Does Life Exist On Any Other Planet In The Universe? Another Look At SETI

Does life exist on any other planet in the universe? This is a question people have been asking ever since we first realized that there were other planets out there.As recently as 80 years ago, there was much speculation about whether there was intelligent life on Mars, given the fact that the best telescopes of that time showed something that looked like giant canals on the surface of Mars.We now know, of course, that not only is there no intelligent life on Mars, but no real evidence for any kind of life at all. (The highly publicized announcement of evidence of life from a Martian meteorite several years ago has been discounted by most scientists, and only direct exploration of Mars is likely to answer the question. ) But what about planets surrounding the myriad of other stars in the universe. Doesn't common sense tell us that surely some must have the conditions right for supporting life?

This thought is one of the main motivations behind a project called SETI (Search for Extra-Terrestrial Intelligence), a project begun close to 40 years ago to search the heavens for some radio signal that would indicate an advanced civilization of some sort. It is also hoped, of course, that contact with a civilization far superior to our own will help solve the world's problems.

The famous astronomer, Carl Sagan, was part of a team that first tried to estimate the number of planets in our galaxy that could have favorable environments for the support of life. By 1966, they had determined that it takes a certain kind of star with a planet located at just the right distance from that star to provide the minimal conditions for life. Working with just these two parameters, they estimated that 0.001% of all stars could have a planet capable of supporting advanced life. While this may sound like a very small number, since there are an estimated 100 billion stars in our own galaxy, that would still mean something like 1 million possible life sites just within our galaxy!Such figures were, of course, the rationale for the spending of huge amounts of money on this search.

It turns out, however, that their estimates were far too optimistic, as they did not take into account a whole variety of other factors that also must be within narrow ranges of value in order for life to be supportable.

The Right Kind of Galaxy

First of all, astrophysicists have shown that only spiral galaxies, such as our own, have a history of stellar development in which a life-supporting star can exist. As only about 6% of all galaxies in the universe still maintain their spiral shape, it is estimated that at least 90% of stars are eliminated from contention on the basis of galaxy type alone.

The reasons for drawing this conclusion are many, but basically they have to do with how "heavy elements" (basically anything heavier than helium, such as carbon, calcium and iron) are formed in the development of the universe. These heavier elements must be present in sufficient quantities for life to exist, and in elliptical galaxies, star formation ceased long before these heavy elements would have time to form in sufficient quantities to later be incorporated into a planet like earth. Likewise, most other types of galaxies and all globular clusters (spherically symmetric systems of stars containing on the order of 100,000 stars and existing around and in between galaxies) have stellar densities that are far too great, resulting in orbital instability and levels of radiation that are far too high for any potential life-supporting planet.

The heavy elements present in the rocks of the earth were formed billions of years ago in giant stars and dispersed into space by supernova explosions. The dust from those explosions was then incorporated into the forming of our solar system in its early development. Such supernova exploding stars need to be quite common in the early years of a spiral galaxy in order for there to be enough heavy elements formed, and rather rare at present, since supernova explosions relatively close to earth in the present era would have devastating effects on life due to the high radiation levels that would be experienced. This scenario fits very well with the best current models of how our galaxy works.

Not only must a life-supporting galaxy maintain it's spiral structure through ongoing star formation into the present, but it also must be a medium-sized spiral galaxy if it is to be a viable candidate for life. Galaxies that are much smaller than the Milky Way will not have a strong enough gravitational field to hold on to the ashes of supernova explosions. Most of the heavy elements blasted out into interstellar space by such supernova explosions in small galaxies end up escaping into intergalactic space and are therefore not available to later planetary formation. On the other hand, in galaxies that are much bigger than the Milky Way, radiation levels are likely to be too high for an earth-like planet to support advanced life.

The frequency with which an earth-like planet would be subject to catastrophic supernova radiation is strongly dependent on its location within the galaxy. Indeed, because of this and other factors, there is only a very narrow region between spiral arms the right distance from the center of the galaxy in which a star/planet system must be located in order for life to exist. The spiral arms of a spiral galaxy are maintained by the ongoing influx of dust and gas and the active star formation this induces. When that star formation ceases, the spiral structure soon breaks down and the galaxy becomes a blended "elliptical" galaxy. It has recently been determined that the sun is at what is called the "co-rotational radius", which means that the sun's orbit around the center of the galaxy (one revolution taking about 200 million years) is such that the sun never gets swept up into the spiral arms with their high stellar density and dust clouds. Stars between the spiral arms that are closer in are moving at faster velocities and thus eventually catch up with a spiral arm, while stars further out than the sun are moving slower and thus are eventually overtaken by a spiral arm. Either would result in likely close encounters with other stars that would disrupt planetary orbits and lead to increased radiation. Such a scenario for earth would mean the end of at least advanced life.

The Right Kind of Star

Not only is a particular kind of galaxy required for life, but also the star around which a life-bearing planet revolves must be just right. It must be a single star of just the right size and age in order to be capable of maintaining a life-supporting planet for a long period of time. About 75% of the stars in our galaxy are combined in groups of two or more stars revolving around each other, and are thus automatically eliminated from contention. This is because a planet cannot maintain a stable orbit around a multiple star.

In order for a star to have a life-supporting planet, it must be of a very specific mass. Stars that are slightly more massive than our sun burn too quickly and too erratically to maintain life-support - even for a planet at just the right distance. On the other hand, however, stars that are slightly less massive than the sun will not work either. The smaller the mass of a star, the less energy it radiates and the closer in a planet must be to be able to maintain a range of temperatures suitable for life. This, however, would result in a different problem, as the tidal interaction between a star and its planet increases dramatically as the distance separating them decreases. (It increases to the inverse 4th power of the distance. ) Thus, bringing a planet in just a bit closer causes a dramatic increase in the braking action that tidal interaction causes on the planet's rotation. Earth's rotation itself is slowing down a small fraction of a second every year, but if earth were a few percent closer to a slightly smaller sun, its rotation would be braked at a considerably greater rate, and our days would have become much longer by now, which would result in temperature differences between night and day that are too extreme for at least advanced life.

In order for a star to have a life-supporting planet, it must also have formed at just the right time in the history of the galaxy. If it forms too soon or too late, the mix of heavy elements suitable for life chemistry will not exist. It is also essential that such a star be middle-aged, since only middle-aged stars can maintain a sufficiently stable burning phase.

Even stars that are of the most stable type and are in their most stable phase of their multi-billion year burning cycles experience changes in luminosity that can be disastrous for life. It is estimated that since life was first introduced onto earth some 3.86 billion years ago, the sun's luminosity has increased by some 15%. All things else being equal, that kind of change would have been more than enough to exterminate all but the most hardy microscopic life. On earth, however, life thrived because this gradual increase in solar luminosity was exactly cancelled out by a concurrent decrease in the efficiency of the greenhouse effect in the earth's atmosphere. The main reason for this is that as life forms increased on the earth, they gradually reduced the levels of C02 and other greenhouse gases in the atmosphere.

The earth's biosphere exists in a rather delicate balance between runaway freezing and runaway heating. Previous "ice ages" and warm spells have been within a fairly narrow range of temperatures. If the mean temperature of the earth's surface were to cool too much (only a few degrees), more snow and ice would form and not melt readily. Since snow and ice reflects solar energy much more efficiently than other surface materials, that would further decrease the earth's mean temperature. This would then result in a vicious cycle of further extension of ice fields and a further lowering of the earth's mean temperature until a very low stable temperature is reached.

On the other hand, if there was a warm-up outside of the narrow range of stability (again, only a few degrees), the opposite kind of vicious cycle would be established, as more water and carbon dioxide would collect in the atmosphere, increasing the greenhouse effect, which in turn would increase the surface temperatures even further until a very high stable temperature was reached.

The fact that the earth has maintained itself in this narrow range of temperatures suitable for the development of advanced life for such a long time in spite of the estimated 35% increase in solar luminosity is truly amazing!This is because life forms were introduced in just the right amounts at just the right time to alter the atmosphere in order to counteract the effects of increased heating from solar radiation. If life originated and developed in strictly naturalistic processes, one wonders how these blind processes could have possibly anticipated the physics of solar burning!If the balance had gotten out of sync, then the earth would have plunged into either runaway glaciation or runaway heating. These conditions could theoretically be reversed by some dramatic event (such as wide-spread volcanic activity), but only very simple life would have survived.

Not only is the type of star critical, but its location is equally critical in determining its suitability as a life-supporting star. As mentioned above, such a star must be between the densely populated spiral arms at the co-rotational radius so that it can always maintain that position far enough away from other stars so that their gravity doesn't disrupt a life-supporting planet's orbit.

Another motion all stars have as they orbit the common center of gravity of the galaxy is a certain amount of up and down motion relative to the mean galaxy plane. The very active, central section of any large galaxy radiates very high levels of hard stellar radiation (x-rays, etc. ) that if directly received would wreak havoc upon advanced terrestrial life forms. Stars systems even as far out from the center of the galaxy as the sun would receive high levels of such radiation if not absorbed by intervening dust clouds. These protective dust clouds are abundant along the galactic plane (which is why our view of the galactic center is so obscured), but if the vertical motion of a star in its orbit is too great, it will be carried outside of this protective zone for certain periods in its orbit. Most stars do, indeed, have sufficient vertical oscillations in their orbits to cause them to undergo such intense radiation exposure from the galactic center at certain points in their orbits. The sun is one of the relatively few that has a low level of vertical oscillation and thus never ventures outside of the protective zone.

The Right Type of Planet

The biochemistry of life requires an environment where liquid water is stable. This means that in order to maintain appropriate temperatures, a planet must be within a narrow range of distances from its parent star. In the case of the earth, a change in average distance from the sun as little as 2% would result in the extermination of any advanced life.

The surface gravity of a planet determines its escape velocity, and the temperature determines how likely it is for any particular molecule to reach that velocity. These two factors, then, are the main factors (along with such things as the solar wind) that determine which atmospheric gases dissipate into outer space and which are retained in the atmosphere. For a planet to support life, it is essential for water vapor (molecular weight 18) to be retained while methane (molecular weight 16) and ammonia (molecular weight 17) dissipate. Therefore, a change in surface gravity or temperature of just a few percent will make the difference between it working and not working.

While earth's surface gravity and temperature are just right to retain water while dissipating methane and ammonia, these two gases (both harmful to life) disappear much faster than their escape velocity energies would indicate. The reason is that the chemical conditions in the upper atmosphere are conducive to breaking down methane and ammonia molecules that haven't already escaped into space.

The rotational period of a life-supporting planet is likewise of critical importance. The earth's 24-hour day is the optimum period for advanced life. If it were significantly slower, then the temperature variations between night and day would become too extreme. On the other hand, if it were rotating much faster, average wind velocities would increase to catastrophic levels (as wind velocity is determined not only by pressure differences but also by the strength of the "Coriolis Force", which is based on a planet’s rotational speed).

It has been estimated that the earth's rotation has been slowing down at a rate of between 2 and 4 hours per billion years. Recent research has also revealed that even this rate of change is an important factor in the development and maintenance of life on earth. This is not only because a slower rate of decrease would have required the earth to be farther away from a bigger sun (which, as mentioned above, would burn too erratically), but it would have also negatively impacted the development of life on the early earth, when a significantly shorter day was an important factor in developing a life supporting environment. With a faster rotation, weather systems would be more tightly wound up, creating smaller, more intense systems. While it is true that this would have caused much higher average wind velocities than on the present earth, this would have had little effect on primitive life in the ocean. The faster rotation would also have concentrated weather systems more towards the equator and reduced the temperature variations between night and day. The net result of this is that the earth was significantly warmer than it otherwise would have been, something that was critical for the development of life.

The Right Moon

The earth's moon also plays a very important role in maintaining life. Our moon is unique among solar system bodies in that it is so large relative to its planet. As a result, our moon exerts a significant gravitational pull on earth. Thanks to this pull, coastal seawaters are cleansed and their nutrients replenished. Without tides, life in the sea would be much poorer. The moon's pull also stabilizes the earth's obliquity (the tilt of the rotation axis relative to the orbital plane). The 23.5 degreestilt of the earth's axis is the optimum tilt to ensure the widest range of latitudes for complex ecosystems to exist. If the tilt were either much more or much less, the climate over large areas of the earth would become too extreme for advanced life to thrive.

The very presence of a moon so large, revolving around a relatively small planet so close to its star, was until recently a great enigma. The various scenarios proposed for the existence of this "dual planet" (forming together out of the primordial disk, being captured by the earth's gravity, or having split off from a rapidly spinning early earth) have, through computer simulation models, all been shown to be utterly impossible. The only known scenario that works is the "giant impact" model, where a Mars-sized body impacts the early earth at just the right angle and speed to produce a huge disc of material around the earth that could later coalesce into a large moon. This scenario is now the favored one as it fits all of the evidence (moon rock composition, etc. ). Such a scenario also helps explain why earth does not have the kind of extremely thick atmosphere that its sister planet Venus has. Such an impact would have blown away most of the primordial (presumably very thick) atmosphere of earth and allowed the much thinner present-day atmosphere to develop. As a general rule, the greater a planet ’s mass is and the farther away from its star, the thicker its atmosphere should be. Thus, according to that rule, earth's atmosphere should be even thicker that Venus', but in fact it is far less.

The Right Planetary Companions

One would not ordinarily expect that the other planets in the solar system would have anything to do with the suitability of the earth for life. However, recent discoveries have shown that particularly Jupiter and to a certain extent Saturn play critical roles for life on earth. Early in the earth's development, frequent collisions of comets and asteroids with the earth provided the earth's crust with the various heavy elements necessary for life. After the introduction of life forms on earth, however, frequent encounters with comets and asteroids would have severely restricted the development of advanced life.

As it turns out, the size and position of Jupiter play a major role in seeing that this doesn't happen. It is estimated that without Jupiter's influence, the earth would be approximately 1000 times more likely to experience catastrophic collisions, such as the one that is believed to have made the dinosaurs extinct some 65 million years ago. Jupiter represents about 71% of the planetary mass of the solar system (some 314.5 times the mass of the earth), and thus its huge gravity acts as a shield to either draw comets into itself like it did in July 1994 or to more commonly deflect them right out of the solar system. Theoretical studies have shown that if Jupiter's mass were significantly less, its ability to shield the earth from frequent asteroid impact would be considerably reduced, while at the same time, a significantly larger Jupiter would also be a problem in that its increased gravity would tend to nudge earth out of the narrow life-support zone.

Another important point is that the extremely regular orbits of both Jupiter and Saturn (which represents almost three-fourths of the planetary mass outside of Jupiter) are important to the earth's being able to maintain a stable orbit. If the orbits of either had any more than the tiny eccentricities they do have, their gravitational influences would disrupt earth's orbit enough over the long term to jeopardize advanced life.

How special are Jupiter and its relation to the rest of the solar system? Recently, tiny perturbations in the positions of a several nearby stars have shown that Jupiter-sized or larger planets do exist, but they typically are much closer in to their parent star, thus eliminating any chance for an earth-like planet to exist in that stellar system. Likewise, all of the extrasolar planets discovered so far have been shown to have highly elliptical orbits. While it is still too early to make a definitive statement, it certainly appears that the nearly circular orbits of the planets of our solar system with no large planets in close is not the typical pattern.

Thus, we see that earth was prepared for life through a variety of finely tuned characteristics of our galaxy, star, planetary companions, planet and moon. Already, over 100 different characteristics have been identified in astronomical literature that must take on narrowly defined values for the system as a whole to work properly, making it possible to maintain life over long periods of time. The following is a partial list of known parameters which determine whether a planet has the ability to support life. The number in parenthesis indicates an estimated probability (on the optimistic side from the standpoint of finding life) of that particular parameter falling within the required range (e.g., .1 means that only about 10% of the galaxies, stars, planets, etc. referred to by that particular parameter are possible candidates for life support based on that parameter alone).

Astronomical Parameters Related to Life Supportability on a Planet

1. Galaxy type (.1)

If too elliptical (and therefore not spiral), star formation would have ceased before sufficient heavy elements could be produced and incorporated into a planet to support life chemistry.

If too irregular, radiation exposure on occasion would be too severe and heavy elements for life chemistry would not be available in sufficient quantities.

2. Galaxy size (.1)

If too small, the gravitational field of the galaxy will be too weak to hold on to most of the heavy elements produced in supernova explosions. The inertia of the ejected particles will carry most of them into intergalactic space, making them unavailable for later planetary formation.

If too large, radiation levels will be too high for advanced life.

3. Supernova rates and proximity (.01)

If too close or too frequent or too late in the development of a galaxy, life on a planet would be exterminated by radiation.

If too far away or too infrequent or too soon in the development of a galaxy, not enough heavy elements would be available at the time a planet forms for it to later be able to support life.

4. White dwarf binaries (a hot dwarf star revolving around a larger star companion).

These stars are the only place in the universe where there exists the kind of nuclear reaction necessary to produce the element fluorine, which is also necessary for life chemistry.(.05)

If too few, insufficient fluorine would be produced to later be incorporated into a planet to allow life chemistry to proceed.

If too many, that would mean that the stellar density is so great that planetary orbits would be disrupted and life could not be maintained for long periods of time.

If too early in the development of a galaxy, not enough heavy elements would have been available for efficient fluorine production.

If too late in the development of a galaxy, fluorine would have been produced too late to be available for incorporation into a developing planet.

5. Parent star location.

For a variety of reasons, only stars located between the spiral arms of a galaxy could have a life-supporting planet. Likewise, the system must be at the co-rotational radius in order to maintain its favored position. (.00001)

If located within a spiral arm, high stellar density would increase radiation and lead to destabilization of planetary orbits.

If much farther out than our sun, the quantities of heavy elements necessary to make a planet like earth would have been insufficient and the star would eventually get swept into a spiral arm as it caught up with the star.

If much closer in than our sun, it would also be impossible to maintain a favorable location for long, as the star would again be swept up into a high stellar density arm.

6. Amplitude of vertical motion away from the galactic plane as a stellar system revolves around the galactic center. (.1)

If great enough to carry the system out of line with the rather narrow band of dust clouds that shield a system from the extreme radiation emitted from the galactic core, radiation levels would greatly rise during the twice per revolution period (about every 100 million years) when the star would be outside the protective zone.Such exposure would be catastrophic to advanced life. The sun is one of the relatively few stars that always remains close to the galactic plane.

7. Number of stars in the planetary system.(.2)

Any planet that is revolving around a double star could not maintain a stable orbit, and thus could not support life.

Likewise, a planet not revolving around any star would obviously be far too cold for life. Therefore, there must be one and only one star around which the planet revolves.

8. Parent star birth date with respect to the parent galaxy.(.2)

If much more recent than our sun, the star would not have been in its stable burning phase for a long enough period to support advance life.

If significantly older than our sun (and therefore a star that developed early in the life of the galaxy), there would not have been enough heavy elements available for the formation of an earth-like planet.

9. Parent star age. (.4)

If either much older or much younger than our sun, the star would not be in a stable burning period and the luminosity of the star would change too quickly.

10. Parent star mass.(.001)

If slightly greater than our sun, the star would burn too rapidly and the luminosity would be too unstable. If slightly less than our sun, the range of distances a life-supporting planet could be from the star would become too narrow; and if such a planet were at that exact distance for proper temperature, the tidal forces involved would be so great that the rotational period of the planet would be slowed down much too fast.

11. Parent star color (which is dependent on surface temperature).(.4)

If either redder (cooler) or bluer (hotter) than our sun, the "bell curve" that represents the radiation coming from the star would be shifted one way or the other.The entire process involved in plants producing food through photosynthesis would be negatively affected, as the percentage of visible light within the total amount of radiation energy (which must be the same in order to have the same temperature on the planet's surface) would be reduced, thus making photosynthesis less efficient.Also, a certain amount of UV light is necessary.This would be significantly reduced with a cooler sun (leading to reduced efficiency in the production of certain nutrients) and significantly increased with a hotter sun (leading to cell damage). (This also, of course, is affected by the ozone shield).

12. Parent star luminosity relative to the introduction of new life forms on a planet.(.0001)

If the rate of introduction of life forms that decreased the greenhouse effect were too slow, the increase in luminosity would have resulted in a runaway greenhouse effect.

If the rate were too fast, then the too rapid reduction in the greenhouse effect would have resulted in runaway glaciation.

13. Albedo (ratio of reflected light to total amount of radiant energy impinging on the surface)(.1)

If significantly greater than the earth's, runaway glaciation would develop.

If significantly less than the earth's, runaway greenhouse effect would develop.

14. Distance from parent star (.001)

If slightly farther out, the planet would become too cold to maintain a stable water cycle (also resulting in runaway glaciation).

If slightly closer, the planet would be too hot to maintain a stable water cycle (also resulting in runaway greenhouse effect).

15. Surface gravity (escape velocity)(.001)

If stronger, the planet's atmosphere would retain too much ammonia and methane, both of which would be detrimental to life.

If weaker, the planet's atmosphere would loose too much water.

16. Orbital eccentricity(.3)

If much more than the 1.6% it is, seasonal temperature differences would become too extreme.

17. Axial tilt(.3)

If much greater than the ideal of 23.5 degrees, the surface temperature differences between summer and winter for most of the planet would be too extreme for advanced life.

If much less than 23.5 degrees, the regions of the earth with climates suitable for advanced life would be greatly narrowed.

18. Rotation period(.1)

If much longer than 24 hours, the diurnal temperature differences would be too great.

If much shorter, atmospheric wind velocities would be too great, as the forces that drive the winds become much stronger.

19. Age of the planet (?)

If too young, the rotation period would be too fast for all but primitive life.

If too old, the rotation period would have been braked by tidal interaction to the point where it would be too long.

20. Collision rate with asteroids and comets during early and subsequent periods of planet's history(.1)

If much greater than earth, there would be too much destruction of habitat and too many species would become extinct.

If much less than earth, the planet would have received too little of the heavier elements necessary for life.In other words, the planet must have high levels of outside material coming in during its early history, and much less later on.

21. Magnetic field of the planet(.01)

If too strong, electromagnetic storms would be too severe (i.e., they themselves, along with a too powerful van Allen Belt, would become sources of detrimental radiation).

If too weak, life on land would be inadequately protected from hard stellar and solar radiation.

22. Gravitational interaction with a moon(.1)

If much greater than that between the earth and its moon, the tidal effects on the oceans, atmosphere, and rotational period would be too severe.

If much less, instabilities in the rotational axis of the earth would cause climatic instabilities; the movement of nutrients in coastal regions would be insufficient.

23. Thickness of the crust(.01)

If much thicker than the earth, too much oxygen would be absorbed in oxidation and then fixed in the crust without being available for recycling within the atmosphere.The crust can be thought of as a layer of "rust" covering the earth.If too much oxygen is taken up into this "rust", then too little is available for the atmosphere in the form of C02, H2O or 02. Likewise, plate tectonics would not operate efficiently.

If much thinner than the earth, volcanic and tectonic activity (the movement of plates that cause earthquakes) would become too intense for advanced life to thrive.

24. Oxygen quantity in atmosphere (.01)

If much greater than 21%, organic material would burn up too easily (fires would start and get out of control much too frequently).

If much less than 21%, advanced animals would have too little to breathe.

25. Oxygen to nitrogen ratio in atmosphere(.1)

If much larger, advanced life chemistry would proceed too quickly.

If much smaller, advanced life chemistry would proceed too slowly.

26. Carbon dioxide level in atmosphere(.01)

If much higher, a runaway greenhouse effect would develop.

If much lower, plants would be unable to maintain efficient photosynthesis.

27. Water vapor level in atmosphere (.01)

If much higher, a runaway greenhouse effect would develop.

If much lower, rainfall would be too sparse for advanced life on land.

28. Atmospheric electric discharge rate(.1)

If much greater, too much fire destruction would occur.

If much less, too little nitrogen would be transferred from the air to the soil.

29. Ozone level in upper atmosphere(.01)

If too high, surface temperatures would be lower, restricting life zones.

If too low, surface temperatures would rise and more importantly, increased UV radiation would be harmful to life.

30. Soil mineralization(.1)

If either too nutrient poor or too nutrient rich, the diversity and complexity of life forms becomes more limited.

31. Seismic activity(.1)

If too intense, the impact on advanced life forms would be too devastating.

If too weak, nutrients on ocean floors (from river runoff) would not be recycled to the continents through tectonic uplift.

32. Oceans-to-continents ratio(.2)

If either much greater or much smaller than the roughly 3 to 1 ratio of earth, the diversity and complexity of life forms becomes much more restricted.

Probability of all necessary parameters occurring on one planet

In order to calculate the probability for the existence of other life supporting planets in the universe, at least two other factors must be taken into account. First of all, it needs to be recognized that a number of the above parameters are interdependent, and thus a simple multiplication of individual probabilities will give too low a figure. Thus, a dependency factor needs to be added in.

On the other hand, the dependency factor is at least partially canceled out by a longevity factor, in that all of the parameters must be maintained within acceptable limits for very long periods of time. In the case of the earth, that means almost 4 billion years!

How large are these factors? Dr. Hugh Ross uses in his estimates 109 for the dependency factor and .0001 for the longevity factor. Putting all of these factors together, that means that the probability of the 32 parameters mentioned above that have estimated probability factors to all come together in one planet comes out to one in 1042 (or 10-42)!As there are a number of other parameters being researched for their sensitivity to the support of life on a planet, the odds are probably many orders of magnitude worse!Even if one is generous and makes the dependency factor a million times greater the odds still only rise to 10-36!

As there are at the most only approximately 1023 stars in the entire universe, it becomes quite obvious that the odds of finding even one star with a life supporting planet is very small - about 10-20 according to the probabilities listed above. One can, of course, argue with some of the probability estimates for specific parameters, as different scientists using different assumptions and rationales will no doubt come out with different probabilities for at least some of the planetary parameters. The figures used above are, according to Dr. Ross, rather optimistic figures, and thus the probability is high that many of these parameters are even further restrictive. Likewise, it should be added that while we can directly observe only the 9 planets of our own solar system, we have literally trillions of stars that we can observe and make measurements on. Thus, the figures for the stellar parameters are far more certain.

At any rate, it should be apparent to any unbiased observer that the universe as a whole is very hostile to life and that it takes some incredibly exacting and fortuitous circumstances for life - particularly advanced life - to exist. Could that all happen by random chance even once (much less countless times!)?

The fact that we are here, of course, means that it happened at least once. Whether that was strictly by naturalistic means or whether it was by the direct design of a transcendent divine being is not a question that science can answer directly. Science can only speak to this issue indirectly. We can investigate the intricate relationship between various astronomical and planetary parameters and the development of life forms, and we can estimate the probabilities for all of the necessary parameters to come together in one place at the same time by natural means alone. That is all the further science can go. After that, it is up to the individual to decide for himself or herself whether it seems reasonable to accept the direct involvement of a divine designer or to eliminate in one's mind that possibility and assign it all to a fluke of random chance.

As for the search for extraterrestrial life, it again comes down to an argument of probabilities. One cannot prove that life does not now nor never has in the past existed on another planet somewhere in the universe. As astronomers have come to understand more about the various parameters involved, the odds of there being another planet like earth have continually shrunk, with the most recent information indicating that you are far more likely to win the national lottery on one ticket than for all of the parameters necessary for life to come together by chance anywhere at any time in the universe's history!The hundreds of millions of dollars spent so far on SETI have no doubt yielded a lot of useful information, and thus, I certainly do not mean to imply that it has all been a complete waste. It would seem, however, that given the scientific and funding realities involved, it would make good sense to switch our priorities to some other more promising fields of astronomical research.

Updated: 2015 年 12 月 12 日,12:20 午後

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