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Despite decades of concerted effort by radio astronomers working on
the project known as SETI, the Search for Extraterrestrial
Intelligence, we've had no sign of beings elsewhere in the universe
that match or exceed us in smarts. That's just one of many pieces of
evidence that paleontologist Peter Ward, coauthor of
Rare Earth: Why Complex Life is Uncommon in the Universe,
points to when claiming that intelligent life beyond Earth must be
exceedingly rare. Since many other scientists believe advanced life
is common out there—Carl Sagan estimated a million intelligent
civilizations in our galaxy alone—NOVA eavesdropped on
astrophysicist and "Origins" host Neil deGrasse Tyson playing
devil's advocate in a conversation with Ward about extraterrestrial
intelligence. Here's what they had to say to each other:
Time is of the essence
Neil deGrasse Tyson: Peter, when you combine the fossil
record on Earth with all that we know of biology and evolution, what
can we expect as we look for intelligence elsewhere in the cosmos?
Peter Ward: There are maybe 30 million species on the planet
today—10 to 30 million. If we look at the fossils, there are
hundreds of millions of species in the past. And one time on Earth
has intelligence arisen to the point where we can build a radio
telescope (which is the definition of intelligence to a radio
astronomer). One time out of hundreds of millions of possibilities.
That's an astronomically small number of intelligences that have
arisen—just one.
Tyson: At last tally, there are several hundred billion stars
in the Milky Way galaxy and perhaps 100 billion galaxies in the
universe. Yet according to your book, we're just cold, dark, and
alone.
Ward: No, we're not alone; we're lonely. There's a big
difference between being alone and being lonesome.
Tyson: But in this big galaxy of ours—hundreds of
billions of stars—surely Earth is repeated many places, many
times.
Ward: We got to intelligent organisms on this planet after
500 million years of animal life. Now, that doesn't say you couldn't
get it sooner at other places, but you still need long periods of
time. And to me that is the major argument against there being
intelligent civilizations. We are on a planet that's very safe, a
planet that has not been repeatedly bombarded by the thing that
killed the dinosaurs—asteroids. How many other planets are
going to have such long periods of time? Not many, I think.
Tyson: But suppose life is common in our galaxy. We are now
asking, of all the planets with life, isn't intelligence surely
going to show up?
Ward: I still think it's a question of time, Neil. I agree
with you, I think life is pervasive in the galaxy. I think that the
way the Earth has extremophiles [organisms that thrive in extreme
hot or cold habitats], the fact that we can have life living in so
many bad environments, suggests that you can have life in really
nasty, nasty places.
But it takes time to evolve into a T. rex or an
Allosaurus, whatever; it takes a long period of time. Now,
our planet has had a very low record of large asteroid impacts.
Jupiter is the major reason for that. One of the great studies of
the 20th century showed that if we did not have a Jupiter, the
impact rate on Earth would be 10,000 times higher than it is.
Location is everything
Tyson: It's not hard for me to imagine a solar system that
forms when the vacuuming up of the debris in the early phases
happened more efficiently than here. For you to base a whole idea
about the likelihood of intelligence in the galaxy on the fact that
we had some extra bombardment seems kind of thin.
Ward: No, that's only a piece of the puzzle. But let's stay
on that one. "Oort Cloud" is the name of the big bunch of comets
around us. There are many studies suggesting that Oort clouds will
be bigger and denser the closer you get to the center of the galaxy.
So it could be that we are in a section of the galaxy real
estate-wise that's great for life, and that as you head more
centrally, where there's more and more stars that form, it gets
worse. You know the nature of galaxies' centers. They are very
dangerous places.
Tyson: They're nasty places.
Ward: And it's not just asteroids. It's gamma ray bursts,
it's supernovae. You've got black holes, you've got energetics,
you've got sterilization events. So the centers of galaxies aren't
going to work. And perhaps half the stars in the galaxy are in the
center. So now we have to move off to the suburbs, where things
start getting spread out. If we go too far out, we run out of the
heavy elements necessary to make a habitable planet.
“I think life is almost everywhere in the universe. But
you’re asking about intelligent life, and that’s a whole
different ball game.”
So we've got a portion of the galaxy that is no good, and another
portion that is no good. We're looking at narrow real estate areas.
How narrow that is is debatable. But this debate we're having would
not have taken place 10 years ago. We have new information just
coming in that seems to be narrowing the areas where habitability
can take place.
Enter geologists
Tyson: Alright, so you're an intelligence pessimist. But
clearly people who professionally look for intelligence in the
galaxy—the SETI people—don't share that pessimism. What
gives them the confidence that you don't have?
Ward: They're not geologists. I think what astronomers have
missed is the record of our own planet. Geology tells us a couple of
very important things. First of all, you need a long period of
temperature stability. That's a geological phenomenon. A very finite
set of conditions on this planet has allowed us to have liquid water
for four billion years. We never went above 100°C. We never
went below 0°C for long periods of time. That, for four billion
years, is a rare thing, I think.
Secondly, the impact rate is also something that causes species to
go extinct. Sixty-five million years ago, millions of dinosaurs were
suddenly wiped out by an impact.
Tyson: Yeah, but that same incident enabled mammals to become
something more ambitious than a tree shrew.
Ward: Sure, maybe that was an instigator towards
intelligence.
Tyson: But you just said it was bad for intelligence.
Ward: Well, what if such a cataclysmic event happens 100
times, 1,000 times, or 10,000 times, and you're looking at things
being knocked out? It took 65 million years after the knockout to
get to the level of intelligence that we have.
Tyson: But as a geologist, is it possible that you're a
little close to the problem? You see Earth in such detail as a
planet that sustains intelligent life, that it's inconceivable to
you how many different kinds of planets could give you intelligence.
You're kind of stuck in a geological paradigm. Maybe it's as much a
handcuff as a liberator of ideas.
Ward: But you astronomers never even considered it.
Tyson: No, we didn't.
Ward: You've been so far out in space. I never heard the term
"plate tectonics" ever used; it's not even in your lexicon. And this
is where I think a whole new view comes from biologists and
geologists and paleontologists to add to the great work you
astronomers are doing. It's not simply a function of how many
planets are out there.
There's an enormous amount of new information coming from planetary
geologists. We know so much more now about how planets form and how
habitable planets work. We have this entire new science called
"Earth System Science" that examines how the life systems of Earth
continue, the carbon cycle, how we have water, how we have
temperature changes, the weather patterns. But all of this in sum
brings together Earth as a planet, which over long periods of time
is able to stay alive. It's like the life of an individual. It's
born, it grows, and it has these systems that allow it to maintain
life.
Animals are rare
Tyson: I don't hear you exploring exotic models for what a
solar system or planet can be. For instance, maybe there are planets
where the water is frozen on the surface, but deep down there's a
place where it's liquid. And wherever it happens to be liquid life
goes to that.
Ward: That's a great idea, and I agree with you entirely. I
think there are so many places in the universe that allow life that
I think life is almost everywhere in the universe. But you're asking
about intelligent life, and that's a whole different ball game. I
think that even animals are rare. The lowest animals on this planet,
worms or snails or whatever, are far from intelligent. Even they are
rare, I think, because it takes a long time for a planet to allow
them to evolve.
Tyson: It took a long time for Earth to do it. Now, when you
say "a planet," you just generalize this single case that you know
for all possible cases that could exist in the galaxy. That's kind
of bold.
Ward: It is bold, but I think time is still the constant. You
need a certain amount of time for evolution to take things from
inorganic to organic, from organic to a cell that combines with
another cell, to eventually intelligence.
Tyson: You're accusing Earth of being rare. But history is
filled with cases of people asserting that our place in the cosmos
is rare. And the march of science has continuously demonstrated the
opposite of that. So here you are now making cogent arguments, but I
see you as one in a long series of people who have come before you
trying to claim that we are the product of something special. So
what makes you the only one in that history of scientists who's
correct?
Ward: Luckily, it's not just me. I'm really cutting short a
lot of my colleagues. This has not been just my work, of course.
Tyson: When I look at the variation in the kinds of planets
there might be, it's not hard for me to imagine that you could have
a planet where all the conditions of that planet promote
intelligence, rather than here on Earth where it's kind of a special
case, or a peculiar branch that happened to eke it out of the
variation in the biosphere. It might be that there are planets out
there where intelligence is rampant.
Ward: There might be. It's a big galaxy. I'll give you that.
Intelligence is rare
Tyson: Suppose that as we start looking for planets around
stars in the galaxy we find out that planets are common, which
appears to be happening. Does that up your estimates a little bit?
Ward: No. Let me use an analogy. Every once in a while a
musical genius arises, and great symphonies come out. Now, this is
one in a million, one in ten million—a really rare
happenstance. But still it takes a long time, even for a prodigy, a
musical genius, to get to the point where he or she can write that
symphony.
“Where are the intelligent martians? They never had a
chance.”
Now, what happens if you change the rules, and all of these kids die
at two years of age, none gets even to 10 years. Maybe once in a
while you've got one rare case. You've cut down the possibilities to
near zero. And our point is, it's probably hard for a planet to
exist long enough as a habitable planet for animals to arise. A few
do, and of those few that do, you then need another long period of
time for intelligence to arise. The numbers get so small.
And by the way, let's get rid of the word "special." Special denotes
that there's some moral force or something. We don't need to think
special; all we need to think is numbers. And we talk about rarity.
We're asking in this huge Milky Way, how many intelligent
civilizations are there? Carl Sagan estimated a million. We know
there's one.
Tyson: It could be zero [laughs].
Ward: So the number is somewhere between one and a million;
maybe it's more than a million. I think the most important point is,
if it's not one, is it two? Is it ten? Is it a hundred? Even if it
were a hundred, there are 500 billion stars. That's pretty rare.
Being Earth-like
Tyson: Suppose we come to learn that solar systems that look
like ours are everywhere. Will that up your confidence in finding
intelligent life?
Ward: Let's examine the concept of an Earth-like planet. This
is the most commonly used phrase, "Earth-like planet." Well,
Earth-like planet is a moment in time. Earth-like planet is Earth as
we know it now. If we went back four billion years, our planet would
be the most hostile-looking place you can imagine; it would not be
an Earth-like planet. An Earth-like planet is not only geography, it
is time, because planets start as uninhabitable. They become
habitable if they're lucky. And they go uninhabitable again. So we
are looking again not just at the number out there. It's where they
are in the whole time frame of things.
Tyson: In principle then, Mars might have had its time in the
sun as an Earth-like planet, when now it's clearly completely
hostile to life as we know it, as is Venus.
Ward: Mars is one of the best examples. Mars probably had
water to begin with. Mars surely had early plate tectonics. It had
everything necessary for life as we know it. And it only lasted
maybe 100 million years or less. Where are the intelligent martians?
They never had a chance. Mars maybe had bacteria and then they died
out. I think that's the fate of most planets. Mars' fate is the fate
of most Earth-like planets. They don't stay Earth-like very long;
they die.
Tyson: If you look at when the dinosaurs went extinct, 65
million years ago, out of the four and a half billion years of
Earth, that's not very long. Yet in that period of time,
intelligence as we know it rose up from whatever existed; animals
went from tree shrews to modern humans.
Ward: But that's not right, Neil, because you needed all the
time that came before. You have to go all the way back to trilobites
and flatworms and everything that came before. You just can't start
65 million years ago and say, "We're starting from scratch. Here's
65 million years to intelligence." What about all that genomic
construction that was necessary over the hundreds of millions of
years prior?
The right recipe
Tyson: What, then, is your recipe for a planet that will
breed intelligence? Just go down the list, what do you need?
Ward: Let me answer by saying, What do I need to breed
animals, which is our complex life; intelligence is a whole other
ball game. Obviously, you have to have a sun that's large enough to
give a constant amount of energy over time, and yet a sun that is
not so large that it burns out quickly. As you know, our sun is
getting brighter, brighter, brighter. A billion years from now, this
planet will no longer be habitable for intelligence, because our sun
will be too bright. So you have to have something that gives you
enough time. Now, the small stars, of course, let you go for a long
time—20 or 30 billion years.
Tyson: They live longer.
Ward: They live longer, but as a planet you would have to be
so close to them as to become totally locked—just like our
moon, the same face all the time. Well, you've got one side that's
really hot, and one side that's really cold. That's not going to
work either.
Tyson: Well, then everyone lives along the border between the
hot and the cold.
Ward: I don't think so. People are studying that right now.
It does not seem to be an option. Now, you know and I know that
three-fifths of the stars in this galaxy—correct me if I'm
wrong—are those tiny stars.
Tyson: Yeah, the majority.
Ward: So we've just thrown out more than half of all the
stars in the galaxy. The big stars, they're gone. The double stars,
they're probably gone because the orbits of the planets get so
irregular that you start throwing things around. You throw out all
those. Now you're really cutting down the number. The centers of
galaxies, they're gone. So we have really cut down to maybe 10
percent of the stars in the galaxy. That's still a huge number, so
let's think about planets.
“I stand by the great Enrico Fermi’s question,
‘Where are they?’”
Earth maintains constant temperature because of the strangest
circumstance. It turns out that plate tectonics causes carbon to
recirculate in such a way that when we get too hot, we pull
greenhouse gases out of the atmosphere and turn them into coral
reefs. When we get too cold, the coral reefs die. Volcanoes put more
carbon dioxide in the atmosphere. This only works because of plate
tectonics. One of the most interesting aspects of studying the
geology of planets is asking the question, How common will plate
tectonics be? We don't know, but all indications are it's rare. It
takes very specific geological conditions to allow it.
Where are they?
Tyson: Getting back to intelligence, if we're not alone,
you're telling me there's other intelligence out there in the
cosmos?
Ward: I cannot believe that we're the single intelligence in
the universe. What I can believe is maybe there's one per galaxy. We
are really scattered, lonesome, very lonesome.
Tyson: So what you're saying is, if we're going to try to
find other intelligence...
Ward: Good luck!
Tyson: ...we need to broaden that search to go well beyond
the Milky Way to the rest of the galaxies in the cosmos.
Ward: There are probably so few intelligences in our galaxy
that to find any of them could be a remote, remote chance.
Tyson: So we should just give up.
Ward: I don't think we should give up. SETI is an interesting
intellectual endeavor. Why not continue it? I just don't think
they'll ever find anything. Or if they do, the separation is so
great, you could never signal back and forth.
Tyson: So in practice we're alone because they're not coming
to us, and we're not going to them?
Ward: I stand by the great physicist Enrico Fermi's question,
"Where are they?" If there were so many, we would have had a signal,
we would have seen somebody, there would have been a visitation.
That's not happened.
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