I have been thinking about about the idea that life will tend to develop wherever there is a large, ongoing energy gradient. Such energy gradients (most often of thermal, but in some cases chemical or other) tend to produce complicated systems which increase the diffusion of energy, reduce the energy differences and increase entropy; life, weather systems and crystals are some examples. The problem is that while an ongoing energy difference is a necessary condition for life, it is not at all clear if that is sufficient, since other conditions may be necessary. For instance, liquids, esp. water, may be needed; no one knows. Perhaps life is possible in liquid methane, but there is no evidence for or against. Perhaps life can exist in a super-hot plasma, such as exists in stars; there is certainly a powerful energy flow outward within stars, but would any life pattern exist long enough to reproduce? I suspect that temperatures of millions of degrees would rapidly destroy any possible “life”, but I can’t be sure.
The field of astrobiology has had a bad reputation as the only field of science without a subject matter, but this is no longer true. Scores of exoplanets are known, and preparations are being made to look at their spectra for signs of O2. Plans are being made to drill through the ice crust of Europa to the water ocean below, to look for life, perhaps at ocean floor vents analogous to those found at the bottom of our planet’s oceans. One needs to be careful about clearly separating the known facts from the speculative ideas about the possible existence of life elsewhere, especially in exceedingly different environments, such as in stars.
Even on planets and large moons, other factors may be necessary, such as liquid water, billions of years of relative stability, single rather than double suns, a moderate range of temperatures, etc. I gather that current scientific thought leans toward life being moderately common in our galaxy, but mainly in relatively simple forms, somewhat equivalent to our bacteria. The organic raw materials for life-as-we-know-it are widely available in space. There is some work on producing chemical configurations with some of the qualities of life, such as making more of the same from available, matching substrates, or using other chemicals for energy, but it is not far advanced. Artificial life from scratch is not going to be invented soon.
Even if life exists on a planet, it may not be very complex. Does life automatically tend to become complex over time? No one really knows. There used to be a tendency among space scientists to assume that life was fairly common in the universe, arising fairly automatically from ordinary chemistry on even vaguely earth-like planets, and that it would become complex and even intelligent over enough time. The great physicist Enrico Fermi pointed out that if that were true, even one technological species would probably colonize the galaxy in a relatively (by evolutionary and astronomical standards) short time; he asked, “Where is everyone?” http://en.wikipedia.org/wiki/Fermi_paradox
The pendulum has now swung to assuming that complex life, and perhaps any life, is rare, but no one really knows. An interesting book on this subject is “Rare Earth: Why Complex Life is Uncommon in the Universe,” by Peter D. Ward and Donald Brownlee; Copernicus/Springer-Verlag, 2000. They conclude that many factors, all of which are not likely to exist together (but do exist for Earth), are needed for complex life to develop, but they think that simpler life is probably common. The uncertainties about technological life forms (those that could emit radio waves we could detect, that could send out starships, etc.), at least, have been summarized by Frank Drake, who developed an equation about the number of technological civilizations likely to exist in our galaxy at any given time. He started with the number of stars in our galaxy,and then sequentially multiplied it by the proportion of stars with planets, the number of planets in the habitable zone (not too hot or cold) per star with planets, the fraction of such planets where life does arise, the fraction of such planets where complex life develops, and then by the percentage of the lifetime of such planets that such forms exist. The numbers, all estimates or guesses, multiplied by each other, give the number of planets with complex life forms in our galaxy. (There are about one or two hundred billion galaxies in the observable universe [the volume small enough for light to have reached us since the Big Bang, 13.7 billion years ago], and perhaps super-huge numbers in space beyond that, though we shall never know, because of early inflation of the universe.) Other scientists have added more terms, relating to other factors possibly needed for complex life, such as a very large moon, like ours, the presence of a Jupiter-sized planet in the star-system, and the lifespan of a technological species (before self-extinction!). The main point is that most of the multiplying factors are still quite unknown, and the estimates of their sizes say as much about the estimators as about the possible planets. Wikipedia has a clear discussion of the Drake equation: http://en.wikipedia.org/wiki/Drake_equation.
Life as-we-do-not-know-it is another question. Science fiction abounds with life forms, often intelligent, floating in gas giant planets, inhabiting intense magnetic eddies in stars, living on the surface of a neutron star or organizing giant clouds in space. I have no idea if any of these exist or can exist; we might not even recognize some of them as life if we detected them. Life considered as any entropy-increasing pattern that can take in energy and raw materials from the environment and process both to continue and reproduce may be overly broad; crystal formation could fit that definition, but is not alive in any realistic sense. Neither is a crystallized virus sitting in a test-tube alive; though it is sort-of alive, almost alive when in our cells, using our reproductive mechanisms to make more of itself. The whole field is strange and interesting.
Tags: astrobiology, astronomy, biology, entropy, science
