I know its questionable whether such an entity could exist, but if there were entities big enough to be mistaken for a planet, how could it reproduce? Would two collide and recombine into three or would it be in a nebula?

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    $\begingroup$ What ideas have you had so far? Organisms reproduce in loads of different ways. What problems arise for those different methods if they are planet sized? $\endgroup$ – Daron Aug 5 '20 at 18:34
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    $\begingroup$ Hi, folks. Please don't write answers to the question in comments; that's what the answer box is for. If you have requests for clarification or suggestions for improvements to the question, that's fine, but comments aren't for answers. $\endgroup$ – HDE 226868 Aug 6 '20 at 14:01
  • $\begingroup$ Why is it hard to see that a planet-sized entity would either reproduce asexually, or have appropriately-sized reproductive organs? $\endgroup$ – Robbie Goodwin Aug 8 '20 at 22:28
  • $\begingroup$ That had better not be serious $\endgroup$ – Joe Smith Aug 9 '20 at 0:19
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    $\begingroup$ What kind of biology does the organism have? How does it grow? How does it eat? Is it entirely alive, and if so how does it maintain any structure at it's core, where a normal planet has enormous pressure and heat? Is it hollow? Can it do anti-gravity or other "magic"? Just what do you mean by "mistaken for a planet", do you mean something like looking at a single photograph, or even after landing and doing some shallow sample drilling? $\endgroup$ – hyde Aug 9 '20 at 5:50

11 Answers 11


If the creatures can be mistaken for planets, then by duck typing, they must have the following properties:

  • They orbit stars,
  • They have sufficient mass to assume hydrostatic equilibrium (a nearly round shape)
  • They have "cleared the neighborhood" around their respective orbits.

This means that they cannot copulate by physical contact. The different orbits would necessitate prohibitive Δv's, and if they did come close they would collapse into a new, larger body.

Therefore, the only way for them to reproduce is to send gametes or spores into space. The absurd size of space would make gametes unfeasible. Check this video of Dr. Becky explaining why when galaxies collide, no star collisions take place! The probability of two gametes meeting within the lifetime of a galaxy would be practically zero.

So the only feasible option is spores. The planet would shoot out a spore with enough speed that it would exceed its star's escape velocity. That spore will travel the galaxy until it finds a nebula or proto-sun being formed. In the case of a nebula, its passage may precipitate star system formation.

Once in these places, the spore will grow into a planet through the usual process of accretion.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – HDE 226868 Aug 7 '20 at 19:37
  • $\begingroup$ @TCooper you do that and the planet wobbles... its orbit around its star will only change shape in a practically negligible way. $\endgroup$ – The Square-Cube Law Aug 7 '20 at 19:37
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    $\begingroup$ I don't think the question was saying that the organism would meet the scientific definition of a planet. The header only mentions size: "How would a planet-size entity reproduce?" $\endgroup$ – levininja Aug 7 '20 at 21:34
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    $\begingroup$ @levininja I agree that this answer makes assumptions not stated in the original question, but once something achieves the size of a planet, it pretty much has to have these properties simply because it's that size $\endgroup$ – Dan Aug 9 '20 at 14:58
  • $\begingroup$ @Dan that logic only really applies to the second requirement, e.g. assuming hudrostatic equilibrium. Achieving a certain size doesn't necessarily imply orbiting a star or clearing a neighborhood around an orbit. There's nothing in the question that would preclude the possibility of this organism travelling interstellarly. $\endgroup$ – levininja Aug 10 '20 at 17:32

A different take on Daron's answer:

They play god--guided evolution. All that junk in our DNA? Some of it is actually the genetic material of the planet-beings. The sentient star travelers are the gametes.

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    $\begingroup$ puts a whole new spin on the term "Mother earth", I like it! $\endgroup$ – Ruadhan Aug 6 '20 at 9:11
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    $\begingroup$ @LorenPechtel: Mind = Blown $\endgroup$ – Daron Aug 6 '20 at 14:25
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    $\begingroup$ @LorenPechtel: There is actually a species that does this already. en.wikipedia.org/wiki/Salp $\endgroup$ – Daron Aug 6 '20 at 14:28
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    $\begingroup$ @Daron we are a species to whom this is happening. Most not-actually-so-called-anymore "junk" DNA consists of self-replicating sequences that can be considered "viruses"(some are) that infected our genome and have high-jacked it for their own reproduction. In the case of humans, specifically, a whopping 10% of our genome is a sequence called Alu. In contrast, our own genes are only ~5% of the genome and the protein coding parts of those genes only ~%2. So this is not only plausible, it's actually happening today. $\endgroup$ – terdon Aug 6 '20 at 17:33
  • $\begingroup$ @terdon Is Alu a virus? $\endgroup$ – Daron Aug 6 '20 at 20:33

Hitch a ride on Spaceships.

Renan mentioneed these creatures cannot possibly make physical contact and live to tell the tale. The answer suggests spores, meaning much smaller particles that get shot off the surface to find other spores in the depths of space.


Space. Is big.[citation needed]

Solution: The reproduction strategy relies upon sentient life evolving on the planet, then inventing space travel to find other sentient life. The spores piggyback on the spaceships.

Of course this might take a very long time. But the creatures live a VERY long time.

  • $\begingroup$ I like this, makes a lot of sense. $\endgroup$ – The Square-Cube Law Aug 5 '20 at 21:43
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    $\begingroup$ @Renan Thanks Dad. $\endgroup$ – Daron Aug 5 '20 at 22:09
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    $\begingroup$ Space-faring sentient beings as pollinators? $\endgroup$ – Alexander Nied Aug 6 '20 at 3:12
  • $\begingroup$ Sentient beings must have evolved to be part of the organism, like mitochondria. $\endgroup$ – Shadows In Rain Aug 6 '20 at 7:32
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    $\begingroup$ @ShadowsInRain The natural question then becomes: Can sentient life develop on planets that are not alive? $\endgroup$ – Daron Aug 6 '20 at 8:14

Homing sperm shuttles

Planetids have a mating cycle. For ten thousand years they listen on the radio waves for mating songs, and send out enormous rocket-propelled winged gametes in the direction of whichever nearby star has the planetid who sings the most beautifully.

Then they stop, and sing their own song for ten thousand years, while another planetid's love dart follows their voice across the void, through the solar system, into their atmosphere and glides right into the most intimate organ hidden delicately under their radio antennae.

A mere dozen years later, a hundred rocket-eggs are launched into the void towards any nearby stars that have no song, each the size of a large building. A few of the larvae that hatch from these eggs ten thousand years later may be lucky enough to find enough light and tasty tasty asteroids to grow into planetids themselves. By that time, their mother has already started making love darts again.

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    $\begingroup$ Any reason why they have to alternate between roles? Why not do both simultaneously? $\endgroup$ – BBeast Aug 7 '20 at 0:43


Some bacteria and other microbes reproduce in this fashion when they don't require a mate. There are many forms of Asexal reproduction that would work. Self-cloning through pathogenesis, cellular division. When there's no chance of a mate or finding one, using its own genetic material to reproduce would be the next logical evolutionary trait.

Mating "grounds".

Now, this would be a simple area of space where the Spheres would come and deliver their genetic material into space, covering several hundred AUs if they have a reasonable population and go on their way every few thousand years. Male and females would do this and sooner or later, the genetics would combine. the newly forming planets would no doubt go rogue at that got bigger, sending them on their new path.

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    $\begingroup$ You probably meant "asexual". $\endgroup$ – RedSonja Aug 6 '20 at 8:18
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    $\begingroup$ @RedSonja yep, sorry, I missed my typo :) $\endgroup$ – SKKennell Aug 6 '20 at 8:56
  • $\begingroup$ Division is clever ! $\endgroup$ – Fattie Aug 6 '20 at 11:29
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    $\begingroup$ I was going to suggest this, you could do some really creative things with division like having the "mother" die when it splits and result in multiple siblings $\endgroup$ – GammaGames Aug 6 '20 at 14:23
  • $\begingroup$ On your latter suggestion, I think it might be extremely generous with the propulsion these planet-creatures have. Unless you have reactionless drives, the planet-creature would have to expel most of its mass to change its orbit to rendezvous with other planet-creatures. But a plausible variation would be to have a designated area where the planet-creatures all send lighter and more mobile gametes, which combine then fly off to find good territory to grow. $\endgroup$ – BBeast Aug 7 '20 at 0:41

Intelligent Design

Perhaps these sentient planets are intelligent enough to design their own descendants. They don't need others of their kind to reproduce.

Imagine that they can, through some sort of delicate telekinesis (or other way) design their own children at the molecular level. Once the child (or children) is developed enough to live on their own, they are released into space. This could be done via shooting them into space, extending a tentacle into space, telekinesis, teleportation, or simply by the original mass exploding and releasing the children.

There might also be a way of a sort of sexual form of reproduction: perhaps two or more of these gigantic creatures consult with one another about the various possibilities. They don't need to exchange matter, just information.

I don't think you'd want a grouping of more than three, because who wants their children designed by a committee? :)


First I'll write a hand waving answer, then the most probable answer which others have also discussed.

The hand waving way. The planet has some kind of internal clock, and some very advanced knowledge of quantum gravity. The reproduction process means writing some information that tells the children planets how to grow and how to reproduce themselves. The information is written in the form of global symmetries or topology of some "seeds", lumps of matter, of size and composition determined by the needs of the story.

Suppose that growing of the planet is done by shooting wormholes into space and accreting mass from stars, gaseous planets or just asteroids. For that, one needs energy which is obtained by all the means that are available to us, humans, plus some.

When the planet reaches a certain size, the clock tells it it's time to reproduce. In its case, that process has a few stages:

  • Shoot a wormhole to bring hot plasma from a star.
  • Bring the plasma to some fusion chamber inside the planet core.
  • Use the energy to create a few small black holes and keep funneling matter from the star to keep them gaining mass until they are big enough to not evaporate immediately
  • Write the genetic information on the black holes by just sending the data (the seed) beyond their event horizon. The information needs to be symmetry, or topology protected so that it's not radiated back by the black holes.
  • Shoot a new set of worm holes and connect each black hole to its own worm hole.
  • Use the energy left to push the black holes through their worm holes, effectively turning them inside out. Such process should normally generate just Hawking radiation, but, you can hand wave some Higgs wormholes, which do not allow for that to happen, and would allow for a percentage of the black hole mass to turn into massive particles.
  • The particle cloud contains the genetic information encoded in some global symmetry properties that the original black hole had. Over a few million years it condenses to some planet, and this is where it turns out it was a good idea to let it be captured by some star.
  • The new planet just grows to become a similar life for as the previous planet, provided no disaster (asteroid or supernova) happens before that.
  • The old planet is a husk now, which looks just like some cosmic dust cloud, but it still contains its genetic code. Provided enough dust floats its way, it may condense again in a live planet, which is why these life forms are so dangerous.

Closer to the science we know. A planet size could simply send something akin to a spaceship to the nearest useful planet and write its genetic code, or whatever its equivalent is on that other planet.

The ship would be some kind of Von Neumann machine which would contain all the genetic information from the old planet as modules. Each module would consume matter from the host to create more units according to the information. Once enough modules have been created, you could have them merge into one entity. The whole process will probably take billions of years, but, at the end, you would have a new grown planet similar to the old one, but different because of different materials that were on the original planet.

This is more or less how we want to duplicate Earth using other planets. We call that process terraforming. Terraforming may take a lot of time, if our modules are just some bacteria surviving the crash of our colony ships.


What is life?

If life is “reaching a certain complexity level” + “being able to reproduce” then once machines reach certain complexity level, and can reproduce in any way. They can be considered life. We are made of thing found in the same universe after all, and must use those things to survive and reproduce.

Other answer suggested a very interesting possibility. We, humans, have a fixed reproductory mechanic, dictated by evolution of known life.

If you can make the concession of accept a planet size machine as something complex enough to be, in a practical sense, a life form, then you gain some fancy possibilities.

As the other answer suggested, this life form doesn’t have to passively accept a fixed reproductory mechanic, it can use its "intelligence" to design an improved version of itself, and that improved version will in time design and improved version of itself, and the loop goes on until universe ends.

If you want your entities to look biological, there is no problem. A complex enough entity, no matter its ancient history, will probably be indistinguishably from biological beings. Elements like carbon are very abundant in the universe. Any "intelligence" will notice this sooner or later, and may choose to design their children to be made of these easy to mine elements. They may prefer a good compromise of repairability plus resistance versus a higher resistance.

Examples of from electronics to biology in fiction: we have Battlestar Galactica (the second series, not the older one) that can be watched on Amazon Video. The worldbuilding has many holes, and the process of transcending to biology isn’t explained, obviously. My point is that the idea isn’t new.

Is ADN the first and definitive space colonizing entity?

Also suggested in another two answers. A real hypothesis, maybe theory, that you can find in the internet. While some bet to the primordial soup (Miller Experiment), others bet on panspermia. But, how panspermia actually happens?

The simpler life forms are the ones that have the higher chances of surviving a space odyssey. Consider extremophiles.

They don’t even require to be radiation resistant (but that would be welcome too). They may use structures present in some asteroids to guard themselves. They can continue metabolizing using things found in the asteroid, gases and other bacteria.

The loop would be like this:

  • Life evolves in an Earth clone planet.
  • Asteroid or anything else destroys that planet.
  • Some fragments carry life out of that solar system.
  • That life survives long enough to colonize other planet.
  • 0,00005% chance of evolving to reach civilization.
  • 0,000000000005% of civilization reach required level to not leave it to chance anymore, then they develop a super bacteria that will improve previous step, on next loop iteration, in a 200% or 400% (and accumulates on previous iterations, it doesn't start at original value).
  • Repeat.

And my favorite fantasy: after millions of iterations, a civilization finally found a way to preserve not only life, but to ensure that life will reach civilization. And finally, a future civilization finds a way to preserve knowledge, or ensure the knowledge that matters will quickly be rediscovered.

While you may argue that "knowledge that matters" implies bias, and that may be better to not reach that point. My answer is not about if it is good or not that those people will do that, but that there is a high chance that they will do that. Let's hope it's a good bias, if such thing exists.


Via explosion.

The planet-sized being has a structure that allows reactions which result in a two or more component explosive. It could also be nuclear fission or fusion. The components are accumulated until the being is ripe for reproduction.

Before reproduction, i.e. explosion, the internals arrange such that children get everything needed and that children are shot onto orbits favorable for growth.

Children grow through collisions, which keep its constituent "cells" unharmed.

The energy for some processes can come from a central star, but the planet beings could also form a system without an overly massive central member.

  • $\begingroup$ This was the plot of a Swamp Thing comic by Alan Moore, titled "Loving the Alien" $\endgroup$ – Shawn V. Wilson Aug 7 '20 at 21:53
  • $\begingroup$ I just looked it up and found two Wikipedia articles: one, two. It is not planet-sized. It does not reproduce via explosion. I don't see the parallel. Oh, I see, there is this one issue #60. $\endgroup$ – Roland Puntaier Aug 8 '20 at 8:48
  • $\begingroup$ There is no known mechanism for a planet to explode. It's not massive enough for runaway carbon fusion nor for its gravity to overcome its electron degeneracy pressure. What mechanism for explosion do you propose? $\endgroup$ – cowlinator Sep 3 '20 at 23:38
  • $\begingroup$ By accumulation over time all thinkable energies (and pressures) can be created for a short time. Long -> short. Global -> local. There needs to be intelligence on the fantasy planets (Maxwell Daemons), that accumulate inner energy from all possible outer sources over a long time to convert it to outer energy in a short time (explosion). Earth has shown to be smarter than other planets, which led to tiny explosions, probes leaving the planet. The cycle could be a a short one, keeping the intelligence, or a long one, destroying intelligence and accumulating that, too. $\endgroup$ – Roland Puntaier Sep 5 '20 at 9:44

I think that the answer is dependent on how the planet sized creature grows. If the creature began its life cycle as a clonal colony like Pando, and matured as it encompassed the entire biosphere of its host world, it could be considered to be a planet sized organism. If the organism's seeds or spores were sufficiently able to hibernate and were sufficiently impact and radiation resistant, they could migrate across solar systems via panspermia. Throw in a bit of the trees from Avatar's Pandora, or the chemical networks in The Uplift War, along with the ability to observe nearby asteroids and you have a plant based intelligence that could gather reproductive material in regions likely to experience a meteorite.


Certain plants can live for years, flower once, then die. Yucca and Hens & Chicks are two examples.

Certain fungi have far more than two sexes. Not all are required to form a new organism.

Lots of bacteria reproduce by fission but also will take in dna from other bacteria.

Lots of fungi and insects have radically different appearances at different stages.

Some will alternate diploid and haploid generations.

Suppose your planet-critter reproduces once.

Scenario: It shatters into a multitude of comet cores that are scattered at faster than the local star's escape velocity. The comets move into the Oort cloud. Stage two reproduction occurs there, using natural comets as food and reaction mass. The comet critter sets course to another star, eventually colliding with a suitable planet there, and converting it to a planet critter.

30 km/s is .0001c So a 10 light year journey is 100,000 years.


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