. But not, say, one central nervous system. $\endgroup$
    – o.m.
    Commented Jun 24 at 4:15
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    $\begingroup$ @encryptoferia the gap between an organism and a colony is fairly fuzzy. The best definition I can think of is that an individual unit of a colony can survive away from the whole, but then I don't think a bee or ant would live very long by itself. $\endgroup$
    – Turksarama
    Commented Jun 24 at 4:48
  • $\begingroup$ Are you a single organism? How about the lactobacilli that let you digest food? How about the mitochondria that build your energy? $\endgroup$
    – fectin
    Commented Jun 24 at 18:09
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It Depends

If we are considering magic into the situation, all bets are off. It would depend on the magic. But as far as basic science is concerned, we have no way of conceiving of a carbon based life form existing at such a scale. Never mind how it got so big, simply existing in that state is impossible. The structures that make up living creatures are frankly delicate, and require a very specific balance. A creature of that size would be crushed under its own weight, even if the core isn't technically part of its body, and would overheat from its shear size.

For example, most attempts to make large creatures start running into more and more problems the bigger they get. The more they move, the more energy it requires and the more excess heat they produce. Now, a creature the size you suggest probably wouldn't be moving much, but approaching anything near 'planet' size would render it immobile by its own weight. The muscles or other mechanisms required to allow it to function would just make it heavier, and would fail in there task. The square cube law, on steroids.

Again, if there is magic involved, or some sort of pseudo science that can wave most of this away, maybe. It's a fun concept to think of a creature like Ego or a Genius Loci, but without a magic system to give us something to work from, all I have is physics, to which physics says resolutely NO.

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Gaia Hypothesis

The Gaia Hypothesis posits that the Earth is a self-regulating complex system, or as some would interpret it, alive. While we may not be able to comprehend what life means to several billion year old planet, discounting that as not life is more of a reflection of our narrow definition of life rather than absolute fact.

If you wanted a creature the size of a planet to actually do something that human-scale creature would broadly interpret as being alive, that would be more difficult. Earth is approximately 1022 times more massive than a human, so if you look the other direction for something 1022 times less massive than a human you end up on the attogram scale, which is proteins. Not even full cells, which are measured in the femtogram/picogram scale. Try to imagine a protein thinking about what life must be like for a human, and you've got approximately the right level of scale there.

So to sum up, it is entirely possible that there are living organisms on the planetary scale in our own universe, we may even be living on one, but it is unlikely that we would ever be able to interpret what life on that scale is like.

If you are writing a story specifically about planetary sized creatures, you could probably ignore anything smaller than a mountain, because that's about the same mass difference as a human to an ant, and most people don't notice things much smaller than that.

Also there are many Rogue planets in our own galaxy, which are not orbiting a star. So there is plenty of opportunity to explore the travels of a planetary sized creature traversing the galaxy, maybe passing through a solar system occasionally, but you'd be writing on the scale of eons, not days or years.

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    $\begingroup$ "there are many Rogue planets in our own solar system" - did you mean in our galaxy or universe? Because obviously they're not in our solar system - one might visit it briefly, but that would cause quite a fuss :) $\endgroup$
    – Syndic
    Commented Jun 24 at 7:05
  • $\begingroup$ Good catch, edited to say galaxy $\endgroup$
    – penguinaut
    Commented Jun 24 at 19:52
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I think so!

Cool question! This answer assumes water and carbon based life; I'm not an inorganic chemist and won't speak to other theories, but I'm sure some plausible ones exist!

An Earth-like planet

As other answers noted, one difficulty here is the pressure. According to this article, life has been shown to survive up to ~1 GPa for an extended period of time. But this is just Earth life which hasn't had much of a reason to adapt to extreme pressures. In the earth's mantle, the pressure ranges from 24 to 136 GPa. I don't think it's on face unreasonable to posit life that can adapt to these conditions; this paper showed that hydrocarbons are relatively stable under deep earth conditions.

The temperatures at these depths would be immense, from 500 - 4,200 K. However, as you can see from this phase diagram, there might be a sweet spot where at deep earth conditions water could exist as a supercritical fluid. enter image description here

Maybe some kind of metabolism could develop under these conditions, but the massive amount of kinetic energy the molecules have would lead to very different reactions and thermodynamics from what we experience.

Smaller and lighter "planets"

Another thing to note is that most planets consist largely of heavier elements like metals and silicon. A planet made mostly of water and proteins would have much less gravity, and therefore pressure and temperature in its mantle, than a normal planet of a corresponding size. Finally, a living "planet" could be pretty huge, but still not exactly planet-sized. Planets, by definition, have to be big enough for their gravity make themselves roughly spherical and to clear their orbit. A living planet wouldn't have to worry about gravity doing these things, as its complicated biological processes could take care of them instead! A death star sized organism, for example, would hardly experience enough pressure to prevent biological activity, even at its core.

Of course, as others have mentioned, the living tissue could just be a shell, equivalent to the Earth's crust. I don't think this means that the planet isn't an organism; plenty of organisms have large quantities of nonliving structural material in them. You wouldn't say coral isn't an organism. In fact, the interior part of the planet might be secreted by the planet itself, and structured in a particular way to interact with the planet's core.

Such a planet might indeed lack a magnetosphere, which you mentioned would be deadly. There are a few options here:

  • The planet has adapted to extreme radiation; perhaps all of its DNA is located way on the interior where the rays can't hit, or it's coated in some kind of hard carapace, or it just has extremely efficient DNA repair machinery. Radiation in space is only deadly to us because we don't put anymore energy than we need to into fighting it. In fact, maybe the planet uses this radiation to produce energy.
  • The planet generates a magnetosphere without a core. This one is a little more far fetched. Instead of a huge molten spinning core producing energy, maybe the planet maintains a gigantic magnetic dipole of some kind in a cool core, which it spins around extremely quickly. This might make sense for a smaller planet.

Planet-Scale Metabolism

Two ways a planet-sized organism could generate energy for itself are by using the heat energy in its core or the energy of its sun. One idea for using the geothermal energy is a large ocean of water between the living tissue and the mantle/core. The huge convection currents of the water could interface with some kind of biological generator to produce energy; they could spin huge rotors or just move "organs" past each other at high speeds. As the cellular level, kinetic energy can be converted to chemical energy in a variety of ways. See ATP synthase, which is basically a wheel that creates ATP, the cell's energy currency, just by spinning. You could have a kind of reverse actin-myosin contraction, where two fibers of proteins smush ATP together when they are forced past each other by the current. Or the convection current could push water through symporters, using the kinetic energy of the water to push protons or what-have-you the other way, setting up a chemical gradient like the one used to harvest energy in all Earth life. As for the sun's light, the planet could just use good old photosynthesis.

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    $\begingroup$ Hydro power is genius! However I don't think convection currents are the best way to collect it. The creature could perspire, let the water evaporate and form clouds, let it rain and then collect the energy through drain pipe-like organs. Cooling + thermal solar power! $\endgroup$ Commented Jun 24 at 18:55
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    $\begingroup$ If you have a big organism, you might just create some electric current around the equator for creating the magnetic field. $\endgroup$ Commented Jun 24 at 23:12
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"Help! My bone's a planet!"

  1. Food: First question of creature design is "what does it eat?". A planet sized organism would need an enormous amount of food, even with a low metabolism. We're talking at least a moonful daily food intake. As a space dwelling creature food is scarce due to the astronomical distances between things. So it HAS to be autotrophic. Photosynthesis and chemosynthesis are the two options available to us. You can think of it as solar and geothermal power respectively. Solar power is pretty straight forward; have it have a large flat surface area to capture sunlight. Geothermal is a bit more tricky (more on that later).
  2. Mobility: Second question is "how does it survive?". Well to survive it would need to reproduce and to do so it would need to move. Due to its sheer size, moving through space will be rather challenging. It can certainly spew gases to steer itself but it's slow, inefficient and it loses mass while doing so. Also as it moves away from a star it will lose its main food source. Alternatively it could shoot spores into space to take root in distant planets and assimilate them. It would need to have a large brain & eyes to aim and calculate the shot. It would also need a seed/egg capable of withstanding the vacuum of space without a protective atmosphere. So the seed would have to be in cryptobiosis to survive the trip. Even then it would take millions of years to reach its target and it isn't even guaranteed to survive the landing.
  3. Respiration How does it breathe? All creatures need to move electrons in order to derive energy to fuel their metabolism. There's aerobic respiration (breathing air) which is good but has limitations. Get too big and you won't have enough oxygen to function. You'd need to be all heart and lungs. Alternatively there's anaerobic options. Some bacteria can respire iron or manganese instead of oxygen. Look up "electric bacteria"! With this method you can have electricity course through your planetary creature while its "skin" would be made of impermeable solar panel-like scales composed of copper and silicon. Since the scales are inorganic they need to be shed and regenerated so the creature would need millions of mouths all over its surface to recycle them. The scales could flip over shed scales towards the nearest mouth like a conveyer belt.
  4. Skeleton: Gravity is a huge problem for a planet sized creature. It would literally collapse under its own weight. The obvious solution is to have it be "planet sized" as in being an organism that envelops an entire planet. So just the surface is organic and it can't be that deep. This opens some options as having a planet, potentially with a molten core, allows it to collect geothermal energy likely through a specialized root system near underwater vents. The organism could even create these vents itself by tunnelling, submerging them and absorbing the nutrients that way. Another benefit of having a planet as a skeleton is having a magnetosphere to protect itself from solar radiation.

Note that if you don't want a colonial organism you can always say it has magical anti gravity powers. However I'm not sure it'll have the energy necessary to feed its insides since the square cube law can be pretty harsh. Unless there's space-krill. Hopefully this was the least bit helpful.

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A planet sized creature would be spherical as its own weight would force it into that form. Such a being would need to a way to generate energy to keep itself alive. If that creature sustains itself through photosynthesis only its surface can produce energy meaning it wouldn't have enough energy to sustain itself. If the center of that creature is however nonorganic then this could. So this creature would be like pando but surrounding the whole world.

If however the entire planet should be an organism I don't think that is possible. The center of a planet experiences so much pressure that any organic compounds would be destroyed. Also it would need other means to generate energy to sustain itself. If at least the core can be nonorganic it could still be a creature covering the surface sustaining itself through geothermal energy.

The biggest creature I can see theoretically existing would exist in gas plants above its core. As photosynthesis would be made harder due to the gas blocking light and the depth of the gas ring yet another form of energy production would be needed like nuclear power. Such a creature would never evolve on its own and be an planet engulfing organic machine created for some purpose. So it might not qualify as a creature.

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  • $\begingroup$ you talked about the surface and the core, but what about the mantle? Would this creature have a mantle? $\endgroup$ Commented Jun 23 at 20:20
  • $\begingroup$ The creature could perhaps have deep root like structures that penetrate into the mantle of the planet. At a certain depth you will have high heat which the creature could use to sustain itself. This can be seen in deep bore projects. However they also show that even steel bore heads can't handle the conditions at these depths so I guess a creature won't get further and probably not as deep. $\endgroup$ Commented Jun 23 at 21:48
  • $\begingroup$ Philosophically it might just be a question of where the center is from. Did the creature form around an existing planet, or did it slowly grow, depositing one layer after another (ignoring the question where the required matter is from). The latter is what any tree does. And while technically most of the inner wood of a tree trunk is long dead, we still consider it part of the living being that is a tree. $\endgroup$
    – mlk
    Commented Jun 24 at 13:35
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It could exist on a gas giant

The red spot on jupiter is famously twice the width of the earth.

enter image description here

Some sort of massive plant like organism could exist on a jupiter like planet with long tubes connecting their body parts, floating around in the oceans and storms. Assuming they were fairly close to a star, they could get sunlight to grow, and get very big. They could have massive tendrils punching deep down to draw up resources and spread across the entire gas giant.

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It sounds like when you say "planet-sized," you're thinking about the mass of a planet. But could an organism be a similar volume as a planet without any considerable mass? I'm thinking of a sphere made of a lattice of "branches" of some sort, with empty space in between.

There would still be questions about how such an organism would sustain itself, but being crushed under its own wait or needing massive amounts of energy to sustain its metabolism would be much less of an issue if it was not very massive, but just "big" in the volumetric sense.

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We are part of the planet-sized organism Earth

Like cells are to us, we are to Earth. Slowly but steadily we are growing up to the point where we can launch off spores towards dead rocks in space to reproduce.

Planetary environments are so far away and so different that often such reproduction involves passing on very basic microbes, which will then adapt to the local environment and slowly grow towards fertility. Panspermia is currently beyond our scientific means to either prove or disprove it.

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