system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40K] Can Exterminatus happen on ships? What would happen if it were to occur?
gpt
There are multiple forms of Exterminatus.  Virus bombing, which is what it sounds like but with a nice firestorm afterwards (metabolizes the dead into explosive gas).  A barrage of Cyclonic Torpedoes, which just nuke the planet and cause seismic instability which makes it uninhabitable (or possibly cause it to break apart). AKA "The Classic".  Rarer "Two Stage" Cyclonic torpedoes with their crust-penetrating ability are the only thing with a "Death Star" result of turning the planet into bite-sized chunks.  And then there's lengthy space bombardment by a fleet of naval ships, which has much the same result as Cyclonic Torpedoes just in a less cinematic manner.

Viral weapons are rarely used by the Imperium, feeding Nurgle and all. But there are some plague marine variants known for boarding enemy ships with the intention of spreading similar pathogens.  Results vary, larger, higher tech ships have some inbuilt NBC countermeasures obviously.

Cyclonic torpedoes are too slow, inaccurate and expensive for use against in regular ship combat.  Doesn't mean they don't see use however, especially against bigass scary stuff like the World Engine or the Planet Killer.  But usually only as a killing blow against an unshielded, crippled target.

Obviously, ships shoot each other all the time.  But ships have shields and a possibility of being missed, wheras a planet either has the former and doesn't get put down so easily, or gets slowly pounded into oblivion.