Solar system combat clarification pls

Without notifications, trying to figure out the combat system is a challenge to say the least. What I'm trying to figure out is this:
When a planet is attacked, do ALL ships and defensive platforms that the defender owns in that SOLAR system participate in the battle?? Or do only the ones around the planet being attacked fight.
May seem obvious, but I'm confused...
Thanks

Just the ones at that planet

There's a Fleet command unit you can research into that supposed to allow the whole solar system to help defend while the attacker can only "attack" the planet defending units at the planet, but I believe it didn't mesh with a combat update so it hasn't been re-implemented yet.