However, if you didn't pass the "Repeal U.N. Charter" bill in the Planetary Council, you'll be expelled from the Council and every faction will declare Vendetta on you. There's no way to negotiate peace after this, since they've labeled you a monster from launching a Planet Buster.
Even if you passed the "Repeal U.N. Charter" bill, they will still kick you out from the Council, team up and declare vendetta against you. Nervestapling and biological/chemical warfare are OK, but if you want to use planetbusters, you better make sure, you can take on them all (and the mindworms, because Planet will hate you, too) or have enough planetbusters!
Within the Alpha Centauri folder is text file (flavor.txt) that explicitly states that the basic Planet Buster is a Mk. 714 Plasma Bomb with an active kill radius of 2000 kilometers, and the explosive force of 296 billion tons of TNT. No wonder Planet goes total bugnuts when you use one. And remember, you get this when you research Orbital Spaceflight; Discover 6. The research tree goes up to 16, making it a literal Disc One Nuke.
The Manifold Nexus. Fullstop. If you have one within your territory, you get +1 Planet for free. If you are lucky, and make Planetfall near it, GET IT. Being a potential third faction (after Deirdre and Cha'Dawn) to catch mindworms, is a huge boon in early game. It's probably the reason why Miriam and the Usurper, who have attacking bonuses, also have -1 on Planet.
While all the quotes are pretty awesome, the Universal Translator and Clinical Immortality quotes in particular make it clear that you're doing nothing less than subverting the will of God - and getting away with it!