Fights around a celestial body only involve those objects in orbit around said body. So, if Player 1 attacks Player 2 at planet 12, even if Player 2 has other ships at Planet 13 in the same system those ships will not be part of the fight unless Player 2 orders them to move to Planet 12, at which point they would auto-engage. Planets also only engage with targets in orbit around them.
Be aware also that planets cannot be conquered as long as the planet owner has defenders in orbit. If the player you attack is also the planet owner, the planet will be beating on the attacking ships for the whole engagement. Even if you beat the planet down to a zero defense, it will not flip until the last defending ship or orbital is destroyed. Planets are also kind of stupid at the moment, they will target more wisely in the near future.
Originally there were options about whether to auto-move, or chase people to other sectors, but we dropped all of that in the interest of a simple UI.
Edit: Just to be clear, -all- of the mobiles which are owned by the players fighting and which are in orbit around the planet/sun will fight. Innocent bystanders in orbit will not be damaged, nor will they engage. They will report to their owner that a fight has started, in case the owner wants to join the battle.