And they would wander into the inner Solar System so frequently that such impacts might completely devastate Earth as often as once every 100,000 years -- enough to make absolutely certain that metazoan life would never have the slightest chance to even start evolving on this planet.

Moreover, the fact that we have gas giants big enough and appropriately placed to serve as our protectors is by no means a high probability, especially given the fact that most gas giants apparently migrate a good distance inward from where they fist formed.

Indeed, Wetherill's simulations have shown that smaller Uranus-sized gas giants at the same distance would instead deflect the orbits of comets just enough to substantially increase the number of comets entering the inner Solar System instead!

In his own Conference talk, John E. Chambers announced still another possible complication.

The growing feeling is that the near-Sun rocky debris from which the inner planets formed was very short on water, and that the water supplies of early Venus, Earth and Mars came mostly from the barrage of icy outer Solar System objects that did hit those planets during the Solar System's early "heavy bombardment" period.

But with Jupiter and Saturn clearing out most of the comets that would otherwise have entered the inner System, the only adequate supply of such icy objects would be the asteroids from the outer half of the Asteroid Belt -- and Jupiter's gravitational tuggings also tended to nudge many of those into entering the inner System and colliding with the inner planets.

Had Jupiter been even moderately farther from the Sun, it would still have protected the inner planets from a huge overdose of comets -- but it might also have deprived them of the early rain of icy asteroids that gave them most of their water. On the other hand, if Jupiter was moderately closer to the Sun -- say, within 450 million km -- its tuggings would have either thrown the inner planets into disastrously eccentric orbits or prevented the rocky debris in the inner System from coalescing into planets at all, so that the Solar System's inner rocky parts would just hae consisted of an asteroid belt.

As Chambers told me, "Jupiter and Saturn are fortunately placed."

So what does all this mean for the chances of complex life elsewhere in the universe? Ward repeated his and Brownlee's belief that there are very large parts of the universe that are "deserts" as far as the development of complex life is concerned.

During the universe's early days, the first generation of stars had not yet converted its initial hydrogen and helium into "metals" (by which astrophysicists mean all elements heavier than helium) -- so there was very little solid material, even ice, from which planets of any kind could develop.

During the universe's distant future, billions of years from now, the supply of uranium, thorium and radioactive potassium -- which provide the internal heat that drives a planet's crustal tectonics and volcanism -- will have greatly shrunk; and without such crustal recycling it will be almost impossible for a planet to maintain the atmospheric carbon dioxide that it needs to maintain and stabilize its climate at a good temperature for life.

(Even large rocky planets will then behave like Mars.)

And even today, small non-spiral galaxies are thought to be poor in "metals", as are the outer regions of spiral galaxies -- including our own -- making planets much rarer in them.