Yep!
Well, that's an interesting point. I disagree with most of what you said, notably because the planet was only in peril because of the PCs' actions. This evolved out of their meddling in a dynamic situation.
A plot is defined as, "The plan, scheme, or main story of a literary or dramatic work, as a play, novel, or short story." (Source: dictionary.com).
This generally aligns with my own definition, which can be roughly stated as, "The events that play out in a story, deriving from the interactions of the characters and the environment."
Now, let's break down some of the assumptions you're using:
1) The planet being in peril was planned by the DM
2) The PCs could not realistically avoid participating, or could only avoid participating with great difficulty
As stated above, the planet was only in peril because the PCs had interfered in a particularly nasty corporate scheme with an insane AI. The corpos decided to glass the planet after the PCs interfered to avoid witnesses (they figured the PCs could be bought off, silenced, or discredited). So I didn't plan anything - my notes never mention the possibility the planet could get ganked. I set up the dominoes, and the PCs kicked them over.
As for assumption 2, the PCs could have bugged out, no problem. And half of them wanted to do just that! Only one player decided to force the issue and intervene. But they didn't have to shoot it down. They could have jammed the bird's tracking, pushed something else in the missile's way, evacuated some of the populace, etc. So they could have very easily avoided participating, even accounting for the one character with a strong moral compass. Hell, they could have just left the system alone, too.
If you're curious about my philosophy on plotting in RPGs, let me know and we'll start a new thread on that. God knows those never blow up in insane wrangling debates.