Less than ten ET civilizations?

There's a new post up on the arXiv blog (a blog that scours the arXiv physics pre-print server for the most interesting physics papers being put up) that's relevant to one of our recent topics of discussion. It concerns the Fermi paradox, which asks why we don't see signs of ET civilizations if they're actually out there. Here's part of it:

Cotta and Morales take a different tack by studying how automated probes sent ahead of the colonisation could explore the galaxy. Obviously, this could advance much faster than the colonisation wavefront. The scenario involves a civilisation sending out 8 probes, each equipped with smaller subprobes for studying regions that the host probe visits.

This is not a new scenario. One previous calculation suggests that in about 300 millions years these 8 probes could explore just 4 per cent of the galaxy. The question that Cotta and Morales ask is: what if several advanced civilisations were exploring the galaxy at the same time? Surely, if enough advanced civilisations were exploring simultaneously, one of their probes would end up visiting the solar system. So that fact we haven't seen one places a limit on how many civilisations can be out there.

The numbers that Cotta and Morales come up with depend crucially on the lifetime of the probes doing the exploring (and obviously on the number of probes each civilisations ends out). They say that if each probe has a lifetime of 50 million years and that evidence of them visiting the solar system lasts for about a million years, there can be no more than about 1000 advanced civilisations out there now.

But if these probes can leave evidence of a visit that lasts for 100 million years, then there can be no more than about 10 civilisations out there.

Of course, we may not have discovered the evidence yet. And when we finally find the black obelisk on the Moon, the paradox will be resolved.

It seems to me that the last sentence is the key bit. Finding an alien probe--assuming one were present in the solar system--would be hard: it's the ultimate needle in a haystack. If it were just somewhere on earth, perhaps buried in some deep layer of rock or something, that would be hard enough. But if it could be anywhere in the solar system you could easily never find it. It could be on our moon, as in 2001, or it could be on one of the other scores of moons throughout the solar system. Or it could be on another planet, perhaps having fallen into the clouds of Jupiter millenia ago. Or perhaps it's just floating in the asteroid belt. Or orbiting outside of it somewhere else in the solar system. Frankly, I can't see how this study could make any sense, given these practical points. Though, in an abstract way, it's a very interesting use of brainpower.