Couple Of Things About This:
- Moons Count.
- It Doesn't Have To Be A Thriving Civilization, Just Landing On The Planet Counts.
- Starting Planet Doesn't Count.
If You Want It As Fast As Possible In One Playthrough:
- Go To The Moon, Terraform It.
- Go To Rock Planet, Grab Cracker Piece, Build Observatory.
- Go To Ice Planet, Terraform It, Grab Cracker Piece.
Save Your Game.
You Can Go For Endings From This Point, Or Go For The 5 And 10 Planet Colonization Achievements.
Achievement Unlocks After Three Total Planets Are Visited Using The Colonization Rocket.
(Again, First Planet Doesn't Count, And Any Moons Do Count)
Good Luck!