RARE
EARTH DEBATE
Rare Earth Debate Part 1: The Hostile Universe posted: 07:00 am ET 15 July 2002
Rare Earth Debate Part 2: Alien Proximity posted: 07:00 am ET 17 July 2002
Rare Earth Debate Part 3: Complex Life posted: 07:00 am ET 22 July 2002
Rare Earth Debate Part 4: Avoiding Doom posted: 07:00 am ET 24 July
2002
Rare Earth Debate Part 5: Elusive ET posted: 07:00 am ET 29 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.
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.
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 Earth’s 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 optimist’s
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 can’t 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
life’s formation. On some
planets, then, life might arise and animals eventually evolve – only to be soon destroyed by a global catastrophe.
Frank Drake:
The Earth’s 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 Earth’s weird qualities.
So it is easy to look at our biosphere, and the way it
so cleverly exploits Earth’s 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 Earth’s, are the only ones
that can breed complexity.
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 Earth’s 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
Earth’s 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 Earth’s 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 Earth’s natural thermostat to maintain homeostasis. Clouds can help, by
reflecting sunlight back to space, but can only delay the inevitable. If you
don’t 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 don’t 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 Earth’s ability to support plants and vegetarians. Like it or not, this is
probably Nature’s 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 doesn’t 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.
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 haven’t
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 haven’t
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 Earth’s 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. It’s 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? What’s 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 doesn’t
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 it’s 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 planet’s 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 aren’t 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 Earth’s, 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!