Dr. Who has always been pretty far-fetched but usually fairly free of elementary errors. Tonight, the Moon was an egg which hatched into some dragon-like creature.
Well okay. There are many hollow-moon theories that could work with that idea. The moon doesn’t have enough mass to account for its size, if it’s all made of the same rock as the surface. There are serious thoughts that it might have big hollows inside, perhaps caused by bubbles of trapped gas when it formed. Nobody knows for sure, that’s why it’s so open to being accused of being a spaceship (no engines, but we’ll let that slide for now because tonight it was a dragon egg).
If the moon was a rock-coated egg then it’s perfectly possible that the contents would have a lower mass density than the rocks on the shell. So far, so good. But if the inside was a fluid could we not tell?
Maybe not. Albumen and yolk are very viscous and since the moon always shows the same face to Earth, it effectively rotates on its axis once every 28 days. It’s not rotating very fast and the fluid inside is viscous and it’s been there a long, long time. If the fluid now rotates in sync with the outer shell, it might not be so easy to detect.
A long incubation period might also be credible given the surrounding temperature and the size of the thing being incubated.
The moon being a dragon egg isn’t yet beyond the bounds of credibility. The story works so far.
The moon increased its mass by several billion tons very suddenly. How? If there is an embryo growing inside then it must be fed by a yolk sac. As the embryo grows, the yolk diminishes but the total mass of the system does not change. Nothing goes in and nothing comes out during this egg-moon incubation so how can it increase in mass by billions of tons – and how can it do that very suddenly?
I could believe that the distribution of mass changes as yolk becomes embryo but that would not be sudden and would not result in an increased mass. It might well result in orbital wobbles and rotational changes so the moon would then sometimes show its other side to Earth. The billions of tons of sudden extra mass though, that breaks the credibility for me.
As for the badger-sized single-celled prokaryote with jointed legs and a mouth full of teeth… no. A spider based on prokaryote cells, well it’s a stretch but not totally incredible, but a single cell that size, a single cell with legs and teeth… nope. Don’t believe it for a moment. Especially since it can live and be fast-moving in vacuum, it shuns the light and prefers the sub-zero shade, and it spins webs. Especially the webs. What was it hoping to catch on the moon? That one was right off the wall.
Finally, the egg hatched and the moon was gone. Then it was back. Exactly as it was before, in the same orbit. How? Well, the creature laid another egg before it left. Another one. The same size. A newly-hatched creature immediately laid an egg the same size as the one it just hatched out of. That has got to sting. A lot. it would also require the creature to put more mass into the egg than it has in its body, since the shell of the original egg disintegrated.
I like Peter Capaldi’s Doctor. Much more un-human than the last few. Much more like William Hartnell’s original.
The storylines though, need a lot more thought put into them. A gradual shift in mass within the moon resulting in it rotating in the sky, threatening to lose orbit (in or out, either would be bad) and signs that it’s breaking up would have been enough. You could even have sudden shifts in mass as the embryo starts moving.
No need to magic up billions of tons of mass from nowhere. What’s going to be next, a space walrus?