emissive armour & problems attacking planets

The "shield" that's stopping you might be his point defense. If they get enough point defense together they can shoot down missiles all day with barely a scratch. That leaves your cruisers with just a DUC, which might not even be firing at that range.

I'm no expert, but I've had some success bringing crowds of big cruisers with Napalm, which counts as Direct Fire and can't be stopped by point defense.

well if u gona get massacred anyway why not just invade? try optimal firing range with 3 or so transports full of troops, while cruisers getn hamerd land troops then retreat... next turn planet yours and stil intact, massive boost to ecconomy makes up for lost ships.
that or as noumenon said use specific glassing wepons or cruisers with 3 nukes at max range, 5 or 6 shuld do it.

I'd agree with invading, but I wouldn't use Optimum Range strat for Troop Transports.

I'm guessing troop tech's low, so build troops with the best ranged weapon you have for them. Armor and engines are less important than range.

If fighters are available take a few of them also in case the enemy has fighters.

Save the game beforehand, as there's a flaw in ground combat: if it ends up stalemated in any way it can't be resolved. For example, if both sides run out of ammo, or if the defender has crippled fighters offmap that can't be reached by units on the map.

Glassing planets is highly inefficient, but it's how some folks like to play. If that's your style choice, just maintain plenty of ships in the system (20 unopposed ships close to the planet will do in most cases) so the population will be in a perpetual state of riot. After 5 or 10 years you should have the tech to blast them any way you please.

If drones are targeting planets these days, you might give them a try. Drones are expensive, but they're cheaper then ships. I'd make a test run with just a few to be sure they'll function properly. There have been problems with drones vs. planets in the past, and everything can have trouble with ringed planets (don't know if that's the case here).

If you can keep them from building (using the perpetual riot strategy), you can eventually just obtain a healthy range advantage with the weapon of your choice and take down the planet without suffering loss using the Max Range strat.

yup u can use strategic, jus make sure the frieghters or watever u use as troop transport has land troops as its strategy and that it has troops in it...
ooh yeh forgot bout drones, u can hav 2 or 3 planets in the sys build drones and launch a few each turn to soften the planet up for glassing or invasion, hah i sometimes have whole systems near front line building drones and launching through warp point at planets i cant be botherd getn ships to, that way can hav proper fight on one front while keeping stalemate at other with almost no effort.
use a mix of anti planet and anti ship drones, if they get to target and nothing there give em new orders till they run out of fuel or blow up

Capturing planets in strategic combat got harder after the last patch, because your troop transports will sit in orbit after they launch instead of turning around and getting out of range. Anyone know how to fix that? But I still do it.

If you just sent in one bombarder with Napalm it probably got blown up immediately. You just send in a ton and the second one they maybe miss a couple times, the third one lives a little longer too and then the rest are blowing them up. This may be overkill, but I think if you send more cruisers than they have weapons platforms you will be guaranteed to win.

Sorry Noumenon, not much can be done. My only idea is a longshot: equip a weapon and hope it gets shot off about the time they go into sitting mode. It might trigger a retreat due to weapon loss. This could backfire if the weapon is lost too early. And I doubt if it would work in any case.

Ships always behave irrationally when they "have no target". This is just another case.

Early napalm has abysmally short range. I'd say Point Blank strat is in order, and a good-sized swarm so some will survive long enough to get in range.

Overall, the planet isn't going anywhere. So you can probably be patient and assemble enough forces to go in with confidence. If they planet has broken off and declared independence, this should show on the diplomacy screen.

One-planet empires can't make any headway because even if they have the right resource values, the AI is too dumb to rebuild them into something useful. It's probably has all its space devoted to its best resource, and nothing else except maybe a spaceport. If there are plenty of ship in the system (other empires count too), the planet won't be able to build anything at all because the population will be rioting.