From the book
RARE EARTH
by Ward and Brownlee
This is taken from the chapter——
ASSESSING THE ODDS
The Importance—and Sheer Chance Occurrence — of Our Large Moon
Although many scientists have been doggedly pursuing the various attributes necessary for a habitable planet—Michael Hart, George Weatherill, Chris McKay, Norman Sleep, Kevin Zahnlee, David Schwartzman, Christopher Chyba, Carl Sagan, and David Des Marais come to mind—one name stands out in the scientific literature: James Kasting of Penn State University.
Kasting notes that whether habitable planets exist around other stars "depends on whether other planets exist, where they form, how big they are, and how they are spaced."
Kasting stresses, as we do, the importance of plate tectonics in creating and maintaining habitable planets, and he suggests that the presence of plate tectonics on any planet can be attributed to the planet's composition and position in its solar system. But one of Kasting's most intriguing comments is related to our Moon. Kasting notes that the obliquity (the angle of the axis of spin of a planet) of three of the four "terrestrial" planets of our solar system—Mercury, Venus, and Mars—has varied chaotically.
Earth is the exception, but only because it has a large moon.... If calculations about the obliquity changes in the absence of the moon are correct, Earth's obliquity would vary chaotically from 0 to 85 degrees on a time scale of tens of millions of years were it not for the presence of the Moon….. Earth's climatic stability is dependent to a large extent on the existence of the Moon. The Moon is now generally believed to have formed as a consequence of a glancing collision with a Mars-sized body during the later stages of the Earth's formation. If such moon-forming collisions are rare…. habitable planets might be equally rare.
We have accumulated a laundry list of potentially low-probability events or conditions necessary for animal life: not only Earth's position in the "habitable zone" of its solar system (and of its galaxy), but many others as well, including a large moon, plate tectonics, Jupiter in the wings, a magnetic field, and the many events that led up to the evolution of the first animal. Let us explore what these conditions might mean for life beyond Earth.
The Odds of Animal Life Elsewhere, and of Intelligence
In the 1950s, astronomer Frank Drake developed a thought-provoking equation to predict how many civilizations might exist in our galaxy. The point of the exercise was to estimate the likelihood of our detecting radio signals sent from other technologically advanced civilizations. This was the beginning of sporadic attempts by Earthlings to detect intelligent life on other planets. Now called the Drake Equation in its creator's honor, it has had enormous influence in a (perhaps necessarily) qualitative field. The Drake Equation is simply a string of factors that, when multiplied together, give an estimate of the number of intelligent civilizations, N, in the Milky Way galaxy.
As originally postulated, the Drake Equation is.
N* X fs X fp X ne X fi X fc X fl = N
where:
N* = stars in the Milky Way galaxy
fs = fraction of sun-like stars
fp = fraction of stars with planets
me = planets in a star's habitable zone
fi = fraction of habitable planets where life does arise
fc = fraction of planets inhabited by intelligent beings
fl = percentage of a lifetime of a planet that is marked by the presence of a communicative civilization
Our ability to assign probable values to these terms varies enormously. When Drake first published his famous equation, there were great uncertainties in most of the factors. There did (and does) exist a good estimate for the number of stars in our galaxy (over 300 billion). The number of star systems with planets, however, was very poorly known in Drake's time. Although many astronomers believed that planets were common, there was no theory that proved star formation should include the creation of planets, and many believed that the formation of planetary systems was exceedingly rare. During the 1970s and later, however, it was assumed that planets were common, in fact, Carl Sagan estimated that an average of ten planets would be found around each star. Even though no extra solar planets were found until the 1990s, their discovery seemed to vindicate those who believed planets were common. But is it so? A new look at this problem suggests that planets may indeed be quite rare—and thus the presence of animal life rarer still.
Are Stars with Planets Anomalous?
We now know that planetary formation outside our own system does indeed occur. The recent and spectacular discovery of extrasolar planets, one of the great triumphs of astronomical research in the 1990s, has proved what has long been assumed: that other stars have planets. But at what frequency? It may be that a substantial fraction of stars have planetary systems. To date, however, astronomers have succeeded in detecting only giant, "Jupiter-like" planets, available techniques cannot yet identify the smaller, rocky, terrestrial worlds. Now that numerous stars have been examined, it appears that only about 5% to 6% of examined stars have detectable planets. Because only large gas-giant planets can be detected, this figure really shows that Jupiter clones close to stars or in elliptical orbit are rare. But perhaps it indicates that planets as a whole are rare as well.
The evidence that planets may be rare comes not so much from the direct-observation approach of the planet finders (such as the Marcy/Butler group) but from spectroscopic studies of stars that appear similar to our own sun. The studies of those stars around which planets have been discovered have yielded an intriguing finding: They, like our sun, are rich in metals. According to astronomers conducting these studies, there seems to be a causal link between high metal content in a star and the presence of planets. Our own star is metal-rich. In a study of 174 stars, astronomer G. Gonzalez discovered that the sun was among the highest in metal content. It appears that we orbit a rare sun.
Other new studies also require us to question the belief that planetary systems such as our own are common. At a large meeting of astronomers held in Texas in early 1999, it was announced that 17 nearby stars had been observed to be orbited by planets the size of Jupiter. Astronomers at the meeting were also puzzled by an emerging pattern: None of the extrasolar planetary systems resembles the sun's family of planets. Geoff Marcy, the world's leading planet finder, noted that "for the first time, we have enough extrasolar planets out there to do some comparative study. We are realizing that most of the Jupiter-like objects far from their stars tool around in elliptical orbits, not circular orbits, which are the rule in our solar system."
All of the Jupiter-sized objects either were found in orbits much closer to their sun than Jupiter is to our sun, or, if they occurred at a greater distance from their sun, had highly elliptical orbits (observed in 9 of the 17 so far detected). In such planetary systems, the possibility of Earth-like planets existing in stable orbits is low. A Jupiter close to its sun will have destroyed the inner rocky planets. A Jupiter with an elliptical or decaying orbit will have disrupted planetary orbits sunward, causing smaller planets either to spiral into their sun or to be ejected into the cold grave of interstellar space.
It is still impossible to observe smaller, rocky planets orbiting other stars. Perhaps such planets—which we believe are necessary for animal life—are quite common. But perhaps this is a moot point. We have hypothesized that animal life cannot long exist on a planet unless there is a giant, Jupiter-like planet within the same planetary system—and orbiting outside the rocky planets—to protect against comet impacts. It may be that Jupiters like our own, in regular orbits, are rare as well. To date, all tend to be in orbital positions that would be lethal, rather than beneficial, to any smaller, rocky planets.
Planet Frequency and the Drake Equation
All predictions concerning the frequency of life in the Universe inherently assume that planets are common. But what if the conclusions suggested by emerging studies—that Earth-like planets are rare, and planets with metal rarer still—are true?
This finding has enormous significance for the final answer to the Drake Equation. Any factor in the equation that is close to zero yields a near-zero final answer, because all the factors are multiplied together. Carl Sagan, in 1974, estimated that the average number of planets around each star is ten. Goldsmith and Owen, in their 1992 The Search for Life in the Universe, also estimated ten planets per star. But the new findings suggest greater caution. Perhaps planetary formation is much less common than these authors have speculated.
To estimate the frequency of intelligent life, the Drake Equation hinges on the abundance of Earth-like planets around sun-like stars. The most common stars in the galaxy are M stars, fainter than the sun and nearly 100 times more numerous than solar-mass stars. These stars can generally be ruled out because their "habitable zones," where surface temperatures could be conducive to life, are uninhabitable for other reasons. To be appropriately warmed by these fainter stars, planets must be so close to the star that tidal effects from the star force them into synchronous rotation. One side of the planet always faces the star, and on the permanently dark side, the ground reaches such low temperatures that the atmosphere freezes out. Stars much more massive than the sun have stable lifetimes of only a few billion years, which might be too short for the development of advanced life and evolution of an ideal atmosphere. As we noted earlier, each planetary system around a 1-solar-mass star will have space for at least one terrestrial planet in its habitable zone. But will there actually be an Earth-sized planet orbiting its star in that space? When we take into account factors such as the abundance of planets and the location and lifetime of the habitable zone, the Drake Equation suggests that only between 1% and 0.001 % percent of all stars might have planets with habitats similar to those on Earth. But many now believe that even these small numbers are overestimated. On a universal viewpoint, the existence of a galactic habitable zone vastly reduces them.
Such percentages seem very small, but considering the vastness of the Universe, applying them to the immense numbers of stars within it can still result in very large estimates. Carl Sagan and others have mulled these various figures over and over. They ultimately arrived at an estimate of one million civilizations of creatures capable of interstellar communication existing in the Milky Way galaxy at this time. How realistic is this estimate?
If microbial life forms readily, then millions to hundreds of millions of planets in the galaxy have the potential for developing advanced life. (We expect that a much higher number will have microbial life.) However, if the advancement to animal-like life requires continental drift, the presence of a large moon, and many of the other rare Earth factors discussed in this book, then it is likely that advanced life is very rare and that Carl Sagan's estimate of a million communicating civilizations is greatly exaggerated.
If only one in 1000 Earth-like planets in a habitable zone really evolves as Earth did, then perhaps only a few thousand have advanced life. Although it could be argued that this is too pessimistic, it may also be much too optimistic. Even so, we cannot rule out the possibility that Earth is not unique in the galaxy as an abode of life that has just recently developed primitive technologies for space travel and interplanetary radio communication.
Perhaps we can suggest a new equation, which we can call the "Rare Earth Equation," tabulated for our galaxy:
N* X fp X ne X fi X fc X fl = N
where:
N* = stars in the Milky Way galaxy
fp = fraction of stars with planets
ne = planets in a star's habitable zone
fi = fraction of habitable planets where life does arise
fc = fraction of planets with life where complex metazoans arise
fl — percentage of a lifetime of a planet that is marked by the presence of complex metazoans
And what if some of the more exotic aspects of Earth's history are required, such as plate tectonics, a large moon, and a critically low number of mass extinctions? When any term of the equation approaches zero, so too does the final result. We will return to this at the end of this chapter.
If animal life is so rare, then intelligent animal life must be rarer still.
How can we define intelligence?
Our favorite definition comes from Christopher McKay of NASA, an astronomer, who defines intelligence as the "ability to construct a radio telescope." Although a chemist might define intelligence as the ability to build a test tube, or an English professor as the ability to write a sonnet, let us for the moment accept McKay's definition and follow the lines of reasoning he sets out in his wonderful essay "Time for Intelligence on Other Planets," published in 1996. Much of the following discussion comes from that source.
McKay points out that if we accept the "Principle of Mediocrity" (also known as the Copernican Principle) that Earth is quite typical and common, it follows that "intelligence has a very high probability of emerging but only after 3.5 billion years of evolution." This supposition is based on a reading of Earth's geological record, which suggests to most authors that evolution has undergone a "steady progressive development of ever more complex and sophisticated forms leading ultimately to human intelligence." Yet McKay notes—as we have tried to emphasize in this book—that evolution on Earth has not proceeded in this fashion but rather has been affected by chance events, such as the mass extinctions and continental configurations produced by continental drift. Furthermore, we believe that not only events on Earth, but also the chance fashion in which the solar system was produced, with its characteristic number of planets and planetary positions, may have had a great influence on the history of life here.
McKay breaks down the critical events in the evolution of intelligence on Earth as shown in the accompanying table.
Event
When It Happened How Long It Possible
on Earth (millions Took to Complete Minimum Time
of years ago) (millions of years) (millions of years)
THE CHART IS CONTINUED AS TO EVENTS AND TIMES WHEN IT WAS POSSIBLE FOR LIFE TO START - Keith Hunt
We can certainly quibble with some (or all) of his numbers, especially his estimate of when life first arose on Earth, for we think it occurred far earlier than 3800 to 3500 million years ago. Yet these estimates are probably not off by orders of magnitude. McKay's point is that complex life—and even intelligence—could conceivably arise faster than it did on Earth. If we accept McKay's figures, a planet could go from an abiotic state to the home of a civilization building radio telescopes in 100 million years, as compared to the nearly 4 billion years it took on Earth. But McKay also concedes that there may be other factors that require a long period:
What is not known is whether there is some aspect of the biogeo-chemical processes on a habitable planet—for example, those dealing with the burial of organic material, the maintenance of habitable temperatures as the stellar luminosity increases gradually over its main sequence lifetime, or global recycling by tectonics—that mandates the long and protracted development of the oxygen-rich biosphere that occurred on Earth. Other important unknowns include the effect of solar system structure on the origin of life and its subsequent evolution to advanced forms.
His inference is that plate tectonics has slowed the rise of oxygen on Earth. But it also may be necessary to ensure a stable oxygenated habitat, just as having the correct types of planets in a solar system is important as well.
In their 1996 essay "Biotically Mediated Surface Cooling and Habitability," Schwartzman and Shore tackle this same problem and reach a different conclusion: They believe that the most critical element in determining the rate at which intelligence can be acquired is a potentially habitable planet's rate of cooling. Their point is that complex life such as animals is extremely temperature-limited, with a very well-defined upper temperature threshold. Although some forms of animal life can exist in temperatures as high as 50°C or sometimes even 60°C, most require lower temperatures, as do the complex plants necessary to underpin animal ecosystems. A maximum temperature of 45°C is probably realistic. It is thus the time necessary for a planet to cool to below this value that is critical, according to these two authors. Many factors affect the time required, including the rate at which a star increases in luminosity through time (which works against cooling), the volcanic outgassing rate (which also works against cooling, because such out-gassing puts more greenhouse gases into a planetary atmosphere), the rate at which continental land surface grows (as continents grow, planets usually cool), the weathering rate of land areas, the number of comet or asteroid impacts and their frequency, the size of a star, whether or not plate tectonics exists, the size of the initial planetary oceans, and the history of evolution on the planet.
With this in mind, let us return to our Rare Earth Equation and flesh it out a bit by adding some of the other factors featured in this book.
N* X fp X fpm X ne X ng X fi X fc X fl X fm X fj X fme = N
where:
N* = stars in the Milky Way galaxy
fp = fraction of stars with planets
fpm = fraction of metal-rich planets
ne = planets in a star's habitable zone
ng = stars in a galactic habitable zone
fi = fraction of habitable planets where life does arise
fc — fraction of planets with life where complex metazoans arise
fl = percentage of a lifetime of a planet that is marked by the
presence of complex metazoans
fm = fraction of planets with a large moon
fj = fraction of solar systems with Jupiter-sized planets
fme = fraction of planets with a critically low number of mass extinction events
With our added elements, the number of planets with animal life gets even smaller. We have left out other aspects that may also be implicated: Snowball Earth and the inertial interchange event. Yet perhaps these too are necessary.
Again, as any term in such an equation approaches zero, so too does the final product.
How much stock can we put in such a calculation? Clearly, many of these terms are known in only the sketchiest detail.
Years from now, after the astrobiology revolution has matured, our understanding of the various factors that have allowed animal life to develop on this planet will be much greater than it is now. Many new factors will be known, and the list of variables involved will undoubtedly be amended.
But it is our contention that any strong signal can be perceived even when only sparse data are available. To us, the signal is so strong that even at this time, it appears that Earth indeed may be extraordinarily rare.
…………………………
WITH ALL THAT NASA HAS DONE WITH PROBES OUT AMONG ARE SOLAR SYTEM, WAY OUT TO PLUTO, NO LIFE OF ANY KIND HAS BEEN FOUND.
WITH THE DECADES UPON DECADES THAT WE HAVE SENT RADIO SIGNALS OUT AND BEEN LISTENING FOR ALIEN RADIO SIGNALS, TRAVELLING AT THE SPEED OF LIGHT [BUT OUR GALAXY IS 100 THOUSAND LIGHT YEARS ACROSS], NO COMMUNICATION FROM OTHER INTELLIGENT BEINGS HAS BEEN RECEIVED OR DETECTED.
FROM A THEOLOGICAL POINT IT COULD BE POSSIBLE THAT SIMPLE MICROBE LIFE IS OUT THERE SOMEWHERE; BUT I CAN TELL YOU QUITE DOGMATICALLY THAT LIFE ON THE HUMAN LEVEL IS ONLY FOUND ON THIS SMALL BLUE PLANET WE CALL EARTH.
THE CREATOR IN HIS HOLY WORD HAS SHOWN THAT THIS EARTH, AND HUMANS UPON IT, ARE PART OF A UNIQUE MASTER-PLAN.
THERE IS A VERY SPECIFIC REASON WHY MANKIND WAS CREATED. I HAVE POSTED UP A STUDY ON THIS BLOG CALLED "A CHRISTIAN'S DESTINY"---- AND IT WILL BLOW YOUR MIND AS TO THE REASON THE ALMIGHTY GOD CREATED THE HUMAN KIND - Keith Hunt
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