How to destroy the Earth
An introductory, scientifically sound guide, as to how damn hard it is to actually destroy a planet.
I strongly suspect that any weapon resembling what you describe, would bankrupt the Federation before it was ever fired.
That's not even getting into how the Feddies could get it to the target.
With that out of the way, the way I would go about producing the in-game effect you describe requires:
2 shďps: called here "Ship A," and "Target"
2 mďsns: one to summon and place each of the two above ships
2 wëaps: a two-stage weapon fired by Ship A.
2 gövts: (possibly; you want Ship A and Target to only hate each other, and no one else.)
Ship A's mďsn is set to "Hyperspace in after short delay"
Target is placed by it's mďsn on top of the planet you want destroyed.
Ship A delivers the weapon to the system.
Target acts as an anchor, so that the weapon goes where you want it to go.
Ship A and Target belong to two different gövts that hate each other, and don't care about anything else. This ensures that Ship A will fire its weapon only at Target.
Both stages of the weapon are seekers.
The first stage of the weapon has an enormous blast radius, enormous mass damage, ignores shields, disables-not-destroy, and has a short lifespan. (2-3 frames). You probably want the firing Ship A to die when using this weapon; you can do this by setting AmmoType to -99. The weapon then submunitions into the second stage.
The second stage should be a fast, planet-type weapon. Since it is a seeker, it inherits it's target from the first stage.
The second stage will home in on Target (which should still be alive, since the first stage was set to disable, not destroy. Since it is a planet-type weapon, it will hit the planet Target is placed on top of, and destroy it.
In the simplest terms:
Ship A pops into the system, and fires at Target. Firing the weapon causes Ship A to die and disappear, quickly. The weapon explodes quickly, disabling all ships in the system, and releases a submunition at Target. Since Target is sitting on top of the planet you want to destroy, and since the second stage is a planet-type weapon, the planet gets destroyed.