From Orson Scott Card Wiki

The Molecular Disruption device', commonly known as MD device or Doctor Device as a play on the acronym, was a powerful weapon capable of destroying a planet. The device, as described in Ender's Game, used the focal point of two beams to create an atomic state in which electrons cannot be shared. When this happens, the molecule splits into separate atoms, which create a chain reaction in inducing a similar splitting state in surrounding mass. The more massive the target, the greater the firepower of the weapon. Hence, the weapon was generally fired on clusters of starships during the Third Invasion. At the final battle of the Bugger Wars, the Doctor Device was aimed at a planet itself, which resulted in the planet's destruction "within three seconds," as the Ender's Game novel describes.

The MD device was also used during the attempted Second Xenocide when Lusitania was ordered to be annihilated by Starways Congress. The weapon, however, did not strike its target, as Jane moved the missile back onboard the firing ship, resulting in its deactivation by one of the crew.