Apparently Stephen Hawking read my book, but not very carefully, because he thinks aliens will come here ala "Independence Day"* and eat up all our resources and move on.
I disagree with him. I think in fact it's more likely that an aggressive alien race would create self-replicating robot probes that will disperse through the galaxy and destroy all life that way.
But more likely still doesn't equate to likely. I've been thinking about this on and off for a few days, in fact, and I suspect a likely answer to Fermi's Paradox -- "Where are they?" -- is simply that intelligent life that is capable of interstellar flight doesn't last long enough to colonize other stars. That would neatly explain why, if stars with planets are common (which we know is almost certainly true), and the conditions for life to arise are relatively common (again, that seems very likely), the galaxy isn't overrun with life. It should be by now; it's had billions of years to have space-faring races evolve and colonize the whole shebang.
So in reality, Hawking's idea and the one I go over in my book are probably wrong. But I'm an optimist, and I can hope that the reason the galaxy isn't softly humming with life (that's Carl Sagan's poetic phrase) is that we're the first, or at least the first in a while. That would mean we still get our chance. It's a big responsibility, really.
And to be clear, that's not snark, even if this post started out a bit snarky. I'm serious. We may be utterly, entirely alone in a galaxy filled with planets that outnumber people on our own planet 50 to 1. That idea gives me the creeps more than the idea of hostile aliens bent on sterilizing each of those planets. But at least it gives us a good chance to spread and see the place a bit. I'd like to think that in a hundred generations, this arm of the Milky Way will boast a thousand human planets. It's a nice thought.