User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5379655-20130217225711

I think I just found a satisfying answer to your question! I was playing ME1 this morning and I was reading the codex entries and stumbled across one for sensors. The reason why you can't shoot at PDC's from the edge of the solar system is because ships have a limited sensor range dictated by the speed of light. Ship active sensors used for targeting information in the ME universe is called LADAR, which is essentially firing a laser at the target that bounces back to original sensors which helps to measure distance and speed of the target in question. However, they have not developed FTL sensors, so the LADAR beams are limited by the speed of light. Pluto is 5.75 light hours from Earth, therefore, you would not be able to get reliable targeting information because wherever you see the earth from the edge of the system is actually where the earth was 5 hours and 45 minutes earlier. That's why battles don't start between dreadnoughts until they have come with a few kilometers from each other when the time lapse is non-existent and that is also why Frigates are still utilized by the armies of the future, because they are still useful for scouting and picket jobs. It's all in the codex for ME1, should answer your question sufficiently.