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There has evolved a species of turtles large enough to maintain an atmosphere of their own and travel interstellar space. We have already discovered how they don't collapse under their own gravity, but we don't know how they reproduce.

Somehow, while still being oxygen breathers, these turtles have grown large enough to carry an atmosphere with them, and enough vegetation growing on them to maintain the oxygen levels in that atmosphere that they became known as the world turtles. Each one carrying a self contained ecosystem of which they are a part.

Turtles however are egg laying species, not normally known for the tender care of their young, but the young won't be large enough to maintain their own atmosphere when they hatch.

Normally turtles go back to the beach where they were born, but planetary landing by thousands of world turtles laying eggs very quickly becomes destructive, so while they still go back to the same system that they initially evolved in, the planet itself is long since gone.

How do young world turtles get an atmosphere? Even if the mother raises them, they'll need to top up atmosphere as they grow.

If you're committing a mother to rearing her children, how big does she need to be and how many young could she support?

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  • $\begingroup$ "planetary landing by thousands of world turtles" waaaait you want to have planets and world turtles in one universe?! $\endgroup$ – Mołot Nov 2 '16 at 9:22
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    $\begingroup$ @Mołot, I'll work on how to stop all the water dripping off the bottom of a spherical world another time. $\endgroup$ – Separatrix Nov 2 '16 at 9:24
  • $\begingroup$ Would you be willing to accept that these Turtles are just an immortal creature? If you never die, you don't have to make babies (or at least take as long as you want raising one) $\endgroup$ – Tezra Nov 2 '16 at 13:47
  • $\begingroup$ @Tezra, even "immortal until killed" creatures suffer losses to the population that need to be replaced, though for a creature of this size, a reproductive cycle measured in thousands of years wouldn't be unreasonable $\endgroup$ – Separatrix Nov 2 '16 at 13:57
  • $\begingroup$ The obvious answer is Naritivium. $\endgroup$ – Durakken Nov 2 '16 at 17:55
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I'd suggest that their babies live on them as part of their ecosystem. They slowly grow until they have a certain size. There could be lots of those baby-turtles. At some time some mechanism should decide which turtle should become an adult. The whole process will take very long therefore I assume, that this mechanism should make sure, that only a "perfect" turtle can be chosen.

I'm assuming, the adult turtle can control it's movement in space somehow. That way it can collect the necessary material and move it in an orbit around itself. First it'll take the material for the body, say from asteroids. The second step would be creating a moon out of that material, by pooling everything together. That moon should have an escape velocity of at least $5 \frac{km}{s}$ to have a stable oxygen atmosphere.

The third step is searching for an gas cloud where it can get oxygen. After that you have to get your baby on that moon. Most likely there is some organ, which does exactly that. Maybe some kind of catapult or something. Maybe it needs several tries. The turtle gets some time to change the moon to become part of itself. It then becomes a real space turtle.

The last thing is the ecosystem. Most likely there are already bacteria-like creatures on it from the landing of the baby. Some should still be alive and start an ecosystem.

After that the now grown turtle leaves it's parent. Possible it will further grow.

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  • $\begingroup$ Turtles swim, world turtles swim in space. Planetary engineer world turtles definitely has a story all of its own in there somewhere $\endgroup$ – Separatrix Nov 2 '16 at 15:14
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The main problem is not strictly in reproduction itself. Bigger problem is growth, and solution to it will determine solution to reproduction.

Baby turtle is small. Grown up turtle is big. Matter must come from somewhere. Obviously, it can't be parents, because that would require their death and cannibalism. Pretty grim. To avoid that, you will need to create food sources big enough to feet new world turtles. Ecosystems larger than worlds. In such ecosystem there well might be a "beach", not literal one of course, but a place safe enough to leave eggs.

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    $\begingroup$ You mean, something like a stellar nursery? $\endgroup$ – Frostfyre Nov 2 '16 at 12:16
  • $\begingroup$ @Frostfyre Something like that, but of course more than H2 would be needed to make turtle grow. $\endgroup$ – Mołot Nov 2 '16 at 12:21
  • $\begingroup$ As far as I know there are plenty of other materials too. Otherwise the earth would only contain of $H^2$. $\endgroup$ – lurch Nov 2 '16 at 20:36
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Based on this NOVA article, I'm going to say that these turtles lay their eggs in a solar system, instead of a beach, and only 1, instead of some hundred. The sun will act as a natural incubator for the new planet sized egg. While it is developing as an egg, The egg can gather its own atmosphere for some millennia before hatching. (Although, based on that article, I think our world turtle would lose it atmosphere if it went to close/far from the sun... So they might have to be star-bound and Hermaphrodites.)

I don't want to imagine them as planet eaters, and diet is technically not part of this question, so I'll leave the 'gain biomass for more babies' to a followup diet question.

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I suggest that a Smoke Ring be the "beach" used for the laying of eggs by the World Turtles.

As a quick description:

A gas giant orbits just outside the Roche Limit of a neutron star. The atmosphere of this planet is siphoned away and into an independent orbit, forming a gas torus around the neutron star. I believe the "gas giant" shepherds this torus to stabilize it. The central portion of the torus is thick enough to be a breathable atmosphere.

Using whatever propulsion methods the World Turtles use, they enter the Torus, slow to a stable orbit, and then lay their eggs one at a time, expelling them with a force to slow the egg's orbit slightly, allowing them to spread through the torus.

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