"Where are the Von Neumann (sp?) probes?"
IIRC, Von Neumann & his contemporaries were involved in a discussion along the lines of...
1) We on Earth are obviously a later (3rd?) generation star system. If sentient life had evolved elsewehere, then surely by now we would have seen some automated probes from one of these earlier civilizations! (exasperatedly)
or
2) If such a civilization has ever existed in the galaxy, they must have destroyed themselves before they had a chance to build the probes.
...which isn't very encouraging. Some people think perhaps no such civilization has ever survived, for instance, their Atomic Age.
I actually think it's a pretty decent argument.
The problem is that we haven’t actually observed population I stars. No pure Hydrogen + Helium stars have been found. We have found population II stars which are typically metal poor.
Which raises the question. If there aren’t any of them around, perhaps they are all gone because they were all large stars and no small ones.
If that’s the case, then it’s entirely possible that none of them would have planets, let alone habitable ones. Especially if we are talking upwards of 100 solar masses.