Meh...
I think I'm with TM on this one. I'm not saying that
every intelligent species will leave its home planet
on its own, but
that it will be forced to do so. Think about it. What will happen if it doesn't?
A) A big chunk o' meteor is bound to come around sooner or later and blow you to smithereens. That's a
certainty. It's just a matter of time. A few million years? That's nothing on an evolutionary time scale. The dinosaurs were around for more than that. (and look what happened
)
B) To avoid (A) you must:
i) have some way of blowing the rock away before it hits the surface. If that is the case, you've left your planet's surface, even with robotic probes, or missiles, or whatever. The tech to leave your planet is there. QED
or ii) Figure out some way to survive after it hits and causes impact winter and so on and so forth. If that happens:
C) Sooner or later, an expanding stellar empire (like the Ancients or the GTVA) will come knocking on your door. Again, a certainty, just a matter of time. It might be i) friendly (in which case you get to be buddies and share techs) or b) hostile (in which case you need to develop space tech to defend yourself. If you don't, you're gonna get beaten to a pulp from orbit).
D) If you share techs and enter into some kind of alliance, in order to compete with the other species economically and politically, you will need to expand.
So, it's fly-where-no-man-has-gone-before...or die.
I think.
If you can point out any flaws in the argument, I'd be glad to debate them...