The planetary politics of an interstellar civilization would be a lot more complicated than science fiction tends to assume.
For one thing, the experience of our own species is that a planet which can send things into outer space is certainly not politically unified on the ground. With the development of cheaper space travel it might be possible to use orbital militaries to unite a planet under one banner -- but unless that banner was flown by a country which had managed to leap a few technological steps ahead, such a Space Force would be met by a Space Force of similar caliber, the resulting war would be horrific, and it might prove inconclusive.
And even if it DOES prove conclusive, you would still have plenty of resentful factions, causing trouble on the ground and trouble above the sky. Claiming to have united a planet, in that case, would be more a statement of opinion than fact.
Or it might just be that, in the face of an extraplanetary military threat, the planet DOES decide to unite, just the same way the nationalist movements of the 1800s saw various peoples each form their own country in defense against other countries. Yet this would not be an easy unity, any more than Italy is, or the United States, or the European Union, or any political entity that brings together multiple regions with a long history of independence and rivalry. Italy might be one country now, but northern and southern Italians still hate each other. The United States is one country, but the southeastern states have been acting like they don't want to be a part of it since the 1830s. On a planetary scale, trying to get the U.S., Russia and China to act like they're part of one big happy family would involve a LOT of forced smiles and clandestine cursing.
So if a spaceship crew is dealing with any allegedly planetary government, they might wind up dealing with a great deal of political factionism and regional chauvinism.
At least for homeworld planets. Colonized planets would more likely have factions based on who got there first, who supposedly put in more effort, who's trying to carry the banner of their home region back on the homeworld versus who wants a fresh start, and so forth.
Plus the possibility that multiple sapient species have colonized the planet at the same time.