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Retrospections Lecture The Drake Equation Revisited Cause for Optimism: Part III
Cause for Optimism: Part III
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Alien Life
Posted:   10/13/03

Summary: The NASA exobiology branch sponsored a public forum in Palo Alto, CA, on Tuesday, August 26, 2003, entitled, "The Drake Equation Revisited." The forum addressed the questions of estimating the probabilities for finding intelligent life in the universe. This is the third in the series of presentations.

Cause for Optimism: Part III

The Drake equation was developed as a means of predicting the likelihood of detecting other intelligent civilizations in our galaxy. At the NASA exobiology forum, Frank Drake, who formulated the equation 42 years ago, moderated a debate between Peter Ward and David Grinspoon.

David Grinspoon is a principal scientist with the department of space studies at the Southwest Research Institute in Boulder and he's the author of Venus Revealed and the forthcoming Lonely Planets: The Natural Philosophy of Alien Life.

In this installment of the series, Dr. Grinspoon responds to Dr. Ward's comments, explaining why he is optimistic about the possibility of making contact with intelligent extraterrestrials.

In part I of this series, Frank Drake discussed the history and content of the Drake equation. In part II, Peter Ward argued that contact with intelligent extraterrestrials is unlikely. Parts IV and V will present the question-and-answer period that followed the opening remarks by Drake, Ward and Grinspoon.Parts 1 * 2 * 4 * 5

"It was commonly believed by serious scientists that there was least vegetation on Mars." -David Grinspoon
Credit: NASA/JPL/Malin Space Science Systems

David Grinspoon: First of all, I'm happy to be here and honored to be sharing the stage with Frank Drake and Peter Ward. Frank Drake has been a hero of mine, I'd have to say, since I was about eight years old, or whenever it was that I gained the sophistication to understand what it was that he does. I just thought that was the coolest. And I still do.

And, of course, Peter Ward, the author of Rare Earth. and I have to say that I think it's a wonderful book. I totally recommend that you all read it, if you haven't. It's got some great descriptions of some really neat science and very current description of a lot of what we know about the history of Earth and the history of life on Earth.

I disagree to some extent with some of the implications of all that knowledge that we can draw for life in the wider Universe and I think that's what we're going to talk about here a little bit. But it's certainly well-argued and well-written and I think that Ward and Brownlee have done us all a favor by instigating what I would call a minor anti-Copernican backlash. There is this wish to believe that we're connected to the cosmos and that there's lots of life out there. Personally, I think that there are also rational reasons to believe that. But we don't want to believe it just because of wishful thinking and so it's great to have somebody come along and kind of challenge that view.

Thinking about the 40 years of advancement in our knowledge - or now I guess it's 42 years - since Frank Drake first wrote the Drake equation on the board at Green Bank what have we really learned that is new and different since that time, that really sheds new light on this question? To my mind there are three developments. There are a lot more, but there are three that come to mind that are significant. And one, I think, is in a certain sense negative as far as the prospects of extraterrestrial intelligence, and the other two are positive.

The one that I would say is perhaps slightly negative is that at the beginning of the 60s there was more optimism about life on our immediate neighbors in our solar system than there is now. It was commonly believed by serious scientists that there was life on Mars, that there was at least vegetation on Mars. You can go back and find papers written in the 50s and early 60s that suggest that the markings that you can see changing on the surface of Mars - and, by the way, this is a very, very good week to see them, as I'm sure you've read, and hopefully have seen - that there was a lot of belief that there might be vegetation. And even Venus, which, of course, now we all think of as just hell, because of the results of the wonderful 40 years of planetary exploration that we've had was at that time thought to be perhaps a watery friendly planet. So our planetary exploration has been a little bit sobering as far as our hopes for life on our immediate neighbors.

Click here for larger image. Europa's ocean.
Credit: NASA/JPL/University of Arizona

But then, of course, as you go out a little bit further, as Frank mentioned, there's the discovery of an ocean on Europa and the broadening of our views about what kinds of planets might support life. So that goes both directions.

But there are two other developments that I think are most significant as far as what is really changed in 42 years. First of all, we've found planets orbiting other stars. We know that they are there. We don't know very much about them yet. We've only found the ones that are easy to see, which as Frank mentioned, are the ones that we know are not going to be like Earth. And there's still the majority of stars, by far - we have no idea whether or not they have planets. So there's still more that we don't know, much more, about extrasolar planets.

And over the next coming decade and decades, it's going to be very exciting as that picture of the demographics of those planets clarify. But, no matter what, I think we have to see that development as very, very encouraging for the prospects of extraterrestrial life, and I would say extraterrestrial intelligence. The planets are out there.

And then, finally, there's so much that we've learned about life on Earth. In particular, you've heard about extremophiles. The range of conditions that life can exist on the Earth, we had no idea, again, in the early 60s, when the Drake equation was formulated, about this incredible array of strategies for survival that organisms on Earth have developed, in terms of the kinds of fuel they can use, what they can eat, what they can breathe, the kinds of temperatures and pressures and acidities they can survive.

And, again, I think that's a very encouraging development. It tells us that life is even more tenacious and adaptable and just clever - not clever in an intentional sense, but clever in terms of the problem-solving capabilities of evolution by natural selection. And you know that we still have not gotten anywhere near the bottom of that well. We're still discovering extremophiles, extreme life on Earth, that surprises us in the range of environments that it can survive in. And that has got to be a hopeful indication of the kinds of environments where life can survive in the Universe.

Now, as far as the rare-Earth hypothesis, let me say why I don't completely agree with the kind of pessimistic conclusion that is drawn in it.

The Rare Earth hypothesis, put forth by Peter Ward and Donald Brownlee in their book, Rare Earth, suggests that Earth-like planets containing complex (animal) life as we know it are likely quite rare in the Universe.
Credit: NASA CERES Project /

Peter Ward: You alluded to the rare-Earth hypothesis. Brownlee and I simply said we thought life would be widespread, almost universal, thinking at that time that life evolved easily. But that complex life would be almost impossible to find. Or at far lower and lower levels.

David Grinspoon: Okay, thanks. So the idea that simple life is going to be more widespread than complex life in the Universe is kind of obvious. It's almost a tautology. If you define complex life as something that sometimes evolves out of simple life and sometimes doesn't, then it is a tautology. It's a little bit like saying there are more acorns than there are oak trees. I mean, we all agree that there's likely to be more simple life than complex life.

The real question is: What is the ratio? In terms of those factors that Frank described, what are those factors fi and L? So it only becomes interesting when you quantify it a little bit. Otherwise it's obvious.

What kind of conclusion can we draw about the history of life on Earth? In Rare Earth, a lot is made out of various historical events that happened to the Earth, and various features of Earth that seem to be unusual. Earth has a large moon; most planets don't have a large moon. There are various ways in which our large moon is related to the history of life here. Earth has a particular climate, has the existence of plate tectonics, and so forth. And, in my mind, there's no question that all of these, what they call the rare-Earth factors, are very closely linked to the evolution of life and the evolution of complex life on Earth.

But I think there's a logical fallacy we make when we look at that history and we say: Only with this set of circumstances could we have arrived at a planet with complex life. I think you can imagine a lot of different kind of planetary histories that are very, very different from the history of Earth, with different kinds of unlikely historical contingencies, and complex life arising on that planet and first looking around and looking back at the history of its own planet and saying, "Well, look at all these unlikely things, and here we are. So clearly only on a planet like this can you have complex life." So the fact that complex and seemingly unlikely set of circumstances leads to complex life on one planet does not mean that the same set of circumstances has to apply.

According to Grinspoon, rare-Earth factors (such as our large moon) are very closely linked to the evolution of complex life on Earth.
Credit: NASA

If you really buy that you must have a moon like Earth's moon, and you must have a planet the same size, with the same kind of water inventory and all of these things have to be very finely tuned, then I think you can make a case for complex life being much more rare. But I think we simply just don't know that.

Another thing that's worth considering is the role of life itself in shaping the Earth and the way that the Earth is. Life is not just along for the ride on Earth. The history of Earth and it's environment is a complex interplay between life and the non-living parts of the planet. It's a give and take. Life has been a major player in determining the composition of the atmosphere, the composition of the oceans. Even, I think you can argue, things like plate tectonics, in the long run, are related to the existence of life and its effect on the hydrosphere and the atmosphere, and so forth. It's all related.

I think there's a way that the rare-Earth hypothesis, at least taken to an extreme, you could call it the Pangloss hypothesis, in honor of Voltaire's character, Dr. Pangloss, who said that all things must be as they are in this, the most perfect of all worlds. I think that the rare-Earth hypothesis verges a little bit on claiming that this is the most perfect of all worlds, and that if you varied any of these factors, you would come up with a less-habitable planet.

The fact is, we don't know that Earth is optimal for life, or complex life. Earth might be not that great a planet for complex life. It seems like almost a heretical thing to say, but we have no idea if this is the perfect planet for life. If you vary some of these factors, if Earth was a little bit larger, had a little bit more water, was within a different solar system with a bigger Jupiter or a smaller Jupiter, so we had a different impact rate affecting the rate of extinctions and evolution, life might have proceeded a lot faster. There might have been intelligent life on Earth after 2 billion years. And by now, who knows what would be here?

So I think the whole notion that Earth's history is somehow optimized for complex life and it can't get any better than this is - to me it seems like pre-Copernican thinking. [So I think we need to get] away from this prejudice of assuming that if a planet isn't like Earth in one of these various ways, that that's going to mean that it's less likely to have complex life. It might make it more likely to have complex life.

This story has been translated into Portuguese.

Related Web Pages

Rare Earth Debates: Complex Life
Life in Extreme Environments
Exploring the Highest Lake on Earth
Astrobiology/Life's Origins

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