In either case, such migrating giants completely sweep away any such small inner planets that form in the star's "Habitable Zone" -- the limited zone of distance in which a planet has a temperature that allows liquid water and life to exist on it.

(Even those planets which might still possess an average distance from the star putting them in the Habitable Zone would be put into wildly eccentric orbits that alternately roast and freeze them.)

Of those stars that do have some sort of planetary system, how many have had that system's chance to sustain life destroyed by this cosmic demolition derby? We don't know for sure.

Only 5 percent of stars seem to have really giant planets -- Jupiter-sized or bigger -- in such close, destructive orbits.

But, as Eric Gaidos has pointed out, many of the remaining stars may lack them not because their Jupiters have stayed at a safe distance, but simply because all their "hot Jupiters" have finished spiraling all the way into the star and been destroyed -- first clearing out all the small inner planets that existed there.

And only in the past few months have we acquired the ability to detect "hot Saturns" -- smaller gas giants that are nevertheless still more than big enough to wreck their inner solar systems. Indeed, Uranus-sized gas giants -- which we can't yet detect at all -- are quite adequate to do that.

Even in those systems where there are terrestrial planets in stable orbits within the Habitable Zone, there are a whole set of possible obstacles to the development of life that have only recently been properly appreciated. First, there is the fact that we don't have any real understanding of the process by which life first appeared (in fact, that lack of understanding is one of the prime motivations in looking for extraterrestrial life).

It is known that life appeared on earth with astonishing speed after the planet's crust became cool enough to allow it (apparently within only a few hundred million years), and the assumption has been that this is strong evidence that the evolution of life out of non-living compounds was automatic and inevitable.

But -- as I noted in my last report -- there is now a serious possibility that life first evolved not on Earth but on Mars (which may have become habitable long before Earth), and was then transferred to Earth by Martian meteorites.

If life took a long time to evolve on Mars, that may serve as evidence that the evolution of life is less inevitable than we had thought.

Even if the evolution of one-celled life is indeed easy, though, it's becoming clear that there are a large number of very serious additional obstacles to that life ever becoming multi celled and complex -- and that laundry list of obstacles was described at the conference by Peter Ward, co-author with Donald Brownlee of the recent famous book "Rare Earth: Why Complex Life is Uncommon in the universe".

Ward and Brownlee think it likely that bacterial life may be very common in the universe, but that "metazoan" (multi celled) life may be very rare (though not non-existent) on other worlds.

As they point out, life itself appeared on earth very quickly, but metazoan life took an amazingly long time to appear -- perhaps three billion years, or two-thirds of Earth's total lifetime up to now.

Why?