How is that even possible???
Some general tips:
- Send 1-2 ships periodically to scout the neighbouring planets (to keep track of enemy fleet size).
- Keep some ships in the captured planets to deter enemy attacks (except when done deliberately to bait the enemy fleet into a pincer attack).
- Build more low-tier factories before upgrading existing factories (since the enemy fleet can blow up the factories when there is no ship to defend the planet).
- Retreat often when capturing a new planet (instead of fighting the initially overwhelming enemy fleet).
- Always attack with around double the enemy fleet size, since more ships will destroy enemy ships faster and thus lose fewer ships in combat.
- If all goes well, this should take 30-40 minutes to capture all planets.