As such there may or may not be errors in it that I have missed, or things I iknow are wrong but I haven't changed. Feel free to point them out or poke holes in them, and I'll either revise or argue them as I can.
I also have things I have to add yet, which is why I've decided to go with the more "ongoing revision" style analysis. I know there will be alot more added (my other dedicated calc threads can probably be added as well.)
Anyhow, as I said, I know I've done some before. But even so I think it deserves its own thread because it sa big, ,huge, complex and varied topic, and because the effects are so varied. I've also come to some other conclusions. So bear with me.
The Exterminatus is well known, as is what it means and the various methods by which it can be delivered. It can be delivered by cyclonic warheads, by virus bombs, or by other conventional firepower (mass drivers, fusion bombs, lance strikes, ,etc.) The effects and durations have been mentioned before:
BFG, page 78 wrote:The attacking fleet is escorting Extermiantors, ships capable of laying waste to entire planetary populations or even obliterating all life on a world in a matter of hours.
BFG, page 96 wrote:All had heard of Exterminatus with fusion torpedoes, virus bombs and mass drivers, but to know the enemy had the ability to destroy an entire planet, not just all life on it, must have been the most chilling thought that any naval crewman had ever faced.
I've done calcs before based on the aforementioned evidence. But it is also my considered opinion that (which I have said before) that those previous calcs were conservative, and likely much higher. On further reflection, they almost certainly ARE much higher. Not only are there examples in other novels to consider (Ciaphas Cain's Caves of ice, multi-teraton ordnance or planetary bombardment missiles in sources like BFG, Rogue Trader, etc.) But there is the fact that the Exterminatus effects themselves point to higher end firepower. The 1e9 megaton figure (from the SD.net planet killers page) is conservative - it merely destroys all "complex" life. But given the evidence (and logic) there is much more to an exterminatus than just destroying all the people and animals.
There is also this from Codex: Daemonhunters:
Codex, daemonhunters, page wrote:In the most dire circumstances, Inquisitors are empowered to declare Exterminatus upon a world which has been lost to the Ruinous Powers, ending the horror of its inhabitants in a catacylsmic rain of fire from ships in orbit.
- The text describes melta torpedoes, barrage bombs (ordannce) and lance strikes being used to conduct bombardments. Given the implication that orbital strikes are the same as Exterminatus just on a less destrcutive magnitude, we can safely conclude that planets can be subject to brute force Exterminatus (if any doubt existed.)
There is yet one more useful tidbit to go on, and that is from Tactica Imperialis:
Tactica Imperialis, page 66 wrote:Exterminatus is the last resort in war, ,used by the Imperium when all other options are exhausted. To declare Exterminatus upon a world is to visit total and utter destruction upon it. It is to sign the death warrant of every living creature upon it, and to render its surface a blasted wasteland, inimical to life for millenia.
There are many forms of Exterminatus, and many ways of delivering it. One method uses a biological agent that renders all living matter to a decayed sludge within hours, releasing a staggering amount of oxygen into the atmosphere as it does so. This oxygen is then burnt off, igniting the entire atmosphere and incinerating everrything on the surface. Another method involves the use of cyclonic torpedoes, a category of weapons that use varying means of scouring the world's surface with nucleonic fire, raw plasma, or deadly radiation. Some worlds might be virus bombed - sometimes the viral agents might be keyed to a specific gene code, so as to target aliens, mutated humans, or simply rebels with a predominant hair colour.
Exterminatus may only be orderd by the very highest level of authority, and any who enact it will be answerable to the High Lords of Terra. While raw manpower remains the Imperium's greatest asset, worlds are not so easily replaced, and those who squander such resources seldom remain in power long enough ot make the same mistake a second time.
Some various points of interest (in order of appearance.)
1 - Exterminatus is a "last resort", and visits "total nad utter destruction" on a world. It kills all life and renders the planet's surface "inimical to life for millenia". Note that while this may suggest that Exterminatus is not "permanant" as I have asserted, note that under most cases, the Imperium Is capable of inhabiting even lifeless/barren worlds (hab domes and such.) or worlds that have been destroyed by bombardment (IE Tallarn and the virus bombing.) Indeed, considering that many hive cities are effectively "uninhabitable" without artificial support is proof of this. What this means is that while certain destructive effects (firestorms, grgoundquakes, ejecta, dust loading, and like) probably die down at some point, the planet probably no longer has the ability to sustain life unassisted. And that the planet cannot be "inhabited" until the destructive effects die down.
2 - "Many forms" of Exterminatus and "manyy ways" of delivering it. This probably does indicate that you can have varying levels of "destructiveness" for Exterminatus (naturally). Some more permanant than others. It probably is quite possible to render a planet uninhabitable via Exterminatus without removing the atmosphere or oceans, for example. And planetary disruption is probably extraordinary form of exterminatus (except for Abbadon's Planet Killer, or specialized ordannce, anyhow.)
As a curious note, Virus bombing i sdescribed as a purely biological attack, wheras certain novels and game sources indicate that the "sludge" and firestorm effects arer part of it. This may be an error, or it may simply indicate that different kinds of "virus bombing" exist, just as different kinds of cyclonic torpedoes exist. (makes sense, since ealrier sources and some novels like the Last chancers do feature virus bombs that don't have the firestorm/sludge effects. Hell you could virus bomb in a tactical manner!.)
Note as well that cyclonics are either "nucleonic" (matter ot energy conversion, probably much akin to a conversion beamer.), radiation, or "raw plasma". The last two hint at a more brute-force approach, although even the nucloonic bvit indicates that Exterminatus effects come about from raw firepower (even if the process in generating the energy is technobabble.) In any event, correlations can be drawn one way or another.
3.) "Worlds cannot be easily replaced" and those who "squander such resources" are disciplined. This again indicates that Exterminatus is, for all intents and purposes a "permanant" action, and that any world redeemed from such a tack must have more limited utility than it had before Exterminatus was enacted.
First, I shall go over each individual "effect" beyond the 1e9 megaton threshold, and explain the underlying logic applying each to the Exterimnatus.
Many of the alien menaces that threaten the Imperium are quite resilient. No matter how many are destroyed, their presence (or taint, if you will) can remain. Spores, microbes, minute traces in ainnumerable forms can survive. (Indeed, according to the 2005 Tau Codex, Tyranids are somewhat resilient to some forms of exterminatus (The Codex indicates the Tyranids either burrow down in case of smaller forms, or largers like Carnifexes may just ride out the attack.) This can mean that the bombardment must be more violent, more thorough, in order to ensure that the taint is completely wiped out. The Tau and Orks seem to be likely canidates for this, given that a number of sources both indicate that spores are employed by both (Orks reproducec by spores, or at least some kinds do, and the tyranids use mycetic spores in their stripping of planets.) Necrons and Chaos, to an extent, also apply, although the reasons for this may be somewhat different (Chaos can arguably infest any form of life, and can also taint inorganic things which may require substantial destruction. The Necrons often have their own facilities kilometers or more beneath teh crust of planets, and their own tech, such as Necrodermis, can have its own invasive, corrupting properties, albeit mainly to inorganic materials. Though C'tan are known to "corrupt" or otherwise influence organic beings as well.) Therefore, it is likely that the firepower required for Exteriminatus is considerably higher (tens of billions of megatons, as per the Planet Killers Page
Additionally, there are Virus bombs. Virus bombs wipe out all life on a planet, no matter how small (the "life eater" virus). As a side effect, they create massive, lethal firestorms (much like a complete serilization will do.) Logically, other weapons ought to be able to achieve similar effects (as per the BFG quote as above.. fusion and mass driver bombardment is brute force, unlike the virus bombs.) A conventional bombardment designed to match the effect of a virus bomb should likewise achieve "complete sterilization" level destruction.
The "tens of billions of megatons" figure I cite from Mike's Planetkiller page is a lower limit, bear in mind. In the research I did in "caves of ice" regarding sterilization, I discovered that boiling and/or vaporization of oceans can be a known side-effect of planetary sterilization, so it could be much higher (and if vaporization occurs, it probably will be.)
Draco, page 90 wrote:For such was exterminatus: the total destruction of all life on the surface of a planet by means of virus bombs delivered frrom orbit.
The life-eater virus, spreading at amazing speed, would attack anything whatever that breathed or grew or crawled or flew as well as anything of biological origin: food, clothes, wood, feathers, bone. The life-eater was voracious. The jungles of Stalinvast would swiftly rot into sludge that would form shallow festering inland seas and lakes, where rot continued to feed so that the very air burned planet-wide, searing the whole surface to ashes and bare rock.
In the cities all protein would eat itself and ooze in a tide into the underbelly, rot eating rot, until the firegas detonated, leaving the cities like mounds of dead, blasted coral.
Draco", ,page 159 wrote:Stalinvast would long have been a scorched husk, its jungles rotted utterly by the life-eater, than cremated by firegas, only the plasteel skeletons of its empty cities hovering above the barren desolation, dead reefs above a dried-out sea. Many cities would most likely have collapsed into tangled, fused ruins when the firegas exploded planet-wide. There would not be an atom of oxygen left in the now poisonous atmosphere; that too would have burned.
The last mentions destruction of the atmosphere ("burning off" level damage") but that may be unusual here (just as cyclonic bombardment can vary, arguably.)
Ultramarines Omnibus, page 367 wrote:Uriel nodded, determined that Chordelis would not suffer the horrible fate of Barbarus Prime. By now there was nothing left of that world but a dead hunk of rock, its people, wildlife, and very ecosystem devoured by these monstrous aliens.
Ultramarines Omnibus, page 377]
Even at maximum magnification, the planet before them barely filled the viewing bay, reflected light from the distant sun rippling across its heaving, fiery surface. Firestorms were raging across the dead planet as flammable gasses released from oceans of decaying organic matter enveloped it, scouring the surface to bare, lifeless rock.
The tyranids themselves could do no more thoruough a job.[/quote]
Uriel compares the destruction described above by the Tyranids to Barbarus prime to what the Virus bombs do. Again, comparable to complete sterilization, minimum. Note that "ecosystem" can include water and soil (and probably atmosphere - all things the Tyranids are known to strip from a planet.) which reinforces my earlier assessment.
Note that the "firegas" triggered firestorms are implied to be "oxygen" based in Tactica Imperialis. This may or may not be so, but it does seem likely that this type of Virus bombing is much more than simple biological attacks - a sort of "technobabble" process that destroys the organic matter and creates the incendiary/explosive "gas" from it.
[quote="Ultramarines Omnibus, page 378-379]
The life-eater virus was quick to act and utterly lethal in its effects. Perhaps some had an inkling of what was being done to their world, but most would probably have succumbed without realising the magnitude of the betrayal visited upon them. The atmosphere would be saturated with mutagenic toxins that attacked the biological glue that held organic matter together, breaking it down with horrifying rapidity. Within hours there would be othing left alive and the virus would be forced to turn on itself in an unthinking act of viral self-cannibalism. The planet's surface would be covered by a thick layer of decayed sludge, wearthed in vast clouds of toxic waste matter. All it would take was a single shot from orbit to ignite the fumes and firestorms of apocalyptic magnitude would sweep the entire surface of the planet bare.[/quote]
- Again, suggestive of at LEAST sterilization level energies.
Depth and Durability of Constructions
Along with complete sterilization, it is known that many Imperium worlds are heavily built up city worlds (forge worlds and hive worlds in particular) much like coruscant (or the continetnal fortresses of Terra.) Many of those worlds may also extend hundreds or thousands of meters underground (shelters and defense installations, mines, other facilities, ,etc. Such as in Necropolis, Execution Hour, Caves of Ice, SHadow Point, etc.) In addition, the construction methods and materials available to the Imperium (to build those shelters, hive cities, etc.) are considerably greater than our own, ,and many facilities (Government capitols, hive cities, Arbites precints, etc.) may have their own forcefield defenses (Execution Hour, NightBringer, Necropolis, etc.) Much like with the Star Wars Base Delta Zero attack can require that signifigantly more firepower be applied to such destruction.
In terms of facilities, Mike's Base Delta Zero Page covers this idea. In that page, Mike mentions that the destruction of hardened facilities in modern nuclear attack simulations use a 2000 psi overpressure to ensure target destruction. The energy input figure he comes up with is 2e25 joules for this. This is only a bit below the "complete sterilization" figure above, and is (as Mike notes) quite conservative given its basis on "modern" material sand construction techniques. It could easily be an order of magnitude greater, much as it would be for Star Wars BDZ.
Additionally, there is the consideration of reaching mines or shelters. As the classical "BDZ" (the 1 hour/1 meter melting depth) calcs have traditonally indicated, reaching deeper into the planet naturally increases the energy requirement (even irerespective of melting or vaporization.) extending 100 meters down increases the energy figure by 100x, and over a kilometer depth by a factor of 1000x. Considering that some sources indicate planets like Cyrene are reduced to a molten state, this is a valid consideration. (Cyrene as a calc will be considered later.) Some may argue that "crust melting" may not be an accurate figure, and this has some merit, but there are a number of considerations to bear in mind. First, in terms of an extinction-level event, indirect effects (heating of the atmosphere by hypervelocity ejecta, for example) and inefficiencies can lead to "melting" of the crust even if the beams themselves do not do it. Asteroid impacts have shown that a substantial degree of melting (and vaporization) also occur with such events. Beam weapons such as lances are even more likely to result in this. Cyrene also indicates "melting" is a valid benchmark, as do other examples where energy weapons are mentioned in bombardment roles (Plasma weapons in execution hour, Plasma weapons melting a hive city in annihilation squad, etc.)
And as mentioned before, some Tyranids are known to bury underground to escape Exterminatus, so more extensive bombardment may be required to root them out. Tyranid infestations in the process of draining a planet of its biomass may also invade deep into a planet's crust. And as mentioned before, the Necrons tend to inhabit tombs buried many kilometers beneath a planet.
Even mundane Imperium structures like hive cities can reach kilometers beneath a planet (Kill Team), and many kinds of facilities (defence stations, mines, bunkers,etc.) can extend many hundreds of meters below ground regardless.
Exterminatus via conventional bombardment has been used on a number of occasions: The most obvious example is Cyrene, of course, but it has also been mentioned with regard to 56-Izar (Xenos). The planet attacked in Fire Warrior too was bombarded by conventional weaponry as well. The sterilization of the planet in Caves of Ice can technically be termed exterminatus even if it was not directly labeled so, and involved conventional bombardment as well. This meshes very well with the Battlefleet Gothic references to mass drivers and fusion bombs being used to conduct Exterminatus (alongside Virus bombs.)
Burning Off the Atmosphere
Bruning off the atmosphere is a very energy-intensive event. Any planet having an atmosphere gnerally has a large escape velocity for its objects (such as the 11 km/s escape velocity for earth.) Just as a planet's mass must overcome said escape velocity for the planet's mass to be scattered (destroyed), the atmosphere as well must achieve escape velocity for the atmosphere to be depleted.
Attacks known to do this are Cyclonic warheads. The Tyranid Codex states:
[quote="Codex Tyranids, 2005, Page 27 wrote:Following Exterminatus by way of Damnatus-pattern, mass-yield cyclonic sturation, the surface of the world was reduced to drifting ash, the atmosphere entirely seared away.
The effects of cyclonic warheads will be discussed separately. The atmosphere of a planet, because it is diffuse, can't be directly attacked by many forms of attack (and others, such as nukes, won't completely effect it. And there are always inefficiencies.) Beam weapons such as lasers or particle beams will easily punch through it without heating much of it. This means generally that planets are heated up by area effects indirectly (either nukes, or nuclear fireballs created by beams or kinetic impactors.) The energy requirement for evacuating the atmosphere of an Earthlike planet (~5.1e18 kg) is roughly 3e26 joules. as a minimum. I say "minimum" because, as I mentioned, atmosphere is diffuse and direct bombardment cannot really heat it up all that well. Heating it indirectly is going to be rather inefficient, and will probably lead to other matter (IE water vapor) saturating the atmosphere (thus needing that much more mass to be evacuated.) The 'minimum" is most likely to apply in technobabble attacks or airbursting detonations (nukes or certain kinds of "plasma" weapons.)
Boiling Off or Vaporising Oceans
There is yet another level of destructive effect to consider. That of boiling (or more accurately, vaporising) the oceans of a habitable planet.
Cetain forms of attack such as cyclonic torpedoes have been known to do this:
(quote originally provided by NecronLord)Akliamor (Eldar-Empire Relics, Destroyed):
White Dwarf #236, The World of the Bloodied Sword, by Gavin Thorpe
"We departed the world's surface and watched from the stars as the akliamor began to activate. It happened slowly to begin with, I remember some of the children standing at the windows, competing to see who could spot the first few blossoms of explosions. Then a chain reaction began to build, as the dark matter at the centre of each weapon was released and began to accumulate. We watched a cloud of death spreading across the face of our ancient home. My vision was clouded with tears and the weeping of our kin weighed heavily on our hearts. Fields were stripped bare, the rocks crumbled to dust, the seas boiled into the air, creating massive electrical storms across the skies, all living things were scattered to the howling winds. We sang the Hymn of Lamentation as we watched Taqamathi dying before our eyes. It did not take long. A final eruption of power shattered the world's skin, causing great volcanoes to spring into life belching rock and dust into the already polluted atmosphere. Cracks and rents ripped across her once beautiful continents. Where the forests of Lietha once spread as far as the eye could see, there was no only rivers and lakes of lava and fire; the Gardens of Maegan were an ash-strewen desert; the majestic Zamua valleys were filled with boiling mud slides and fountaining geysers. At that final moment Taqamathi again retained something of beauty; the savage and powerful beauty of the universe unleashed and returning to its natural form, for Taqamathi had always been tamed to our will and now she was retuning to her wild origins. Satisfied that nothing could ever live on Taqamathi..."
"What are cyclonic torpedoes?"
"Planet Killers," answered Uriel. "They will burn the atmosphere of Pavolnis away in a storm of fire, scouring the surface bare until there is nothing left alive. The seas will boil to vapour and your world will become a barren rock, wreathed in the ashes of your people."
- Ultramarines Omnibus, page 232
To my knowledge, no other form of attack (save the Planetkiller) actually does this to a planet, but this does not neccesarily mean Imperium warships would be incapable of it via conventional bombardment. It is known that the Tyranids frequently consume not just the organic matter of a planet, but also the components essential to it, such as the atmosphere or the oceans. On more than one occasion (those enacted by Inquisitor Kryptmann himself are the most notable) the Imperium has been known to preemptively conduct Exterimnatus on planets to deny those resources to the Tyranids (Again Ultramarines omnibus and the Tyranids Codex.)
An earthlike planet generally has a large quantity of water (Earth's surface is 70% ocean, massing some 1.4e21 kilograms). Boiling off and/or vaporising that much water requires approximately 3.9e27 joules of energy. (a bit more than an order of magnitude more than estimated for blowing off the atmosphere.)
There IS another dimension to this level of destruction, with regards to "blowing off the atmosphere". Vaporisation of so much water can, under some circumstances, lead to the atmosphere being saturated with the superheated vapor. While this will heat the atmosphere, it will also make it effectively heavier as wel, meaning that the energy requirement to "blow off the atmosphere" could also increase.
Under most circumstances, the above is most likely to occur by "direct fire" attacks like energy beams or impactors and ground/subsurface detonations. Airburst explosions (espeically high altitude ones) and the like could possibly result in much less energy being injected than attacks that directly strike or strike close to the surface of the planet. (heat from ineffieicnies and bombardment of other partts of the planet could also lead to some vaporisation, but due to the properties of water and the sheer amount involved, oceans would make very good heat sinks as well.) In any case, evacuating the entire oceans of the planet (in the form of vapor) requires around 8.47e28 joules of energy, at least.
The issue of Exterminatus removing or destroying the atmosphere of a planet deserves further elaboration. As many 40Kers no doubt already know, Tyranids attack habitable planets and consume all the biological matter and materials on the planet. This not only includes all the plant and animal life, but can include the biosphere, the oceans, and the atmosphere as well. While not directly "biological", they are elements vital to life (our bodies are 75% water, after all. And air can be useful for things other than construction materials.)
In addition, certain other elements seem to be consumed from a world as well. This probably includes iron and silicates, because Tyranid carapaces are frequently described as "stony" or "ceramic" like, and some metallic elements like iron ARE essential biological components. a certain amount of destruction of such materials is bound to be expected as well.
Quotes specifically pertaining to the concept:
Codex Tyranids 2001, page 41]
As resistance is overcome more and more of the planet's surface is stripped bare by continuous harvesting until the hive fleet concludes its actions by draining the planet's atmosphere and seas.[/quote]
- Reference to removal of atmosphere and oceans.
[quote="Codex Tyranids 2001, page 42 wrote:The world was almost drained now, within a few revolutions the last of its biomass would be consumed and the hive ships would cluster in tighter. They would strip away its atmosphere and drink its oceans, covering their mile-long bodies with frozen sheaths of oxygen and hydorgen, nitrogen and chlorine in preparation for the journey ahead. When all was done, when Tethris had been reduced to an infertile ball of rock, the hive fleet would move on and begin tis hunt again, ,searching for a new prey-world to feed upon.
- again, oceans and atmosphere removed, along with other elemtns.
Codex, Tyranids 2005, page 6 wrote:In ancient surveys, these particular planets had been loged as supporting life, but more recent Explorator scans and expeditions reported them to be barren airless rocks.
As time passed, the Technomagi found that even worlds that were known to have thriving ecosystems had been transformed into barren planetoids.
...almost a year had passed and at first he could not equate the dead, ,barren planet he found to ocean-bound Tyran.
Tyranids leaving habitable planets (including "ocecan bound Tyran" airless and "barren".
Codex Tyranids, 2005, page 20 wrote:His plan was that a band of worlds should be evacuated across the path of Leviathan's main advance, with many of them razed to the ground in order to deny the hive fleet any further raw materials for its ships. This would slow is advance long enough for Battlefleets Solar and Tempestus to muster. Any worlds already under invasion within the bounds of this cordon were to undergo Exterminatus just at the poitn when the Tyranids descended to feed upon the doomed populace. Kryptman theorised that in this manner the swarms would expend great resource to claim a world, only to have every living thing upon it reduced to ash by barrages of cyclonic torpedoes and virus bombs.
The description of Kryptmann's "Biomass Denial" tactics. Note mention of the use of Virus bombs as well as Cyclonics. Given that Cyclonics can be brute force (See Tactica Imperials) as well as the fact that brute forcee attacks can apparently emulate Virus bombing (Battlefleet gothic), its quite likely that brute force attacks can achieve similar destructive capability (although it may be more complicated.)
Codex Tyranids, 2005, page 26 wrote:To summarise, our strategy in this regard has been the doctrine of 'Biomass Denial'. A Tyranid hive fleet must expend a staggering amount of energy in taking one of our worlds provided our forcecs are able to mount an effective defence. Frequently, we have been able to draw the hive fleet into committing ever increasing reserves of energy into taking the world, at which point, we attempt to withdraw the bulk of our forces, ,and deny the hivefleet its price, by way of enacting Exterminatus upon the target world. This has the effect of critically draining the hive fleet's resourcecs, as it has no biomass with which to replenish the energy expended in taking the world."
Again, "Biomass denial" via Exterminatus. One can infer, potentially, that fleets of vessels were capable of simialr effects via brute-force bombardment (within an order of magnitude, for example.)
Lastly, we have this link here concerning Tyranoforming, which again confrims the "removal of atmosphere and oceans" bit as well.
Bombardments and Side Effects
Its worth noting that none of the above are neccesarily mutually exclusive. Inefficiencies will creep in, as energy is radiated back into space or into other targets (such as deeper into the ground, or into oceans, or the air.) as a starship attempts to blast hive cities or underground facilities into destsruction. It is therefore possible that in the process, that the air may be heated to many thousands of degrees or burned off, oceans vaporised/boiled away, ,or the crust substantially motlen/vaporised as mere side effects of something as simple as destroying cities (Indeed, in most cases, you cannot effectively burning off the atmosphere directly with most attacks.) As noted in the "sterilization" research from the Caves of Ice calcs, you can easily cause boiling or vaporization of most or all of the planet's oceans in the process of sterilizing the planet (depending on method of Exterminatus conducted) so those can hardly be treated as "mutually exclusive" in all cases. And as I mentioned with the "burning off the atmosphere" entry, air can be hard to remove directly, and doing it indirectly can complicate the removal (and thus increase the yield.)
A good example of course is the bombardment of fortified structures (as above) and its relation to complete sterilization or crust melting. All involve energies substantially higher than 1e9 megatons (well into the e25 joule level of destructtion - many billions or even tens of billions of megatons). And given the conservative nature of the "destruction of hardened targetS" example above, compared to advanced tehc, its quite possible that the feat of destrtoying cities or shelters would also burn off the atmosphere. So then you can argue that the energy input logically extends well into the E26 joule range simply by concluding that 40K materials/construction tech are at least several times better than what we have already! And that doesn't even include the shielding.)
It is therefore quite likely that multiple effects (ocean boiling/vaporization, crustal melting/vaporization/pulverization, destruction of shielded structures, and sterilization, for example) are going to be occuring simultaneously, and thus teh energy requirements add up. (Another example would be ocean/crustal vaporization and removal of atmosphere.)
We know from Battlefleet Gothic that the Exterminatus can be exectued (By a single ship) in a matter of "hours". This means at least 2 hours, but less than a full day (fewer than 24 hours.)
It can be noted that while many effects or results could arguably be the same frfom the various kinds of bombardment (IE mass driver. lance, and fusion bombardment could yield effects similar to virus bombs or cyclonic warheads) the other factors (timeframes, or number of ships) can differ (A cyclonic warhead, for example, could blow off the atmosphere or boil off the oceans of a planet much more quickly than conventional bombardment could.) From the viewpoint of "brute force" attacks, this can also make sense (complete sterilization takes less energy than blowing off the atmosphere or vaporising the ocean, so naturally the timeframe ought to be less.)
Vehicles of Destruction
As per Battlefleet Gothic, the vessels capable of Exterminatus are referred to as "Exterimnators" - these included Abbadon's Planetkiller, the Blackstone Fortresses, and any capital ship modified to carry a prow-mounted "armageddon weapon." In-game, this is clearly meant to suggest that the capability to exterminate planets is not a standard capability for ships, but this can be considered silly for several reasons:
1.) A number of game and literary sources (novels and game books alike, such as Codex: Daemonhunters) indicate that conventional weapony (lances and torpedoes) on a ship are quite capable of bombarding and damaging a planet, up to and including Exterminatus level-destruction.
2.) Even ignoring the above, we know from the self-same Battlefleet Gothic source that exterminatus can be conducted by fusion and mass-driver bombardment. Either is quite evideently a brute-force attack, and would indirectly indicate (plasma warheads are at least the equal of a fusion warhead) or directly (the acceleration of mass-driver projectiles would necessitate the firing ship provide the power) that the "exterminator" carry the requisite firepower even if it DIDN'T carry the armageddon weapon (Hell, a Nova cannon IS for all intents and purposes a glorified mass driver... and a prow mounted weapon at that...)
Note that "capital ship" refers to both cruisers of all classes, as well as battlecruisers and battleships. This means that a single cruiser (even a light cruiser!) should be capable of rendering a planet totally uninhhabiable (read: at least sterilized, if not having the atmosphere and/or oceans boiled off) in a matter of hours. A battlecruiser (or battleship) could do likewise, but since they are bigger (and more powerful) they arguably should be capable of doing it much more quickly.
Another possibility not covered in Battlefleet Gothic but mentioned by other sources: groups of ships can work together to perform Exterminatus on a planet. This has a benefit of reducing the timeframe to accomplish the feat drastically (most useful for brute-force attacks by a starship's armaments.)
Abbadon's Planet Killer
Abbadon's Planet Killer deserves a special, unique mention, both because it qualifes as an "exterimnator" ship and because of the sheer magnitude of its destructive capability. The Planet Killer's firepower has already been well established previously (by me), and need not be repeated here. I will, however, add one thing. Given the recent analysis performed by myself regarding sterilization here, my calcs for atmospheric removal were probably conservative.
While the attack by the Planet Killer almost always results in the destruction of the planet, it should be noted that for the most part this is not due to the "brute force" application of firepower, unlike the other effects. The manner of destruction (the factors causing it), the duration and effects, all mitigate against this (although at least one incident could arguably be brute force.)
Because it is so fearsome, and because its capabilities are considered (and have been stated) to be well beyond what the Imperium is capable of, it is worth noting that the Planet Killer represents the upper-end firepower of all IoM and Chaos warships. Likely the same for Tau or Eldar ships, but probably not for Necrons.
The BlackStone Fortresses/Talismans of Vaul
While it is not their original purposes, the Blackstone Fortresses captured by Abbadon and created by the Eldar Smith-god Vaul to combat the C'tan are also capable of exterimnatus-level destruction.
In practice the effects of the Blackstone fortresses may seem less destructive than what the Planetkiller can achieve. This may seem contradictory, but remember that the Blackstones are not really meant for brute-force exterimination attacks. Its danger (to ships or other targets) comes from its largely technoabble ablities, especially in groups (because of its connection to the warp - they WERE designed to fight the C'Tan, after all.) the sheer amount of energy they generate can be considered more of a side effect than anything. (and note that they still have tremendous firepower, especially given their ability to bypass void shields and the warp-based anture of the damage mechanism..) Brute force attack by raw energy (or force) is more of a secondary element (unlike 40K weapons batteries or lances.)
Special note: Cyclonic torpedoes
Cyclonics deserve their special place because they are in many ways the "upper end" of the Imperium's destructive capability with regard to Exterminatus. It has previously been conjectured that they are technobabble weapons, working on some sort tof exotic, "chain reaction" much like a phaser. However, ,given the abilities to "burn off" the atmosphere or "vaporise" a planet, (as well as the fact some sourcecs do indicate they are brute force weapons or may accompany brute force weapons during bombardment, such as the Last Chancers omnibus) they can likely be considered "brute force" weapons with "technobabble/chain reaction" energy generation mechanisms ( That is, the way the effects are generated are technobabble, but they still are "brute force", at least in terms of their effects on the planet. They may or may not be effective against other targets.)
I am aware that some examples of Cyclonic bombardment seem to result in the destruction of a planet. I will not address those, because I know little of them. We do know from Tactica Imperialis (as described above) that there are many forms of Cyclonic bombardment, and that they are quite brute force (particularily evinced by plasma bombardment.) Brute forcee methods like plasma and radiation probably require multiple bombardments (like Ariadne V and Tethris, mentioned in the Tyranids codex. Ariadne V was the one that had its atmosphere strtipped off by "Damnatus-pattern mass yield cyclonic saturation." Tethris was destroyed by cyclonic torpedoes launched by a Cobra-class destroyer squadron, with 1/3 of the torpedoes being intercepted, the rest striking. The bombardment cremated the entire Tyranid invasion force. Neither example is specifically described as "Nucleonic", although the "nucleonic" cyclonic warhead unleashed in Tactica Imperialis only required a single cloaked vessel delivering a single torpedo (to wipe out the entire Tyranid invasion force.) This is different from the other two examples, so we can safely conclude that nucleonic bombardment was not used in either case (probably brute force.) Another possible factor against nucleonic warheads is their implied rarity - the "process" (according to tactica Imperialis) ws supposedly only known to the Techpriests on Mars, which implies very specialized knowledge. So the "brute force" methods may be more common.
In Ariadne V's case, it woudl require at least e26-e27 joules. Tethris had been "nearly drained' (including atmosphere and oceans) by the Tyranids, and this implies over 1e21 kg of biological matter. To vaporize/incinerate that much mass requires around 3e27 joules. If this is brute force bombardment, this could very well imply a squadron of Cobras could carry a fraction of that firepower as standard in its ordnance: We dont know how MANY torpedoes were launched, but ti seems likely that they used the Cobras own torpedo tubes to do it, which implies that the cyclonic warheads could not have been much larger than regular plasma torpedoes (although they may have sacrificed propulsion or guidance for greater payloads, given that a planet is not that hard a target to hit from orbit and accuracy isn't an issue where Exterminatus is concerned.) We also have no idea HOW many torpedoes were launched, but even assuming they emptied their entire torpedo loadout in cyclonics (assume thousands) that's still many tens or hundreds of teratons of firepower "per torpedo".
In any case, planetary destruction is well beyond the ability of the vast majority of Exterminatus (including hard bombardment) except in extraordinary cases (volatile planet, like the Night Lords bombardment or the destruction of Caliban by the Dark Angels), since we know the Planet Killer's effects were considered so horrifying. It may also be that other factors play a role in cyclonic bombardment as well (maybe there is somethign volatile on planets destroyed by cyclonic bombardment that the attack triggers inadvertantly.) A distinct possibility is that the "planet destroying" cyclonic torpedo was a very large/powerful nucleonic warhead (single missile, and if it converted enough of the planet's mass ot pure energy it could very well blow it up.) Given what we know, I'm inclined to believe that nucleonic warheads can blow up planets, especially given the implied rarity of the nucleonic warheads (a planet-shattering nucleonic cyclonic torpedo would probably be even rarer or harder to build.)
I'm sure that everyone is very bored by this point by all of the above, so I'll just finish up by covering just what the numbers mean in terms of capital ship firepower.
From above, we have approximately four effects, with associated energy ranges:
Complete Sterilization - 4e25 joules
(I know that the "sterilization" bit was a bit broader than that 2e25 to 8e25+ joules, but I decided to approximate an average.)
Burning off the Atmosphere - 3e26 joules
Vaporising the Oceans - 3.9e27 joules
Burning off Ocean Vapor + Atmosphere - 8.5e28 joules
We also have timeframes of between Two and twenty four hours Two hours will be the upper limit, twenty four the lower limit.
24 hours - 4.63e20 watts
2 hours - 5.56e21 Watts
Burning Off the Atmosphere
24 hours - 3.47e21 watts
2 hours - 4.17e22 watts
Vaporising the Oceans
24 hours - 4.52e22 watts
2 hours - 5.42e23 watts
boiling off atmosphere and ocean
24 hours - 9.84e23 watts
2 hours - 1.18e25 watts
Okay, so we have the sustained firepower rates now. As mentioned before, if you wanted to be conservative this would represent battleship firepower (since we know a Battleship can be an exterminator) but in reality this probably represnts what a cruiser can do. (and even a cruiser or heavy cruiser is probably somewhat consevative). A battlecruiser and Battleship, therefore, are arguably much more powerful (at least an order of magnitude, if not two.)
On the other hand, Abbadon's planetkiller is STILL more powerful than an individual 40K warship (upper limit.)
Hopefully, I'll try working to refine some more concrete firepower distributions, but it is worth noting that based on this, its safe to say that 40K battleships are probably in the "high teraton-low petaton" sustained firepower range.