RARE EARTH DEBATE

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Rare Earth Debate Part 1: The Hostile Universe posted: 07:00 am ET 15 July 2002 1

Rare Earth Debate Part 2: Alien Proximity posted: 07:00 am ET 17 July 2002 3

Rare Earth Debate Part 3: Complex Life posted: 07:00 am ET 22 July 2002 4

Rare Earth Debate Part 4: Avoiding Doom posted: 07:00 am ET 24 July 2002 7

Rare Earth Debate Part 5: Elusive ET posted: 07:00 am ET 29 July 2002 9

 

Rare Earth Debate Part 1: The Hostile Universe
posted: 07:00 am ET
15 July 2002

When the book "Rare Earth" was published two years ago, it raised a great deal of controversy among astrobiologists. Written by Peter Ward and Donald Brownlee, the book's hypothesis suggests complex life is rare in the universe, and may even be unique to Earth. If life does occur elsewhere, the authors contend, it will only be in the form of single-celled microbial life such as bacteria.

This debate, a 5-part series beginning today, will cover a variety of topics prompted by the Rare Earth hypothesis. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.


Michael Meyer: Thank you for joining the first in what we hope will be a series of Great Debates. Before delving into the vagaries and specifics of planetary and biological evolution, and into a discussion of whether we are unique or common, it might be useful to set a baseline for at least one prerequisite for complex beings -- life itself. This leads to the first question:

Other than on Earth, is there life in our stellar neighborhood?

Peter Ward: There is a cultural assumption that there are many alien civilizations. This stems in no small way from the famous estimate by Frank Drake -- known as the "Drake Equation" -- that was later amended by Drake and Carl Sagan. They arrived at an estimate that there are perhaps a million intelligent civilizations in the Milky Way Galaxy alone.

The Drake and Sagan estimate was based on their best guess about the number of planets in the galaxy, the percentage of those that might harbor life, and the percentage of planets on which life not only could exist but could have advanced to culture. Since our galaxy is but one of hundreds of billions of galaxies in the universe, the number of intelligent alien species would be numbered in the billions.

Surely, if there are so many intelligent aliens out there, then the number of planets with life must be truly astronomical. But what if the Drake and Sagan estimates are way off? If, as could be the reality, our civilization is unique in the galaxy, does that mean that there might be much less life in general as well?

In my view, life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake and Sagan envisioned. However, complex life -- animals and higher plants -- is likely to be far more rare than commonly assumed. Life on Earth evolved from single celled organisms to multi-cellular creatures with tissues and organs, climaxing in animals and higher plants.

But is Earth’s particular history of life -- one of increasing complexity to an animal grade of evolution -- an inevitable result of evolution, or even a common one? Perhaps life is common, but complex life -- anything that is multi-cellular -- is not.

Chris McKay: There is no solid evidence of life elsewhere, but several factors suggest it is common. Organic material is widespread in the interstellar medium and in our own solar system. We have found planetary systems around other sun-like stars. On Earth, microbial life appeared very quickly -- probably before 3.8 billion years ago. Also, we know that microbial ecosystems can survive in a variety of environments with liquid water and a suitable chemical energy source or sunlight.

These factors suggest that microbial life -- the sort of life the dominated Earth for the first two billion years -- is widespread in the stellar neighborhood.

David Grinspoon: It is always shaky when we generalize from experiments with a sample size of one. So we have to be a bit cautious when we fill the cosmos with creatures based on the time scales of Earth history (it happened so fast here, therefore it must be easy) and the resourcefulness of Earth life (they are everywhere where there is water).

This is one history, and one example of life. When our arguments rest on such shaky grounds, balancing a house of cards on a one-card foundation, we are in danger of erecting structures formed more by our desires than the "evidence."

Frank Drake: I think this is an occasion where that old principal of good science, Occam's Razor, is helpful. Apply Occam's Razor to the question of the origin of life on Earth. We look at the Earth, and with regards to that origin, as best we know, no special or freak circumstances were required. It took water, organics, a source of energy, and a long time. Deep-sea vents are the current favorite and a reasonable place for the origin.

But even if they weren't the culprits, the chemists have found a multitude of other pathways that produce the chemistry of life.

The challenge seems to be not to find the pathway, but the one that was the quickest and most productive. The prime point is that nothing special was required. There will be a pathway that works, on Earth and on similar planets. Then, by Occam's Razor, the origin of life on Earth is nothing more than the result of normal processes on the planet. Furthermore, life should appear very frequently on other Earth-like planets. There will be microbial life nearby the solar system.

Donald Brownlee: While there is hope and even expectation of nearby extraterrestrial life, the goal of "Rare Earth" was to point out that the universe is fundamentally hostile to life. Most planets and other places in the universe clearly could not support any type of Earth-like creatures. The universe is vast, so there may be many Earth-like places, but they will be widely spaced, and if they are too widely spaced they will be isolated from each other.

What fraction of stars harbor Earth-like planets with Earth-like life? Is it one in a hundred, one in a million, or even less? Even the most optimistic have to admit Earth-like environments must be rare.

In our book "Rare Earth," we suggest that extraterrestrial life is likely to be near but that complex animal-like life is rare and will probably not be found close to us in space. A major question about life relates to the environments needed for its formation and long term evolution. Unfortunately Earth is our only successful example. Predictions of life elsewhere are problematic; presently there is no detectable life elsewhere in the solar system.

David Grinspoon: I am not convinced that the Earth’s carbon-in-water example is the only way for the universe to solve the life riddle. I am not talking about silicon, which is a bad idea, but systems of chemical complexity that we have not thought of, which may not manifest themselves at room temperature in our oxygen atmosphere. The universe is consistently more clever than we are, and we learn about complex phenomena, like life, more through exploration than by theorizing and modeling. I think there are probably forms of life out there which use different chemical bases than we, and which we will know about only when we find them, or when they find us.

An obvious rejoinder to this is, "But no one has invented another system that works as well as carbon-in-water." That is true. But to this I would answer, "We did not invent carbon-in-water!" We discovered it. I don’t believe that we are clever enough to have thought of life based on nucleic acids and proteins if we hadn’t had this example handed to us. This makes me wonder what else the universe might be using for its refined, evolving complexity elsewhere, in other conditions that seem hostile to life as we know it.

Frank Drake: All evidence of the most primitive steps in the first 700 million years of chemical evolution on Earth is apparently lost. We grope towards understanding of that profound gap in our knowledge by working backwards, hypothesizing that there once was an RNA world based on self-catalyzing RNA. But this system evolved from something else, and led to the esoteric DNA-protein world.

As David Grinspoon rightly points out, we are not remotely smart enough to hypothesize ab initio the system of the DNA-protein world, or even the RNA world. It was handed to us on a silver platter. This should be a strong warning that we are over our heads when predicting what might have taken place on other worlds.

Give us knowledge of another independent origin of life in space, and the doors to great progress in this field may open.

 

 

 

Rare Earth Debate Part 2: Alien Proximity
posted: 07:00 am ET
17 July 2002

This five-part debate will cover a variety of topics prompted by the hypothesis of "Rare Earth," a book by Peter Ward and Donald Brownlee that suggests complex life may be unique to Earth.

Part 1 wrangled with the question of whether life could originate and exist anywhere except on Earth. The general consensus was that simple (microbial) life, at least, may be common in the universe. The focus on microbial life continues today in Part 2 as the moderator asks where we can expect to find life in our solar system and beyond. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.


Michael Meyer: If there is life out there -- either microbial or complex -- where can we expect to find it?

Peter Ward: Life might have originated on Mars and Europa early in the solar system’s history (and may live there still). Many of us think that, at best, we’ll find evidence that life once existed on Mars and may or may not have started on Europa.

My guess is that the Earth is the only place in the solar system where there is existent life -- but we might expect to find a rich fossil record of extinct life on Mars.

Of all planets beyond the Earth, Mars is by far the best known. It has been poked, prodded, examined and measured by a variety of Earth- and space-borne instruments, including those many that have successfully and unsuccessfully either landed or crashed on the surface of the red planet. An enormous amount of information now suggests that early in its history, while our Earth was still a chaotic and uninhabitable world of magma oceans and unceasing asteroidal impacts, Mars may have been a benign world, of equable temperatures and almost planet-spanning oceans. It may, as well, have been a world with an atmosphere that included oxygen.

All of these factors lead to an inescapable conclusion – that the early Martian conditions would have been favorable for the development of life. Some scientists have even suggested that life arose on Mars, and then was transported to Earth.

For several hundred million years or more these benign conditions may have lasted, and in that time span evolution could have worked wonders. Perhaps the first geologists sampling Martian sedimentary rocks older than 4 billion years in age will find not only the fossil remains of bacteria, but also the remains of more complex organisms.

Perhaps the fossils of animals will be found.

What would that scene be like: the swing of a rock hammer against a Martian outcrop, splitting a piece of ancient Martian shale, and the heart-stopping joy of finding a mollusk look-alike or the bones of a fish-equivalent? Yet even if life did attain such a rapid rise in complexity on Mars, it did not last, for Mars as an environment for life died early.

Even as bacteria on Earth were readying for the rush to higher grades of life, Mars was dying or was already long dead – assuming that life originated there at all. On Mars, the oceans seeped back into the planet or were lost to space, the oxygen in the atmosphere bound itself to rocks, and life died out.

David Grinspoon: I agree with the belief that Mars is currently lifeless. My impression that Mars today is dead is derived from the stale atmosphere (no signs of biological disequilibrium yet discerned) and the lack of internally driven geological activity. I think that to support a biosphere over billions of years, a planet needs more than isolated pockets of water.

Don't get me wrong -- I am a big proponent of Mars exploration. No matter what we find there, we will learn a lot. And if Mars is lifeless, this gets us off the hook because there won’t be any difficult ethical choices about human activities there. But all opinions about life elsewhere are just that. We need to go and look.

Donald Brownlee: If no evidence for life is found on Mars, then the formation of life probably is neither easy nor common in the solar system. We already have seriously negative results from asteroidal meteorites.

There are now over 30,000 asteroidal meteorites in captivity, and none of them show compelling evidence of alien life. Many of these rocks came from bodies that were much richer in water, carbon, and nitrogen than Earth, and many had warm and wet interiors that lasted for millions of years. Life apparently did not form in the asteroids. Presumably this is because asteroids did not have the right environments even though they did have the right building materials.

Creation of life apparently needs a richer diversity of disequilibria than can be found inside wet organic-rich interiors of asteroids. Probably what is needed is something akin to environments that occurred on early Earth and hopefully other planets as well.

David Grinspoon: We need to keep an open mind for possible bio-signs in unexpected places as we explore the entire solar system and beyond. If we relax our (understandable) attachment to "life as we know it," other intriguing possibilities become worthy of our consideration.

For a planet to foster the origin of life and maintain the necessary conditions, I believe that the most important requirement is a planet with continuous and vigorous geological activity over billions of years. Watery conditions are needed for our kind of life, but any chemical environment where complexity can flourish might do, and we don't know enough about planets and about chemical evolution to place good limits on these environments.

Although my hunch is that currently Mars is lifeless, I am still holding out for Venus: nice conditions in the clouds, energetic flows, strange UV absorbing pigments, unexplained particle populations, etc., if you don't mind a little acid. Europa, and possibly Titan or Io, also may harbor life.

[Titan is a moon of Saturn; Io is moon of Jupiter.]

Frank Drake: In places like Io and Titan, we may find the first evidence of other biochemistries that are beyond our powers of prediction. I am a little on the pessimistic side with regards to Io -- it has no substantial atmosphere.

But Titan! Wow! A prodigious organic chemical factory, some kind of solvent, even an atmosphere. It sounds better than primitive Earth. Sure, it is very cold there, but chemistry still happens easily if more slowly at Titanian temperatures. Could it be that one creature's arctic clime is another creature's balmy tropical island?

Don Brownlee: My prediction is that the nearest alien neighbors live in feces and food scrap left on the Moon by the six Apollo missions. Even though it’s been three decades, there is a good chance that hearty bacteria live and can reproduce inside encapsulated small damp places and survive the monthly cycles of heat and cold as well as the effects of solar flares, ultraviolet light, and hard vacuum.

If born-on-the-Moon organisms are not living in food scraps (and worse) there are probably dormant terrestrial organisms trapped inside vast numbers of components -- wire harnesses and tape interfaces that are parts of the lunar lander, back packs, surface experiments, rover, etc. Somewhere out there is Allan Shepard’s unsterilized golf ball, which is likely to carry a small zoo of terrestrial microorganisms. Beyond our Moon, my great hope is that microbial life or at least fossil evidence for its prior existence will be found on Mars, Europa, or some other solar system body.

If we find life elsewhere in our solar system, and show that it is not a distant cousin of terrestrial life, this will greatly support the idea that formation of life is easy and commonplace, given the right environmental conditions.

Rare Earth Debate Part 3: Complex Life
posted: 07:00 am ET
22 July 2002

This five-part debate will cover a variety of topics prompted by the hypothesis of "Rare Earth," a book by Peter Ward and Donald Brownlee that suggests complex life may be unique to Earth.

In Part 2, the participants discussed how far (or near) alien life might be. Today they examine complex life and the possibility of its occurrence in the universe. Complex life is generally considered any living thing with multiple cells -- as opposed to single celled, microbial life -- and, on Earth anyway, includes everything from the simplest slime molds to human beings. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.


Michael Meyer: I presume that we are in agreement that microbial life, at least, may be common in our stellar neighborhood and even may be present on other planets in our solar system. That being the premise, the probability of complex life elsewhere is then dependent on the probability of the transition from slime to civilization. It happened here, so why not elsewhere? Do you think that complex life should develop on a sizeable fraction of worlds around other stars?

Christopher McKay: As David Grinspoon pointed out earlier, the Earth is our only example of planetary life. This makes it difficult to unravel what is universal and what is accidental about the nature and history of life. Still, one data point is better than none, and when we look at the question of complex life, our one data point seems to say that complex life arose as a result of the rise of free oxygen. If we take this as being generally true, then we can ask the geophysical question: On what types of planets will free oxygen arise and how long will it take to reach high enough levels?

On Earth it took billions of years for oxygen to rise to present levels. Partly this is because the Earth is efficient at recycling by plate tectonics. This recycling keeps the Earth habitable by cycling the essential elements, but it also would have been a barrier to the buildup of oxygen. Earth probably is not the best possible planet for complex life development, since less plate tectonics would allow a faster rate of oxygen build up.

Mars took this to the extreme. With no plate tectonics, a shallow ocean, and only 38 percent of the Earths gravity, Mars might have built up oxygen much faster than the Earth. But the lack of plate tectonics doomed Mars to lose its atmosphere through mineralization. We might find that complex life arose on Mars only to be extinguished later. Perhaps the optimal planet for complex life would be an intermediate between Earth and Mars.

There may be a range of planet types on which oxygen could arise -- and therefore complex life. I would hazard a guess that most maybe two-thirds -- of terrestrial planets with life go on to develop complex life at some stage of their history. An optimists view.

Simon Conway Morris: The problem in my view is, why did complex life take so long to evolve on Earth? Evidence from oxygen data is frankly equivocal. Maybe the redox state of the Earth's mantle was peculiar in comparison with other similar planets. Alternatively, ocean chemistry may have put the lid on things.

There could be other dimensions that could explain why there was such a brake on the evolution of complex life -- why there were no Meso-Proterozoic dry martinis, but on the other hand, once microbes, then NASA.

David Grinspoon: Planetary biospheres are complex entities whose histories are fraught with contingency, accident, and luck. Therefore, the time it took for complex life to arise on Earth is probably much faster than some and much slower than others.

We cant stand a mystery without a chief suspect, so we pin the rise of complex life on the rise of oxygen. This may well have factored in, but as Chris pointed out, there is no reason to believe that oxygen rose on Earth as quickly as it might have elsewhere. The rate of plate tectonics is one variable that will change atmospheric history - there are countless others. For example, if Earth had formed less rich in iron, then oxygen would have risen much more quickly because there would not have been as much iron to devour the oxygen.

So in other planetary systems that are less metal-rich, creatures might have evolved to levels far beyond our current state.

Peter Ward: On Earth, evolution has undergone a progressive development of ever more complex and sophisticated forms leading ultimately to human intelligence. Complex life and even intelligence could conceivably arise faster than it did on Earth. A planet could go from an abiotic state to a civilization in 100 million years, as compared to the nearly 4 billion years it took on Earth.

Evolution on Earth has been affected by chance events, such as the configuration of the continents produced by continental drift. Furthermore, I believe that the way the solar system was produced, with its characteristic number and planetary positions, may have had a great impact on the history of life here.

It has always been assumed that attaining the evolutionary grade we call animals would be the final and decisive step. Once we are at this level of evolution, a long and continuous progression toward intelligence should occur. However, recent research shows that while attaining the stage of animal life is one thing, maintaining that level is quite another. The geologic record has shown that once evolved, complex life is subject to an unending succession of planetary disasters, creating what are known as mass extinction events. These rare but devastating events can reset the evolutionary timetable and destroy complex life while sparing simpler life forms.

Such discoveries suggest that the conditions allowing the rise and existence of complex life are far more rigorous than are those for lifes formation. On some planets, then, life might arise and animals eventually evolve only to be soon destroyed by a global catastrophe.

Frank Drake: The Earths fossil record is quite clear in showing that the complexity of the central nervous system -- particularly the capabilities of the brain -- has steadily increased in the course of evolution. Even the mass extinctions did not set back this steady increase in brain size. It can be argued that extinction events expedite the development of cognitive abilities, since those creatures with superior brains are better able to save themselves from the sudden change in their environment.

Thus smarter creatures are selected, and the growth of intelligence accelerates.

We see this effect in all varieties of animals -- it is not a fluke that it has occurred in some small sub-set of animal life. This picture suggests strongly that, given enough time, a biota can evolve not just one intelligent species, but many. So complex life should occur abundantly.

There is a claim that "among the millions of species which have developed on Earth, only one became intelligent, so intelligence must be a very, very rare event." This is a textbook example of a wrong logical conclusion. All planets in time may produce one or more intelligent species, but they will not appear simultaneously. One will be first. It will look around and find it is the only intelligent species. Should it be surprised? No! Of course the first one will be alone. Its uniqueness -- in principal temporary -- says nothing about the ability of the biota to produce one or more intelligent species.

If we assume that Earths are common, and that usually there is enough time to evolve an intelligent species before nature tramples on the biota, then the optimistic view is that new systems of intelligent, technology-using creatures appear about once per year. Based on an extrapolation of our own experience, let's make a guess that a civilization's technology is detectable after 10,000 years. In that case, there are at least 10,000 detectable civilizations out there.

This is a heady result, and very encouraging to SETI people.

On the other hand, taking into account the number and distribution of stars in space, it implies that the nearest detectable civilizations are about 1,000 light-years away, and only one in ten million stars may have a detectable civilization. These last numbers create a daunting challenge to those who construct instruments and projects to search for extraterrestrial intelligence. No actual observing program carried out so far has come anywhere close to meeting the requirement of detecting reasonable signals from a distance of 1,000 light years, or of studying 10 million stars with high sensitivity.

Donald Brownlee: But how often are animal-habitable planets located in the habitable zones of solar mass stars? Of the all the stars that have now been shown to have planets, all either have Jupiter-mass planets interior to 5.5 AU [1 AU, or astronomical unit, is the distance from Earth to the Sun] or they have Jupiters on elliptical orbits. It is unlikely that any of these stars could retain habitable zone planets on long-term stable orbits.

On the other hand, many of the stars that do not have currently detectable giant planets could have habitable-zone planets. But even when rocky planets are located in the right place, will they have the "right stuff" for the evolution and long term survival of animal-like life? There are many "Rare Earth" factors (such as planet mass, abundance of water and carbon, plate tectonics, etc.) that may play important and even critical roles in allowing the apparently difficult transition from slime to civilization.

As is the case in the solar system, animal-like life is probably uncommon in the cosmos. This might even be the case for microbes: how can scientists agree that microbial life is common in our celestial neighborhood when there is no data? Even the simplest life is extraordinarily complicated and until we find solid evidence for life elsewhere, the frequency of life will unfortunately be guesswork. We can predict that some planetary bodies will provide life-supporting conditions, but no one can predict that life will form.

Frank Drake: Only about 5 percent of the stars that have been studied sufficiently have hot Jupiters or Jupiters in elliptical orbits. The other 95 percent of the stars studied do not have hot Jupiters, and just what they have is still an open question. The latest discoveries, which depend on observations over a decade or more, are finding solar system analogs. This suggests that 95 percent of the stars- - for which the answers are not yet in -- could be similar to our own system. This is reason for optimism among those who expect solar system analogs to be abundant.

David Grinspoon: I think it is a mistake to look at the many specific peculiarities of Earth's biosphere, and how unlikely such a combination of characteristics seems, and to then conclude that complex life is rare. This argument can only be used to justify the conclusion that planets exactly like Earth, with life exactly like Earth-life, are rare.

My cat "Wookie" survived life as a near-starving alley cat and wound up as a beloved house cat through an unlikely series of biographical accidents, which I won't take up space describing but, trust me, given all of the incredible things that had to happen in just the right way, it is much more likely that there would be no Wookie than Wookie. From this I do not conclude that there are no other cats (The Rare Cat Hypothesis), only that there are no other cats exactly like Wookie.

Life has evolved together with the Earth. Life is opportunistic. The biosphere has taken advantage of the myriad strange idiosyncrasies that our planet has to offer. Not only that, life has created many of Earths weird qualities.

So it is easy to look at our biosphere, and the way it so cleverly exploits Earths peculiar features, and conclude that this is the best of all possible worlds; that only on such a world could complex life evolve. My bet is that many other worlds, with their own peculiar characteristics and histories, co-evolve their own biospheres. The complex creatures on those worlds, upon first developing intelligence and science, would observe how incredibly well adapted life is to the many unique features of their home world. They might naively assume that these qualities, very different from Earths, are the only ones that can breed complexity.

Rare Earth Debate Part 4: Avoiding Doom
posted: 07:00 am ET
24 July 2002

This five-part debate will cover a variety of topics prompted by the hypothesis of "Rare Earth," a book by Peter Ward and Donald Brownlee that suggests complex life may be unique to Earth.

 

In Part 3, the participants discussed complex life and what it takes to make it. Today they examine whether life on Earth is doomed to extinction due to the changing nature of the Sun. If the Earths habitable zone does have a time limit, does the same necessarily hold true for other worlds? The moderator is Michael Meyer, the NASA senior scientist for astrobiology.


Michael Meyer: As our Sun grows ever brighter, the Earths habitability will be reduced. How long can life last on Earth? Do you think all life in the universe shares a similar fate?

Peter Ward: Our Sun has about another 7 billion years before it enters the Red Giant phase. Surely, then, we could expect a long period of habitability. But the reality is that it takes more than the correct amount of solar energy to make a planet habitable. This is especially true for complex organisms such as animals, which have a very narrow range of temperatures and nutrient requirements compared to microbes.

The presence of complex life on the Earth will end in no more than a billion years (and perhaps much sooner), due to a sequentially predictable breakdown of habitable systems on our planet. The systems in question are those that serve to regulate the Earths temperature and atmospheric carbon dioxide content.

New models suggest that over the next billion years, we can expect atmospheric carbon dioxide to drop to levels that can no longer support photosynthesis. This will be followed by a temperature rise on the planet to above 50 degrees Celsius [122 degrees Fahrenheit]. Both of these factors will spell the end of complex life on Earth. When the global temperature rises to about 70 C [158 degrees F], the oceans will be lost to space, and this might spell the end of all life on Earth. [More on this Theory]

David Grinspoon: Above a certain level of solar input, an oceanic planet must lose its water. This is a robust conclusion of fairly simple physics, so unless the Sun is a very weird star it will keep heating up and, unless someone intervenes, Earth will lose its oceans.

Long before that the carbon dioxide needed by plant life will be drawn down into the rocks in a futile attempt by Earths natural thermostat to maintain homeostasis. Clouds can help, by reflecting sunlight back to space, but can only delay the inevitable. If you dont believe me, just ask Venus.

The specific details and timing of how this heat death will come about are much less certain, as they depend on climate modeling. Climate modeling for our own atmosphere is an imprecise art (if you dont believe me just read the newspapers), and becomes more uncertain when we apply it to the atmospheres of far-future Earth, or other planets.

So, there is a lot of slop in these dates.

Peter Ward: The period of time that one can expect complex life to exist will vary from world to world. Our "Rare Earth" hypothesis is that on most planets, this will be too short a time to allow complexity to arise at all.

Perhaps the fates of Earth and the other planets in our solar system are not typical at all.

But still it is certain that all planets as abodes for life age through time, and as they change they eventually lose the ability to sustain life. Sometimes they do so over immense periods of time, sometimes it might be fast. Some die of old age and some are killed off by cosmic catastrophe. But all end eventually. This salient fact must be considered in any reflection about the frequency of life in the cosmos.

For our own star, the flaring into a red giant will be followed by a stellar retreat into a dwarf stage that will last untold billions of years. As astronomers gaze out into the heavens with their powerful telescopes, they see billions of such stellar tombstones. The galaxy is littered with dead stars, the markers of how many dead planets, and of how many dead civilizations that for a time circled these stars when they were young and vigorous?

The presence of these stellar graveyards are thought-provoking reminders that any estimate about the frequency of life in the universe must take into account the fact that once evolved, life has a finite life span on any world. And, like the varieties of ages of individuals, the life span of life-covered planets depends in large part on a whole slew of characteristics.

David Grinspoon: If complex life sometimes leads to sentient life with powers slightly greater than our own at present, then it need not accept "natural" climate evolution as inevitable. Right now we are in the stage of inadvertently altering our global climate, but it is not inconceivable that we, or someone else, could advance to the stage of purposefully altering climate for the benefit of the biosphere. If that happens, then reports of the death of the habitable zone are greatly exaggerated.

We should at least ponder the possibility that sentient life, once it arises, will not let its planet become uninhabitable quite so easily.

Assume for a second that humans, or our sentient descendants, do not wipe themselves out any time soon, and solve the problems of asteroid impacts and other threats to long term survival. How hard would it be, with the technology of even 100 years from now, to say nothing of 10,000 or several million years from now, to put up a sunshade and keep the Earth cool from our warming star? Or move to Mars for a while?

Once complex life gets just slightly more advanced than we are now, then it becomes quite possible that sentient creatures can alter the habitability of worlds and planetary systems.

Christopher McKay: Based on our own experience, we know that civilization and technology radically change the rules. Even extrapolating 1,000 years into the future (a brief instant on the scale of the age of the planet) we can imagine the transforming effect of intelligence on the distribution of life in our own solar system and possibly even our region of the galaxy.

Frank Drake: Once a species has developed high technology, there are many strategies for dealing with the changing brightness of the home star. It has even been suggested by Gregory Benford that the main sequence lifetime of stars can be greatly extended by developing a technology which stirs the star, bringing fresh hydrogen to the core -- after all, about 90 percent of a star's mass is intact when the giant stage is approached.

A far out idea to be sure, but it reminds us that clever technologies may be as yet unrecognized by us.

The luminosity of the Sun-like stars changes very gradually, over millions of years. This is enough time to mount a massive technological program to move outwards in the planetary system. Perhaps to terraform Mars, or the satellites of Jupiter; perhaps to utilize material from asteroids to build a constellation of space colonies. There is plenty of time, and the motivation will be there. As the Sun collapses from the super giant phase, the creatures can move inward, eventually to huddle close to the white dwarf Sun. There they will finally be at peace with the cosmos, with a supporting star whose lifetime will be many billions of years.

Donald Brownlee: There is a common belief that life will always find a way and that the universe itself is bio-friendly. An extension of this line of thinking is that life will usually solve its problems, travel the universe, and perhaps even evolve to something far beyond our "wet life" based on cells, genetic codes, and complex chemical processes.

On Earth, life so far has indeed "found the way" and after 4 billion years it has evolved to what we now consider to be normal.

But was Earth lucky to get this far? Will its diverse biological communities be able to survive long into the future? Unless the universe actually is bio-friendly, our planet will have barely reached its present state before the ever-warming Sun begins to degrade Earths ability to support plants and vegetarians. Like it or not, this is probably Natures way. Even on the best of planets, advanced life only flourishes for a relatively short period of time.

If advanced life only rarely evolves and doesnt last long when it does, it will be rare in the universe at large. The only way that I see that animals are likely to be common in the universe is if interstellar travel actually is so easy that the Noahs and Johnny Appleseeds of the cosmos just spread things around.

I personally doubt that this happens.

I believe that it is most likely that organisms as complex as animals only occur in transient cosmic oases widely separated by space and time. Planets form, they may develop life, but eventually the planet and its life perishes. This cycle repeats endlessly in the cosmos. Likewise, civilizations form, they may send SETI transmissions or even launch time capsules, but they will never make direct physical contact.

Rare Earth Debate Part 5: Elusive ET
posted: 07:00 am ET
29 July 2002

This five-part debate has covered a variety of topics prompted by the hypothesis of "Rare Earth," a book by Peter Ward and Donald Brownlee that suggests complex life may be unique to Earth.

In Part 4, the participants discussed whether all life, here or anywhere, is ultimately doomed by the fact that stars swell and then die. Today in the final installment they examine why we havent found complex intelligent life, if indeed it does exist elsewhere in the universe. A thread of this debate also picks up on a comment made by Donald Brownlee in Part 4 -- that interstellar space travel may be impossible. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.


Michael Meyer: If there is intelligent life out there, why havent we found them yet?

Chris McKay: This is Fermi's paradox: Where are they? Or phrased differently: why aren't signs of galactic-scale intelligent life obvious in our telescopes? The simplest explanation for this is that we are the only, or at least the first, intelligent species in the galaxy.

Can anyone give a good argument for why our type of civilization would not be obvious over much of the galaxy after a million years?

David Grinspoon: If civilizations like ours were all over the galaxy, it would not be obvious. We are only listening, not broadcasting. We are not doing astroengineering.

True, we are leaking sitcoms and beer commercials, but these are not easily detectable over most of the galaxy and certainly would not be interpreted as signs of true intelligence.

So, in order to have an obvious presence, "our type of civilization" must become something quite different. Perhaps this is very rare or difficult. However, being a constitutional optimist, and considering the unimaginably vast reaches of time and space, I tend to think that sentient, long-lived civilizations should be out there somewhere. So, where are they?

The reasoning behind Chris's (and Fermi's) question implicitly assumes certain things about the behavior of advanced civilizations. It assumes they will keep expanding their populations and increasing the size of their civil engineering projects. Looking at the history of our civilization and extrapolating to our future, I understand why you could draw such a conclusion.

But it may be that truly sentient societies realize there is no future in unlimited expansion. We cannot keep expanding our population at our current rate. Even if we were somehow able to move out into space at the speed of light and colonize all available planets, we would still run out of space and resources and experience mass starvation within a thousand years. True minds will realize that such expansion is a dead end.

Of course, the problem with this kind of explanation for "the great silence" (Fermi's paradox) is that it must apply to every single civilization out there. It is hard to believe that every society that ever forms will transform themselves into sustainably living, granola munching, navel staring, contemplative Buddhists before creating some observable signs of their presence. So, we must search for another answer.

Frank Drake: A parallel question to this is: how long will the Earths technology be detectable? A few decades ago we thought the visibility would last a long time -- ever more powerful TV stations and radar installations were being built, and these are the strongest signs of our existence.

But there is only so much bandwidth in the useful electromagnetic spectrum. To transmit ever-increasing amounts of information, portions of the spectrum must be shared. This is only possible if signal strengths are reduced so that transmissions on the same frequency do not interfere with one another. The textbook example of this paradigm is the cellular phone system. This signal reduction means we are well on our way to becoming invisible.

So if the transmission of a rich cornucopia of information is what advanced civilizations do, they may become invisible. This is a rather counter-intuitive result, but a real one. This means that the detectable lifetimes of civilizations may be shorter than we have estimated, and hoped, alas.

David Grinspoon: Another possibility is that they may not want us to know they are there. Its hard for us to fathom the possible motivations and behaviors of societies millions of years older than ours. It seems reasonable, however, to suppose that the differences between their capabilities and ours will be so great that it will be up to them, not us, how and when some kind of detection or contact is made.

It is possible that they have decided it should be against the law to let us know they are there (The "Zoo Hypothesis" or the "Prime Directive"). This might be because they are protecting us, studying us, protecting themselves from us or what we might someday become, or waiting until the time is right to initiate us into the Galactic Club.

The simplest explanation -- that we are the only, or the at least first, intelligent species in the galaxy -- requires an extreme violation of the Copernican Principle (which says the Earth is typical and common). This is especially true when you consider the generations of stars -- with possible habitable planets -- that lived for billions of years before our star and planet were even a twinkle in the eye in our parent molecular cloud. There has been so much time for someone to come along and achieve intelligence. Why should our present time be so special? It comes down to which unjustified pillar of scientific reasoning you prefer to violate: Occam's Razor (things are simple) or the Copernican Principle (our place is not special). Take your pick.

Frank Drake: Every discussion of alien intelligence assumes that they will come visit us. But the expense and danger of space travel are formidable. A strong reason why such enterprises are not carried out may be that radio communication works so much better, is far cheaper, and you get your answers at the speed of light.

Any reasonable transport of creatures across space calls for travel speeds that are a substantial fraction of the speed of light, otherwise it takes too long to go even to the nearest stars. But this exposes the spacecraft to serious hazards. Probably the most serious is the potential for collision with debris -- and we are learning that space is full of debris.

At relativistic speeds [approaching the speed of light], even a collision with a particle of a few grams results in something close in energy to a nuclear bomb blast. Not good news for the space travelers.

Also the energy requirements are ridiculous, at least to us. To send a spacecraft the size of a small airliner at one-tenth the speed of light requires as much energy as the U.S. now produces in more than a hundred years. And that just gets you someplace -- it doesn't provide for a landing or a return home.

To put it another way, it takes 10 million times as much energy to move a small space colony to another star as it takes to establish the same colony in the home system. And there is plenty of room at home. It is easily calculated that the energy of the Sun is enough to sustain more than ten thousand billion billion humans. That seems like enough. Why go to the great expense and danger of going to other stars? Truly intelligent life would laugh at the idea. The only ones who might try are the dumb ones, and they don't know how.

David Grinspoon: I agree that, given the time and energy constraints, any intelligent creatures would have to be nuts to attempt interstellar travel. But you would also have to be nuts to attempt to cross the ocean in a rowboat, and people have done that.

Why do we need to go one-tenth the speed of light? Whats the hurry? So what if travel times are thousands of years? From the perspective of an individual human life at this stage in our evolution, this seems like a long time. But will the galaxy never, ever, anywhere, produce a creature or cultural entity that doesnt find this span of time daunting? Even at these slow speeds, if someone decided to start spreading across the galaxy they would be able to spread across the whole Milky Way in a few hundred million years, tops, which is still short compared to the life of the galaxy.

I also agree that radio communication makes much more sense than any form of interstellar travel for almost any purpose. Except its still more fun to go to the game than watch it on TV. I doubt we'll ever achieve warp drive or anything that makes interstellar travel so much faster, better, and cheaper that we can visit a new star system with shapely natives every week like Captain Kirk.

Still, isn't it extreme to declare that no one will ever travel the interstellar distances?

Donald Brownlee: I have always loved space travel in science fiction, but I take a very dim view of the likelihood that we will be able to send people more than just a short distance away. I know that a future without interstellar travel is a minority view, but it is not at all clear that technology could be developed to transport living humans to habitable places beyond our solar system.

I think that it is odd that so many people are sure that we will inevitably evolve to a Star Trek society, able to zip across the Galaxy like we drive to the next state. Beaming up and all that stuff seems so easy on TV. Our best bet with foreseeable technology is to use antimatter fuel, but even if we could build the hardware it would take all of our planets energy production for over a century just to make the fuel. Besides, there are additional problems in technology, funding, and human organization. New discoveries involving navigation and maneuvering are required to get to other earthly oases in space on a comfortable and timely basis.

Can all the UFOers really be wrong? Time will surely tell.

David Grinspoon: As many brilliant thinkers have pointed out, if a civilization survives to a certain point they could easily become immortal. That is, if they learn how to avoid asteroids and other natural disasters, tame any self-destructive instincts and learn to live sustainably, their lifetime effectively becomes the lifetime of the universe.

Yes, I know there are nasty things like gamma ray bursts and other hazards we haven't even discovered yet, but we are talking about technology and an understanding of nature, and of self-understanding, that are many millions of years beyond our own. Migrating between stars to stay alive will not be a hurdle for these "old ones." Comparing this idea to Star Trek or UFOs is a cheap shot that ignores the serious literature on this topic.

If you don't insist on making the trip within the current human life span, there are no huge technical hurdles.

Donald Brownlee: I am sorry that David considered my previous comments about Star Trek and UFOs to be a cheap shot, but I really do believe that the difficulty of practical interstellar travel is horrendously underestimated. In my opinion, the public is being bilked by wishful thinkers that like to write books and muse about futures that we would like to believe are our logical destinies.

Perhaps I take too much of a hard-nosed and practical view of this, but doing even simple things in space is difficult, unforgiving, and exceedingly expensive.

I am aware of the studies of anti-matter rockets, beamed energy, interstellar ram jets, etc., but all of these ideas have severe problems. As I see it, known physics will never deposit living people on Earth-like planets around other stars. Doing so would require "warp speed" and/or harnessing exotic phenomena such as wormholes or zero-point energy. Unless such radical developments occur, mundane ideas such as anti-matter rockets will not do the job.

We have gone to the Moon, we can go to Mars, but that is likely to be the limit that our resources and foreseeable technology will allow. At our current rate of progress, humans may not even make it beyond the International Space Station. Our bounds in space may be as limited as they are on Earth. We have covered the Earth but it seems highly unlikely that we will ever live more than a kilometer above or a few kilometers beneath its surface.

The suggestion that organisms could easily become immortal if they live long enough is intriguing. There are a number of issues here, including whether "immortal" means "relatively immortal" or "actually immortal." Forever is a very long time -- I suggest that nothing physical can ever be immortal. Infinite time is something that the universe cannot keep up with unless things like child universes pop up from time to time to refresh the landscape. If things arent miraculously refreshed, the universe just runs down over long time scales.

According to new information, the expansion of the universe has accelerated. Lawrence Krauss of Case Western University says that an accelerating universe "would be the worst possible universe, both for the quality and quantity of life All our knowledge, civilization, and culture are destined to be forgotten. There's no long-term future." A most bleak forecast and at the totally opposite end of the spectrum from predictions of immortal beings.

David Grinspoon: I define "immortal" as lasting for the rest of the life of the universe, which may not be "really immortal" but may have to do.

If we accept the idea that some civilizations can solve the problems which threaten their survival, attain peace, stability, control their populations, learn to intelligently engineer their solar systems, etc., then "immortality" happens. By definition it is an irreversible transition, so the immortals must slowly be accumulating. None of us know, but my sense is that the universe is bio-friendly. I doubt there are any other planets with a peculiar history and biosphere closely resembling Earths, but I predict many, many inhabited worlds, and a large number with intelligence far in advance of anything we can even conceive of.

Don't you love predictions like this? It cannot be proven wrong!

 

 

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