Survival on another planet does not require that it be exactly like the earth. It would need to be very close in characteristics, but not identical.

For example, gravity. If the gravity on this hypothetical other earth were, say ten percent stronger than on earth, it would be a little bit of a strain getting around at first, but nothing a normal person couldn't get adapted to. Future generations of earthlings would probably be a bit stockier and more strongly muscled, not through any particular evolutionary changes, but just growing up in a somewhat stronger gravity would accomplish that.

Or if the gravity were lighter. Easier to get around. Which would make returning to earth a bit of a strain to redevelop one's muscular strength. Offspring might be more slender—not unlike the natives of "Pandora," the planet in the movie Avatar, which appears to actually be a satellite of a gas giant rather than an independent planet. And the huge, tall trees tend to indicate a lower gravity.

Considering the different temperature ranges that people live in here on earth, a warmer or colder planet should be no problem, provided the differences are not too extreme.

The concept of the "Goldilocks" planet. Different from earth perhaps, but not too different for it to be "just right."

Ozone layer, yes, that would be a necessity. But if a planet had oceans from being bombarded early-on by ice-bearing comets and meteorites as the earth was, oxygen-producing sea life would more than likely develop, the oxygen would interact with radiation, and an ozone layer would ipso facto undoubtedly develop. As to that sea life (pond scum from which we all evolved), I've heard biological scientists say that, given the kind of primordial soup of which the early earthly bodies of water consisted, plus the input of a bit of energy—sunlight, volcanic activity—and voila!!—organic compounds!

Life has developed in the most unlikely of places: in the deepest depths of the oceans, near "black smokers" (volcanic vents), where the surrounding water reaches 700 degrees (possible because of the immense pressures at that depth). Sunlight doesn't reach that deep into the sea, but the creatures that live there derive their energy from the heat around the black smokers.

[Where do I get this stuff? The University of Washington, just up the pike a ways, has an "Exobiology Department," and a good friend of mine works there.]

The 23 degree axial tilt actually doesn't have anything to do with the moon, it just sort of happened. Almost all planets have some axial tilt. And I don't really see that the moon has anything crucial to do with the development of life on earth. It does provide the tides, which may have initially "stirred the pot" some, but it's debatable whether that's a necessary ingredient for producing life. It IS an extraordinarily large satellite relative to most, and some have said that the earth-moon system really should be considered a double-planet.

A planet, they say, that is the right distance from its primary (its sun) to be in a zone temperate enough for there to be liquid water, and for there not to be at least some form of primitive life, would more than likely be a statistical anomaly.

Mars would be a bitch to try to live on. Too many things humans need just to survive for a few minutes. Any length of time and you'd better pack a lunch. And a fair number of other things.

Venus? Hotter than a pizza oven! No joy there. The late Carl Sagan, however outlined a plan to possibly "terraform" Venus. Large quantities of blue-green algae dumped into Venus's upper atmosphere would gradually alter the atmosphere, start producing oxygen which would interact with the hydrogen already present, eventually lead to rain, which in turn, would cool the planet and produce oceans, plus a number of other things that would make it livable for earthlings.    But it would take several centuries for this to be accomplished, so don't pack your bags yet. Also, it would be very tropical due to it's proximity to the sun. So if you're a snow bunny, forget it!

Sagan did raise the problem of ethics, however. It would not be ethical to "redecorate" an abode to our tastes if "someone else" already occupied the property (although, historically, that sort of thing hasn't stopped us from wiping out the indigenous population and colonizing the place anyway).

As to the physical aspects of intelligent aliens, I've heard many scientists say that even intelligent aliens would undoubtedly be so different from us that we may not even recognize them as any kind of intelligent beings. But then, I've heard other scientists point out that if intelligent life evolved on a planet similar to (even if not the same as) earth, they might very well look a fair amount like us. There are distinct advantages to biological symmetry. Four limbs seem a quite efficient way of doing things; three limbs would not be quite as stable and five would be a bit redundant. The placement of sense organs such as eyes and ears close to the "central processing unit" or brain is a matter of simple efficiency, and if the intelligent aliens had evolved in an arboreal setting as we did, binocular vision would be necessary to judge distances, so their primitive forebears could swing from limb to limb with less danger of misjudging and falling long distances and landing on their keisters. And standing on one's hind legs allows one to look further distances to spot prey or predators. Survival mechanisms that should work well in sufficiently analogous environments.

In a few Star Trek episodes they mentioned "The Theory of Parallel Evolution" to cover for the fact that Central Casting would have an easier time finding humans to play humanoid aliens than they would finding an actor with two heads (one for linear thinking and the other for gestalt thinking), tentacles instead of arms, and wire wheels instead of legs (although that would have been quite a snort—Jack Nicholson could have possibly brought it off!). The Theory of Parallel Evolution is not necessarily a purely science fiction concept. Gene Roddenberry was no dummy.

Quandary:    If we know that dolphins are quite intelligent, but can't figure out how intelligent, and we haven't figured out how to communicate concepts of any complexity with them, we might have some difficulty even with aliens who may look very much like we do. Physical characteristics are one thing, but cultural differences can make a big difference as well.

Douglas Adams could be right! The reasons we assume that dolphins aren't that intelligent and that we are may be exactly why they think they're intelligent, and we aren't! We invented digital watches, cell phones, politics, and war. And they didn't!

Also, GfS mentions the matter of "inter-dimensional travel." Try reading some of the works of Michio Kaku, particularly Parallel Worlds.

What if the Big Bang were a common occurrence? When a massive star goes supernova and implodes into a black hole, where does all that matter go? Could it be that it creates another "universe" somewhere else? Could it be that the Big Bang that began our universe was the result of a supernova imploding into a black hole in some super-universe? Perhaps this is a common occurrence. Each universe spawning other universes.

Perhaps "universes" (more precisely, "multiverses") would look to some super-entity outside of it all (God? Nah, let's not go there!) a bit like a bunch of grapes of infinite number, each one growing out of another, which, in turn, spawns others.

And closely packed, so that they contact each other, separated by "branes" (membranes).

And sometimes there might be "leakage" through branes!

Along with this bit of weirdness, Kaku points out that there are some eleven dimensions (the math exists to prove this, but that's way beyond my comprehension!). We have direct awareness of three, with a sense of the fourth. But the other seven are there, interlaced all around us—and through us.

Possible fleeting awareness of another dimension, or leakage through a "brane" between universes could account, Kaku ruminates, for a lot of what is assumed to be "supernatural" manifestations.

Ah, yes!! The universe (however many there might be) and the future and its possibilities are truly fascinating places!! Or are they one and the same?

Just a few ruminations. Slow day at the Skunk Works.

Don Firth