In my previous reports on the First Astrobiology Science Conference April 3-5 at Ames Research Center, I talked about the possibility of life elsewhere in our Solar System -- specifically, Mars and Europa -- and the techniques that may be used to detect it.

But there is no hope of finding anything more complex than one-celled microbial life in this solar system. To have any chance of discovering multi-celled life -- let alone intelligence -- on other worlds, we have to look at the planets of other stars.

This is a staggeringly difficult undertaking, but over the past decade we have finally begun acquiring the technologies that will let us make a serious effort to do so.

Over the past five years alone -- even using the preliminary tools we have now -- we have confirmed the existence of over 30 planets of other stars. Planets are indeed common in the universe.

But at the same time, our first look at the structure of these alien solar systems has produced a series of major surprises -- and, along with other recent scientific research, it has begun to raise another serious question: just how many extrasolar planets are actually habitable for life -- and especially for advanced, multi-celled life? In this report, I'll give a rundown on the Conference talks and posters on these issues.

It has become clear beyond doubt in recent years that planetary systems around other stars are very common indeed.

In one talk at the Conference, David Koerner described his recent microwave and infrared mapping surveys of "classical T Tauri stars" -- very young stars still surrounded by thick dust and gas clouds -- in which he examined 20 such stars whose clouds contained more dust than that which has been theoretically calculated as being necessary to produce a solar system a big as our own (these make up about one-third of classical T Tauris, which in turn are extremely common).

In every one of them, the dust cloud turned out to be a rotating flat disk of just the sort which is supposed to become a planetary system.

Moreover, he found that many of the disks had gaps or concentrated rings in them of a sort which strongly indicates that giant planets are already forming in them.

Koerner also pointed out that at least 15% of nearby mature stars have much fainter but still detectable dust disks around them -- and it seems likely that such dust can only come from comets and asteroids colliding or drying up near such a star.

Many astronomers think that over the next few years, as our instruments become more sensitive, as many as half of all mature stars will turn out to have such dust disks -- that is, half of all stars are orbited at least by very large numbers of comets and asteroids.

It seems certain that a very large number of such stars also have planets; we may reasonably hope that as many as one-third of our neighbor stars do.

But as we've actually detected the first known extrasolar planets and determined their orbits, it's quickly become clear that "solar system" and "habitable solar system" are not at all the same thing.

The only techniques we have had up to now for planet detection have been sensitive only to giant planets (Jupiter-sized or larger) close in to their stars, where no one seriously expected to find them.

It was thought that only in the cold outer reaches of the disk-shaped nebula out of which the planets formed -- where the nebula's relatively sparse rock grains were accompanied by much larger quantities of ice grains -- could solid proto-planets accumulate that were large enough to pull in massive additional quantities of the nebula's gas and become giants.

But about half of the new planets found thus far have been incredibly close to their stars -- usually far closer than Mercury. And most of the remainder have not only been at the same general distances as Venus, Earth or Mars, but they are in wildly eccentric orbits -- sometimes almost twice as far from their star at aphelion as at perihelion -- unlike the orbits of any of our own Sun's planets.