Anatomically Correct Night Land Monsters

Question

The Night Land features a far future landscape, where the sun has gone out, which is made treacherous by the presence of various non-human "forces" and "powers", and monstrous creatures waiting to attack the unwary traveler.

Of course, William Hodgson didn't know about nuclear fusion or stellar evolution--and really, a future Earth orbiting a dead sun would be orbiting a white dwarf, with a surface temperature somewhere around 40-50K. Rather chilly to support creatures based on contemporary biology.

But suppose something evolved to live there and threaten spacesuited travelers walking the surface. How might such creatures function and survive? (If necessary, assume that sometime in the preceding billion years, humans found it convenient to do some biological engineering and introduce artificial lifeforms that could survive on the cold Earth, which have since evolved out of control; they do not need to be based on carbon and/or water.)

Details:

• The atmosphere of the dead-sun Earth is very thin, and composed on hydrogen, helium, and neon.
• Water, oxygen, nitrogen, and carbon dioxide are common surface minerals.
• Illumination from the white dwarf sun is about 1/1000th of what the contemporary Earth receives--so full sun at noon is still 400 times brighter than a contemporary full moon. Plenty bright enough to see by, and to support (slow) photosynthesis.
• Some creatures must be mobile, but need not be extremely active; if an individual can chase down a lone human in a burst of energy once every thousand years, that's just fine.

This question is part of the Anatomically Correct Series.

Answer 1

Plutoforming.

https://www.americaspace.com/2016/03/23/pluto-once-had-rivers-and-lakes-of-liquid-nitrogen-new-horizons-data-suggests/

It got too dark out on Pluto. But the inner planets cooled off nicely and so the Plutonians moved in. Plutoforming Venus and Earth made them in some respects nicer than Pluto ever was for the relocated Plutonians.

The Plutonian predators big enough to threaten an unwary human are all sessile ambush predators. Life forms are mostly small and so no predators are huge. They might take a leg. They might unsessile enough to come get the rest of the human where it fell a little ways off. That should be enough to keep that one sated for a few years.

Answer 2

Hydrothermal Vents

There are already ecosystems on Earth that are separated from the sun. For example tube worms feed off the energy released by hydrothermal vents.

Of course a tube worm cannot snatch a human out of its shoes. But it's not just tubeworms down here. Look closer -- there is a ghost crab and some small orange Pokemon hiding in there.

Your Night Land Monsters have as the bottom of their food chain something akin to smokers. I leave it to your imagination how the smokers got so close to the surface that the denizens ever encounter humans.

Answer 3

Your best bet would probably be for some form of silicon based life. They could withstand much colder temperature than anything carbon based, and are much more likely to form on a planet with no gaseous oxygen. The base of the food chain would probably be comprised of chemosynthetic organisms, and may use some compound such as liquid methane or ethane. They could also potentially have incredibly slow metabolisms, and could have very strange compared to carbon life.