[Suicide Junkie]

Give the fleet "Capture planet" orders.

The warships will attack the planet until all of the platforms are dead.
The warships will then back off and left the transports drop troops.

If the transports fail, then the warships will glass the planet.

If the planet has a very low population, and can't survive the splash damage from the destruction of the platforms, send in transports only. You can make a bunch of faster, heavily shielded, decoy transports to draw off fire if you like.