[Templar Mikeel]

Exterminatus

Exterminatus is typically only used as a last resort, when the price of holding or retaking the world is deemed too high or the threat of mutation or heresy too great to contain by conventional means. Thus Exterminatus has very rarely been used on vital or valuable planets to the Imperium. Typically, the order may only be given by a high-ranking Imperial Warmaster, a member of the Inquisition or a Space Marine Chapter. Exterminatus can be carried out in many different ways, as outlined below
Note that with the exception of those listed under other, the rest are deployed from space.

Orbital Bombardment
Orbital bombardment is the simplest means of carrying out an Exterminatus, as it can be done solely through the conventional firepower of the Imperial Navy. The power of an orbital strike can range from small precision strikes, not unlike that of modern Surface to Ground missiles, to thermonuclear blasts. Typically however, special weapons are used, designed to destroy all life on a planet's surface and potentially any entrenched in the planet's crust.

Virus Bombs
Virus Bombs are potent weapons of mass destruction, and were commonly used to carry out Exterminatus in pre-Heresy times. In this role they have largely been superseded by Cyclonic Torpedoes, however Virus Bombs are still retained by some Imperial arsenals. They contain a virus deadly enough to kill all life in a certain region, and if enough bombs are dropped the entire planet could be sterilized. These are only used in the direst of situations as it takes time for the virus to die down and kills off all the life in the planet, removing the potential work force. Virus bombs release a special virus designed to quickly spread and destroy cell structures, reducing all planetary life into organic slime. This produces huge volumes of flammable gases as a by-product, which is later ignited and turns the atmosphere into a firestorm that usually takes out whatever is left. Only those who may have escaped into especially-deep underground shelters can survive a Virus Bomb attack. The Tallarn population managed to escape complete destruction this way, but the once-verdant world was forever transformed into a desert. Virus bombs are known for their extreme speed; in the fiction surrounding the Horus Heresy, they completely wiped out Istvaan III's entire population of 16 billion in just minutes. Virus bombs were used on Armageddon during the Second War for Armageddon. Von Strab had a secret stash of them, put there by his family when the planet was colonized. They wreaked a terrible havoc on the invading Orks but had lost their potency over time and did not have exactly the desired effect. The virus also has a tendency to blow with the winds, and during the Armageddon deployment, it washed back over the defending Imperial Guard, killing as many defenders as attackers.

Cyclonic Torpedoes
Cyclonic torpedoes are much more immediate in action, having enough power to directly ignite a planet's atmosphere. They are the primary means of enacting Exterminatus. Usually deployed by Inquisitorial vessels specially designed for the task, Cyclonic Torpedoes are designed to penetrate the outer crust of a planet and detonate near its core. The detonation of multiple high yield nuclear warheads near a planets core is usually enough to trigger massive volcanic eruptions, and tears open the tectonic plates. The blast radius is large enough to be seen from outer space.

Two-stage Torpedoes
Two-stage torpedoes are the most common of a special class of rare Exterminatus weapons, designed for use against planets without an atmosphere or biologically-void worlds (Necron Tomb Worlds being the main example). These have two-stage warheads: The first stage is a melta charge that bores straight through a planet all the way down to its core. The second stage is a modified cyclonic charge that destabilizes it, in most cases physically destroying the planet.

Other
During the 13th Black Crusade, Lord Castellan Ursarkar E. Creed ordered a massive nuclear reactor on the prison world of St. Josmane's Hope to be overloaded when it seemed that Chaos would finally take the world. Rather than a colossal meltdown, the reactor exploded and blew the entire planet apart.

Scources: Wikipedia, Lexicanum