Uh, Guys? Exterminatus is never a good or profitable option by Imperial or Rogue trader standards! It renders all of the planet's recourses at least temporarily unavailable. That's why the Imperium will only do it as a last resort when they believe they Can not take the world by conventional means and destroying it will deny it to an enemy! BOTH of these criteria must be met before the Inquisition will sanction exterminatus. Firing a Nova Cannon at a Human populated world without inquisitorial sanction would be Heresy! Firing one at a Xenos world would at least cause one's warrant to come under review!

Radwraith said:

Fortunately, the Nova Cannon is not considered Exterminatus weaponry. Also fortunate that Imperial laws don't really apply the same outside of actual Imperial space - like the Koronus Expanse. Much more likely that an objection might come from other Rogue Traders as they have much more say in such areas.

I'd also point out that using the Nova Cannon is a bluff that will likely only get called once. After devastating one group of proud fools and making the outcome known, few would be willing to call that bluff again.

Cryhavok said:

The resources you suggest are FAR beyond what a typical colony in the Expanse will have for defenses. As for your 'Army of Light' (B5 reference) defense, that's so far way from what we typically see in WH40K canon - where factions just don't come together like that - that I can't take it seriously. Sometimes, the players really do have the biggest stick in the area and they can swing it how they will.

We have very little afaik on planetary defences. However, a planet with a good industrial base should be able to make a generational investment in system defence.

Find a suitable asteroid. Fit it with basic essential components (not warp drive or geller field). It might have 2 broadside slots and a void shield - perhaps a modest drive to give it Move 1. A dozen of these orbiting a world and you with need a fleet to take it.

If a world could build one of these every 50 years, in the timescale of the Imperium the average industrial world should be very tough.

Fresnel said:

Funny enough, a colony can grow every 90 days per Stars of Inequity, meaning a small outpost could grow remarkably quickly. Absurdly so.

Cryhavok said:

WELL BEYOND WHAT A COLONY SHOULD HAVE!!

Seriously.  No colonly should be able to even remotely threaten a Grand Cruiser.  Do you not have any idea of how difficult industrial production is in this setting??

If there's a Grand Cruiser that can be seriously threatened by a colony when it moves into position to bombard it, the Grand Cruiser is Doing It Wrong. Seriously, you've got so much space to play with, there's virtually no reason not to take (for example) the extra flak turrets. Fit that to an Exorcist and wow, suddenly you have a turret rating of what? 4-5? Plus, of course, fighter support from the embarked squadrons.
Combine that with the fact that ground-launched small craft are going to be extra vulnerable (as they're stuck on a fairly predictable trajectory) as they climb out of the gravity well, and (per Battlefleet Gothic) a single macrobattery "hit" will result in a squadron (actually, the entire formed wing, so potentially many squadrons) being mission-killed at least, plus the fact that ground-launched small craft are going to be less capable in void combat than ship-launched (again, to do with the fact they have to claw their way up to orbit), and we see that it should take dozens of squadrons, at least, to pose a threat to a serious warship over their world.

That said, if the colony can produce orbit-capable shuttles, they can produce planetary defence bombers, fighters and assault boats (basic engine requirements are the same, but the fuselage and total mass is going to be smaller, giving some more manueverability). They may not be up to IN standards one-on-one, but when you potentially have the resources of a planet to draw on, and aren't as limited by storage space and consumables, then given time they can produce enough to threaten/deter even a squadron of capital ships from taking position in their orbitals. Of course, once the attackers have the orbitals, it becomes much harder to launch against them without having your bombers/launch sites destroyed, so there's a narrow window for launching and having a hope of your bombers doing something before being destroyed. But if the defenders hit that window, they can do serious damage.
Hell, if they have the technology to produce ICBMs, they can deter a ship from taking the orbitals without being careful and launching small craft from further away, as even a chemical rocket booster can loft a half-decent torpedo (ok, Poor Quality, but that can still do some nasty damage) into the guts of an unwary ship.

For that matter, what on Terra is a grand cruiser doing without escorts? Really, the largest ships that should be allowed out alone are (maybe) battlecruisers. What happens if they don't is they get picked on by lots of smaller, more maneuverable vessels until they get crippled/dead. Look at what happened to the Planet Killer (at least battleship sized, if not a dreadnought/superdreadnought)- taken out by 3-6 Cobra destroyers who just "parked" themselves behind it and pumping torpedoes up the aft.

Cryhavok said:

I can think of dozens of examples of planets that do not have starship tech but suitably tough ground and orbital defences that have cropped in my games over the years. We had one where the locals had a nuclear satellite defence grid and nothing else in orbit, but they had excellent ground and air defences when we finally tried to invade. 
Remember that to profit from a world you have several routes:
A) Trade agreements - you discuss with the leading trade faction or rulers of the planet about what you want from them and what you can offer in exchange. This could be a like for like trade of goods, it could involve services (offering priests, doctors, techpriests etc in exchange for a rare local commodity, or even taking pilgrims from a world to a distant shrine they have ancient tales of but have never been able to visit).

B) Covert profiterring - this could be as simple as sneaking up on a valuable asteroid ring in a system you are considered hostile in, or as complex as sending undercover agents down to a planet to hijack operations and funnel profits or valuables back to your ship.

C) Invasion - Assuming you can't work the first two options, then invasion is your final calling card. You have to rock up in system, take out hostile ships, knock out system ships and orbital defences, get air-superiority on the planet, land forces and start taking over territory and resources. You could be stopped at any time along this route, or if you are really lucky you could be going up against a primitive culture who while incredibly aggressive simply do not have advanced tech enough to oppose you.

A well run (and played) campaign will have a mixture of opportunites for profit and plunder, and there should never be a situation where you can 100% predict exactly what sort of defences a planet will have, or how trade negotiations are going - if this is the case then the GM needs to vary things more and stop letting the players get away with pulling the same tricks over and over.

The Nova Cannon is  still going to be ideal for rapidly stripping away orbital defenses and it can do so from outside of the range that ground-launched torpedoes can threaten.

If a system can build 30 squadrons of bombers/fighters, it can probably build an orbital platforms to launch them from. Build the bays into asteroids and 'run silent', and you might get the drop on invaders.

I am sure people can think of other ways to make invasion challenging too. How about a Nova cannon in an asteroid?

Fresnel said:

Well, yes, they probably could build bays into asteroids/moons. Trouble is, you then start to run straight into the problems of space and consumables, just as you would with a starship. Life support, munitions, spare parts… all the stuff that's relatively easy to produce, store and transport on a planet (even assuming you need to provide life support to the launch site/bay/base, which on a planet with a habitable atmosphere, you don't) suddenly becomes an issue for a void-based facility. Admittedly, it's not necessarily a major issue, particularly as a colony matures, but it does mean that your "secret" orbital bases will need regular (if not necessarily frequent) resupply. It'd be a stroke of bad luck to have attackers arrive to spot that resupply run, but it could well happen, with fairly predictable consequences.

Other tricks? SDF bases in close orbit of a local gas giant, alerted by listening stations on the system's edge (admittedly, that's something that requires a mature colony). Orion drives on asteroids to disrupt/annoy attackers. Mass drivers on the moons (stuff like the Lathe Grav-Culverin, ideally). Hell, a SAPL system is not necessarily that tricky to build (although in 40k terms, keeping your collector and targeting mirrors aligned and on target would be an absolute bitch). Minefields. Orbital tugs dropping mines. Deadfall torpedo salvoes in the L4 and L5 points of a colony's moon. One-shot bomb-pumped lances fitted to asteroids, or in an oversized torpedo bus.