Would it be possible for a living being the size of a planet to survive? [closed]

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

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.

Answer 5

"Help! My bone's a planet!"

1. Food: First question of creature design is "what does it eat?". A planet-sized organisms daily calorie needs would be so great that it would die before reaching a meal, therefore it has to be autotrophic or self-sustaining. Capturing solar energy can give just about enough energy to sustain a thin layer of flesh on the surface. Bottom layers will have extremely low metabolisms to compensate.
2. Respiration There's aerobic respiration (breathing air) which is impractical at such a size. Alternatively there's anaerobic options; some bacteria can respire iron or manganese instead of oxygen, meaning they can live off of weak electric currents. 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 layered deposits of copper and silicon. These materials exist in cells and can be deposited into layers similar to how teeth form. (However since it's anaerobic, oxygen will be toxic to it.)
3. 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 meaning the planet functions as the 'skeleton'. A skeleton like this will form naturally as bottom layers die and minerals accumulate.
4. Vacuum adaptation: Being planet-sized means its gravity can allow it to form its own protective atmosphere. However if it's small or is faced with strong solar winds it'll have to form an impermeable chitin or silica layer to keep itself pressurized together with melanin to block harmful solar radiation.
5. Motility: Due to its sheer size, moving through space by spewing gases will be slow and extremely inefficient. 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. A sort of tardigrade pod.

In conclusion: It's a plant/fungus/corral that doesn't really move and only the surface layer is alive. If you want the creature to be entirely organic it'll be no bigger than an asteroid at best.

Answer 6

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.

Answer 7

It could exist on a gas giant

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

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.

Answer 8

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.

Answer 9

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.