But as they point out, one side of the planet would still be shrouded in darkness for months at a time, making it much harder for photosynthetic plants to survive.

(Kasting, I should add, disagrees with the statement I made in "SpaceDaily" last year that such planets would be so thickly shrouded in frozen CO2 clouds that it would be hard for green plants to get enough light to survive on them in any case.

He belives that their cloud cover may not be much more extensive than Earth's.)

Then, of course, there's the problem of giant asteroid and comet impacts, of the type that is now known with near-certainty to have killed off the dinosaurs.

It's surprising that it took paleontologists so long to realize this fact -- it's been known for decades that objects several kilometers across, big enough to have a devastating effect on Earth's ecosystem, exist in large enough numbers to hit us every hundred million years or so.

It would be amazing if impact-caused mass extinctions had NOT occurred -- and as Peter Ward said in his Conference talk, it's now known that there have been at least seven mass extinctions since the start of the Cambrian Age half a billion years ago.

Only the one at the end of the Cretaceous that did in the dinosaurs has yet been proven by physical evidence to be impact-caused -- but plain probability indicates that many of the others were.

And just a few days after the Conference, it was announced that a huge meteor crater has been confirmed in Australia that seems to be the right age to explain either the gigantic extinction at the end of the Permian Age that first ushered in the era of the dinosaurs, or the somewhat smaller but still great extinction that marked the end of the Triassic Age (the first of the three ages of the Mesozoic Era).

These, as I say, were devastating.

The Cretaceous impact apparently wiped out two-thirds of the existing metazoan species on Earth -- and to do that, it must killed almost all of the metazoan plant and animal individual living things on the planet.

The Permian extinction -- whether it was caused by an impact, or by some more gradual phenomenon such as a vast chance outbreak of volcanic activity that disastrously fouled up Earth's climate -- was even deadlier: it killed off an incredible 90% of the metazoan species on Earth at the time.

The fate of all metazoan life on Earth must have hung by a thread during a few of these episodes.

Moreover, we now have reason to believe that with a differently structured Solar System, they could have been much, much commoner.

George Wetherill -- who delivered a Conference talk on the effects that Jupiter could have had on the formation of the asteroids -- has stated that his orbital-mechanics computer simulations indicate that during the Solar System's formative days, Jupiter and Saturn served as cosmic bodyguards of the inner planets.

Their powerful gravity, during the first few hundred million years after the planets had formed, flung almost all of the trillions of comets still inhabiting the planetary parts of the Solar System either completely away from the Sun, or into orbits that take them trillions of kilometers away into the "Oort Cloud", from which they re-enter the inner Solar System only every few million years at most.