Orrin Judd emails me with a link to a very nice (and long) essay in the Atlantic on exploring space from your computer.

I haven't read the whole thing, but glancing over it, this paragraph jumped out at me:

A ghostly mass of battered rock, Earth's satellite is an archetypal solar-system object, with surface features echoing those of many of the planets and moons arrayed in far-flung archipelagos around the Sun. But it's much more than that?at least in the human context. The longer one considers it, the more its tidal influence grows. Without that luminescent lure would there even have been a pull to leave this planet?

The Moon is much more than that. Without it, we might not have developed the mathematics needed to get us to it. But more fundamentally, it's possible that we wouldn't exist at all--without the tides to periodically strand creatures in the shallows, none of them might have ever transitioned from the ocean to the land.

I remember reading an essay by Asimov many years ago, in which he described all the ways that the Moon might have been critical to the development of both life in general, and to intelligent life, and our own development in particular. In fact, I believe that he argued that such a large secondary body might be a necessary prerequisite for LAWKI (at least for it to develop naturally), and that this added one more factor to the Drake equation, further narrowing the odds of finding company in the universe.

My opinion is that the physical tides affecting Earth may have been a necessary force in bringing forth life on our planet -- which leads me to think inhabited planets may not be as common as the Golden Age science-fiction writers speculated, unless the second sun in a binary-star system can take the place of the second planetoid in a binary-planet system like ours (or, on another scale, the Jupiter-Europa relationship).

If so, I wonder if SF writers in a binary-star solar system write stories about what life would be like in a one-sun solar system, and whether they consider the binary-planet model at all?

(After seeing the paragraph about Asimov) Dang it -- that'll teach me to read the whole post before commenting...

Kevin, take a look at the book "Rare Earth" by Ward and Brownlee. They make a compelling case that the evolution of complex, as microbial, life requires an extraordinarily rare set of conditions, such as a planet with plate tectonics, a certain ratio between water and land mass on the planet, a moon large enough to stabilize a planet's axis of rotation, larger outer planets to be "comet catchers," enough oxygen in the atmosphere to support life but not enough to support runaway combustion, etc.

Ward and Brownlee believe that microbial life may be common throughout the universe and indeed in our solar system. But intelligent life may be much more rare, and indeed we may be unique in our galaxy or even the universe!

The part about stabilizing the rotation may have some merit, but I think the tides being responsible for land animals is not compelling. Somebody please correct me if my information is outdated (benn a lotta years since college), but I recall learning that most vertebrates, including the teleost fish and all land vertabrates, are likely descended from fresh-water lungfish. Fresh water certainly lends itself to colonization of land without tides.

For the rest of Harry's comment, some or all of those conditions may (or may not) be required, but are likely common anyway. Plate tectonics is largely a function of the planet size. Large outer planets may be the norm for planetary systems (they certainly appear to be common enough with recent discoveries).

And oxygen is not required to support life, it is a waste product of photosynthesis (which, of course, is almost certainly necessary for multicellular life). The oxygen level is self-limiting, since too much leads to burning off the land plants that form it (conceivably, I suppose, the sea plants could do it - anybody have an opinion on this possibility?)

I've read that not only are we the right distance from the galactic core (not too far out, not too close in) but we are also in the right place in the spiral arm. Apparently most stars are not able to support life (I don't remember the reason given for this to be so, just the fact that the Earth happened to be in the right place for life to be possible.)

This doesn't prove anything of course, however I find it deeply satisfying. I'm still trying to fathom the cosmic joke that makes the sun and moon's apparent size such that total eclipses occur.

Ken, you're right that most stars can't support life. The two biggest reasons for this are (1) most starts are in the galactic core, and (2) a large fraction (may be a majority) of starts are binaries which don't form planetary systems.

But even so, there are more than a billion stars in the Milky Way alone, so even if only a small fraction are in the spiral arms, and only a small fraction of those are not binaries, there are plenty capable of having planets.

And you're right about eclipses - how could we be so lucky?

Another reason why many star systems may not be capable of supporting life (or at least not much of it, and then only the simplest) is their lack of heavier elements, most of which are produced in supernovae. Young galaxies are extremely metal-poor.