A. Orbital Bombardment is done by non-transport warships.
B. Ships CAN NOT bombard a planet with defending ships in orbit. That is, like transports, ships must attack and defeat defending ships first.
C. The amount of damage done by bombardment is determined by the amount of damage points the ship/fleet is capable of. That is, a ship with more weapons, OR a ship with more advanced weapons does more damage when bombarding. Another example being that a fleet armed with lasers and stingers would do a TINY fraction of the damage done by a ship with phasors and photon torpedos.
D. Bombardment damage is incremental. Meaning it is dealt out over a period of turns, the amount of damage per turn being decided by "item C." above.
E. Bombardment damage is equated to Population, Planetary Improvements, and Planet Quality.
F. Bombardment damage affects Planetary Improvements more than Population, and Population more than Planet Quality. That is, once Bombardment damage reaches a certain point, X number of improvements are destroyed, Y amount of population is killed, and Planet Quality drops Z amount. It takes more damage to kill a certain amount of people than it does to destroy improvements, and even more damage to drop a planet's quality.
G. You cannot bombard a planet with any given ship/fleet more than once per turn.
H. You can determine what percentage of your ship/fleet's power is used in the bombardment via a slider. For example, you can bombard with 25%, 50%, 75%, or 100% of total power.
Alternatively you could bolster a planet's bombardment defenses with improvements, like is done with a planet's invasion defenses now. For example at some point you could develop planetary shields that could absorb a certain amount of bombardment damage each turn, or perhaps a certain amount of damage before colapsing and being destroyed, this could even take several turns.