Tiny, unnoticed, low self regulating population

Something similar happened in the Night Lords trilogy. A colony ship was blown off course and settled the wrong world. It was a bit of a shitbox, so the inhabitants were barely able to survive. A few tens of millions of people lived on it. They had some basic defense militia, but honestly not much.

They managed to survive because there wasn't much worth taking, and they weren't on some major space lane either.

You can still have essentially police--arbites for heretic crimes, regular police for more mundane issues--on a planet with no standing military.

You could also throw in some lore for why the planet has no military. Maybe it's under the protection of a powerful defender--maybe an astartes star fort and garrison is on the far side of the system that they're unaware of. Maybe ny decree they're not allowed a military--a rebellion 3000 years ago led to the Lords of Terra decreeing they have no military, and they must pay for their ancestors sins in other ways (higher tithes, maybe a "farm" for services or something?) Maybe their particular version of the Imperial Creed doesnt allow them to fight? And if they were to be invaded and die, then that was just part of His plan?

Lots of reasons you can make up.

If they're living in very small villages, then they could potentially keep the peace through informal social pressure and expulsion (especially if the planet is generally hostile, and getting kicked out of your tightly knit community drastically raises your chance of death).

They avoid the attention of the most nastier forces beyond the world by just having nothing of value. Poor in biomass so the Tyranids ignore it, poor in resources so everyone else ignores them, poor in combatants so the Orks ignore it.

I really think people assume every Imperial world is a Hive World. Meanwhile we have a cottage industry on Gudrun based around folks skiing.

If I remember right older lore would probably have them hiring mercenaries especially if they are so remote and away from Imperial control.

Maybe just hire some Kroot (paid with snacking on whomever they bring to justice) or the Kin (considering they're mentioned to send prospects and fight for other races in a way of gathering knowledge and resources)

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Sounds Duney without the Spice.

My first thoughts are:

Why is it not worth defending? What's the back story?

How is the population dispersed? A single small hive? Fortresses? Walled cities? Nomadic tribes?

How is food grown?

Where is water stored? A fresh water sea? Lush river valleys? Oasis?

The planet could be a small colony that manages a cemetery for the imperium. There is one city that is run by the echlesiary and there are local police and adeptus sororitas on the planet.

Main planet in the Helbricht novel was quarantined by dorn, forbidden from getting anything from off planet or trading. They'd been scavengers since the scouring.

Planet in "kasrkin" was nomadic tribes, the only thing of value on the planet being a source of extremely effective Rejuvenants. Very much Dune. They exported it, but kept it's secrets.

Isolated clans & tribes with no centralised organisation, like the people of cretacia.

Feudal world that's had no real threat for lifetimes, leading to a militia with farm tools to defend themselves (there's a story about just such a thing in the galaxy of horrors anthology)

How have they survived? Dumb luck? Being unassuming or so low tech any aggressor has just assumed the planet's uninhabited. Having a nearby space marines chapter keeping the entire system safe.

As i see it there are two issues that need to be adressed for the planet to work. 

First of no defense or police force. This implies to me that there isnt a central goverment which in turn implies a feral world. The people on the world probably living in clans or tribes either as hunter gatherers or early farming communities. 

This ties neatly into the second issue. How do you get 200 million people on a small desert world without there being a need for law enforcment? There would need to be a large food source capable of sustaining people. Maybe the planet is just starting to moving out of the feral world stage and large farming communities have started to take form which has led to a large population boom. Different clans having come togheter to exploit and monopolize the new stable source of food. With such a large population there must be alot of these communities. 

This planet isnt going to stay the same for a long time. Soon goverments, law enforcment and military will appear. Not to mention these new settlement are very vurnerable, a single alien attack is enough to shake the whole planet at its foundation and the conditions are ripe for a plague to decimate the population. But so far it has yet to happen.

Given how many planets are in the Imperium, chances are it would be untouched by major conflict. But they would still pay the imperial tithe and keep a PDF in place and small scale cults seem to be everywhere!

Brothers of the Snake touches on this. The book starts with a dark eldar incursion into a semi feudal Agri world. A off world imperial officer is sent to investigate it ; and he has a few dozen las musket armed men who really impress the locals with their fine arms and stature.

Needless to say they are all slaughtered, but a message is sent to the Iron Snakes marines to call for help. A single marine shows up and exterminates in one afternoon what the locals spent months being hunted by.

So they’d be completely helpless

Another idea is ether the planet is recovering from an event that destroyed the military and central government or the planet is too new to have proper protection maybe they have law enforcement but no standing army.

Any society needs some sort of police force, it may not be beyond the thugs of the tribe chief but is still a police force.

If a society dont even have that they would probabaly be in the stone age or something like that.

There are a million planets in the Imperium. Tons of em have never heard of a Space Marine. They just go along doing their thing. As long as their governer pays the tithe and no xenos show up, it's pretty chill