|Class Name :
|Unit Run :
Unknown 10,000 built in total.
||Length : 250 m 
Beam : 88 m
Maximum Diameter : 176 m
443,000 metric tons
Unknown; possibly one individual 
1 x Energy weapon of unknown type, capable of destroying planets in concert with 8 bioships , total output 7,500,000 TeraWatts
|Defence Systems :
||Standard Bioarmoured  Single hull.
(TNG scale) :
|Strength Indices :
(Galaxy class = 1,000)
|Beam Firepower : 150,000
Torpedo Firepower : -
Weapon Range and Accuracy : 7,500
Shield Strength : -
Hull Armour : 600,000
Speed : -
Combat Manoeuvrability : 6,320
|Overall Strength Index :
|Diplomatic Capability :
|Expected Hull Life :
The Federation has relatively little knowledge of species 8472 and its vessels, and what little information there is concerns the standard bioship. The only information regarding the planet destroyer is the little Voyager was able to gather during the attack it observed on a Borg planet at the end of 2373.
The planet killer is somewhat larger than the standard bioship, but is of a generally similar configuration. The major external difference is the ring of eight apertures around the midsection. The main weapon of the planet killer can apparently only operate in concert with a group of standard bioships; these fly in formation with the planet killer and fire beam weapons into the apertures. The planet killer then fires a single beam from the nose at the target.
In the incident Voyager observed, a single discharge of approximately ten seconds was sufficient to destroy a borg planet. Since significant heating of the crust also took place, this would require the beam to have imparted in excess of 3.6x1032 Joules. 
This is a most impressive feat for such a small vessel; generating this energy from a M/AM reaction would require over 4 trillion tons of reactants! Starfleet can only theorize as to the mechanism used for generating this energy; possibly the energy discharge from the smaller bioships is used to trigger a zero point energy discharge, in much the same way that a quantum torpedo uses a M/AM charge to trigger a ZPE explosion. 
But it is just as likely that the technology involved is far beyond anything Starfleet knows of. It seems unlikely that this weapon could be used practically against another starship, and it is not included in the strength indices above; the output given is for the standard beam weapon which the 8472 Bioship carries, it being presumed that the Planetbuster mounts a similar weapon. Voyager managed to make peaceful contact with Species 8472 after their war with the Borg, forestalling a possible attack on Earth. Given the level of firepower possessed by this species, that is undoubtedly one of the intrepid little ship's greater accomplishments. 
The 8472 planet killer was seen briefly at the end of "Scorpion, Part I", where it destroyed a Borg world in concert with eight of its smaller cousins. The specs table mostly consists of unknowns, while the notes mostly focuses on the planet slaying role.
The energy required to completely destroy a world - or any body in space for that matter - is that needed to impart escape velocity to its entire mass. An Earth-type world has a mass of about 6x1024
kilos, and an escape velocity of about 11 km/s. Using the kinetic energy formula of 1/2mv2
, we get an energy required of 3.63x1032
Joules. The true figure would be a little above this since the Borg planet was also heated considerably in the blast, as evidenced by the fact that most of the crust was glowing by the time it exploded.
The timescale for the destruction is not completely clear; the bioships fire an eight second blast, then we cut to Janeway inside one of the cubes for a reaction shot before cutting back to see the planet exploding. So the strike on the planet could last anything from 8 to 12 seconds. Taking the midpoint of 10 seconds, the planet killer would output about 3.6x1031
The 'official' stuff on how a quantum torpedo works comes from the DS9 tech manual. As usual regarding this source, take it with some caution.