SESAS - Alien Studies

There are many arguments about the forms alien life might take. Some people argue that life everywhere would be based on the same chemistry we "know and love": carbon, and probably DNA. Others say this is too restricting and that life forms based on silicon might be possible (and, no, we don't mean computers, but rather "organic" compounds which have their carbon atoms replaced by silicon). Still others say even this is too restrictive and that life forms in the universe may not be physical at all, but might be made entirely of energy.

Obviously, until we discover such life forms (or they discover us), we don't know. In fact, we don't know if life exists at all outside of the Earth. But there are some basic science principals that can help us narrow down the question of what forms such extra-terrestrial life might take, and what kinds of planets might support life.

In the most conservative view, for a planet to support life it must have gravity somewhere between 50% and 150% of the Earth's, a day of 12 to 72 hours, an atmosphere with a substantial chunk of oxygen, very little carbon dioxide, lots of liquid water, a mean temperature somewhere above freezing and somewhat below boiling, and an atmospheric pressure consistent with all of these other requirements. How many planets in the universe might meet these requirements? Well, we only know of one for sure... Earth. But, then, the only planets we know of are those around our Sun. What about other stars? We are only now starting to obtain evidence for planets around other stars, but we have no direct observations to base any statistics on. However, using what we know of the output of stars, we can make reasonable guesses about what range of distance a planet would have to be from various types of stars to have the appropriate temperature. But even then these distances are not absolute.

For example, if the Earth didn't rotate, but were instead tidally locked, with one side always facing the sun (the way the moon always faces the Earth) it would probably be far too hot on the sunny side, and far too cold on the night side... maybe. You see, although the fact that the Earth spins means that the temperatures diurnally swing about a nice medium, a tidally locked Earth might also stabilize globally at a fair temperature. If the atmosphere of the Earth had a very warm half and a very cold half, we'd be setting up quite an ordered system. Entropy plays havoc with ordered systems, and the energetic warm air would quickly move to the cold side, and vice versa. Probably resulting in extremely violent weather, but perhaps a fairly normal temperature: the revolving Earth is exchanged for a revolving atmosphere.

So, you see, changes in the rotational speed of a planet might totally change our guesses about the distances a planet would have to be from its star... or it might not. Without further examples (or better models) we'll never know.

There are probably some limits on temperature. For instance, at extremely high temperatures, many chemical bonds just can't happen. Sometimes the electrons just get stripped from their atoms. Science fiction not-withstanding, I think we can discount living beings in the sun. Many people with more liberal views of alien life can come up with plausible scenarios for how life forms could have adaptations to living in certain extremes. However, the acid test is: could it have evolved this way? What steps would have to be taken to get there? And would they survive the same conditions? If a fictional life form has all the right adaptations to survive in environmental extremes, did it have to always be that way? In that case, we can assume it's not possible, or that such a creature had "outside help"... religious or otherwise.

Extremely cold temperatures might not be possible either. Certain chemical reactions need a minimum temperature to occur, or they occur terribly slowly. Such life might not evolve fast enough to be advanced at this point in the life of the universe.

How about some simpler questions: like once life exists, can we predict how it might evolve? The answer is a tentative yes.

For instance,
Sorry - ran out of time. Will upload the rest when I get a chance.