We all have our suppositions, our scenarios. The late astronomer Carl Sagan estimated that there are a million technological civilizations in our galaxy alone. His more conservative colleague Frank Drake offers the number 10,000. John Oro, a prominent comet researcher, calculates that the Milky Way is sprinkled with a hundred civilizations. And finally there are skeptics like Ben Zuckerman, an astronomer at UCLA, who thinks we may well be alone in this galaxy if not in the universe.
All estimates are highly speculative.
The fact is that there is no conclusive evidence of any life beyond Earth. Absence of evidence is not evidence of absence, as various pundits have wisely noted. But we still don't have any solid knowledge about a single alien microbe, a solitary spore, much less the hubcap from a passing alien starship. Even if we convince ourselves that there must be life out there, we confront a second problem, which is that we don't anything about that life. We don't know how truly alien it is.