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Rare Earth Debate Part 4: Avoiding Doom

Space.com, July 2002

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Summary: 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. Today the participants 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?

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.

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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.

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