Ted S. Frost
“Intelligent life means being able to make a radio telescope.”—Woody Sullivan
Green Bank Radio Telescope. photo courtesy Andy Clegg, NSF
A daily highlight sixty-five years ago was arrival of the evening newspaper—Buck Rogers and rocket ships, ray guns, robots, and aliens galore. Flash Gordon on the planet Mongo heroically facing Ming the Merciless. Flights of fancy transporting this young boy from the humdrummery of a plebian lifestyle. Other worlds seemed not so fanciful in view of UFO sightings, SETI, and the Copernican principle1: the assumption that Earth, the Sun, our solar system, and the Milky Way are average. If nothing is unique about our place in the universe, why should anything be special about life here on it, including complex life, intelligent life, and human technology. With gazillions of stars, isn't it reasonable to assume others are out there? Many a starry night I've looked at the heavens and wondered if strange humanoids could be staring back at me.
Alas, comes now old age and disillusionment: after attending astrobiology classes at the University of Washington the past two years, and reading prominent scientists, advanced civilizations seem problematical. Buck Rogers and Flash Gordon might find it pretty lonely. Microbial life can thrive in hostile environments. But complex multi cellular organisms with complicated information processing systems require more benign environments. There are many constraints on intelligent life in the universe.
Cosmological Constraints: When considering extraterrestrial life in the cosmos, astrobiologists are concerned with metallicity, the measure of elements heavier than hydrogen and helium. (Astronomers consider anything heavier than H and He to be a metal. ) Since the Big Bang produced only hydrogen and helium in large quantities, metallicity, the raw materials for life, had to accumulate from subsequent supernovae. This could have taken several billionyears2. But if metallicity gets too high, it may be detrimental to earth-like planets due to formation of giant gas planets close to the host star3.
Messier 81 Spiral Galaxy
Recent discoveries of large gas planets show many orbiting so close to their stars that there’s no room for earth-likeplanetsFormation of the Moon/Giant Impactor Theory. Image Credit: NASA.
There appears to be a “Goldilocks” selection effect for metallicity: a limited period of time where the appropriate levels exist. Galaxy Constraints: Scientists also have concluded that galactic regions are limited as to their suitability for life5. It appears that only spiral galaxies, such as the Milky Way, have stable and collision-free regions. But even spiral galaxies have extensive regions that are unsuitable for life. According to recent analysis, our Milky Way galaxy has a habitable zone: a very restricted region of the Milky Way's thin disk. The thick disk, bulge, and halo regions have too much radiation, too many cosmic collisions, and too little metallicity for development of intelligent life. And on the thin disk, only the middle portion is suitable. The inner regions are too dangerous because of collisions while outer regions are too poor in metallicity. Finally, the fact our Milky Way even has a habitable zone maybe unusual. Eighty percent of the galaxies in our local universe appear to have less metallicity than the MilkyWay5.
Planetary System Constraints: Once we have picked a habitable site within a spiral galaxy with suitable metallicity, certain attributes are critical. First, it should be a single star system. Planetary orbits in the more common binary or multiple star systems are undoubtedly too erratic for long term evolution of intelligent life. Even if stable, differences in luminosities of the multiple stars would probably make living there very difficult. And this is assuming planets are able to form around the multiple stars in the firstplace6.
The star should be close to the size of our sun. The Sun's life span is estimated to be 1010 years, time enough for the 4.5x109 years it took our planet to form and for intelligent life to evolve. A star's life and luminosity is dependent on mass, with the life of a main sequence star varying roughly as the inverse third power of its mass. Thus a star two times the Sun's mass would have a life ˜ 1010x(1/23) = 1.25 billion years, not nearly enough time.
A star one half the Sun's mass would live approximately 80 billion years. That sounds attractive, except its luminosity would only be about 6% that of the Sun making it pretty dim. So dim an earth-like planet would have to orbit extremely close in order receive enough rays for liquid water. So close it would get zapped by solar flare radiation, and so close one side of the planet would perpetually face the star. It sounds like we'd better stick to a host star close to the size of our Sun, which eliminates 93% of all stars7.Another important attribute is existence of Jupiter sized planets outside the system's habitable zone. These intercept incoming comets and asteroids, reducing the chances of inner planets getting clobbered with extinction-causing missiles8. Finally, to maintain peace and tranquility, planetary orbits should be stable and not too elliptical.
How are we doing so far? Multiplying the foregoing constraints Drake Equation-wise gives us approximately one one-thousandth of one percent of all stars having the potential of harboring planet Mongo and Ming the Merciless. A very small percentage but, in view of the humongous number of stars out there, still a large number.
Planetary Constraints: Contrary to the Copernican principle’s expectations, there’s nothing ordinary about Earth. As pointed out a few years ago by Peter Ward and Donald Brownlee9, Earth has a number of unusual features making it conducive to life and, in particular, the evolution of complex life forms.
First, Earth resides within our solar system’s continuously habitable zone—the circular region around the sun where liquid H2O can exist—far enough out to avoid being boiled away like Venus, but near enough in to avoid freezing like Mars. This is a rather narrow band, estimated to be between .95 and1.25 A.U’s10. Fortunately, Earth’s orbit is nearly circular, keeping it well within this zone.
Earth is big enough to retain its internal heat, atmosphere, and water, but not so large that its gravity retains a runaway greenhouse atmosphere, or is excessively attractive to asteroids and comets. Earth was born early enough to have a generous inventory of radioactive elements to keep its interior hot and liquid during the course of life’s evolution on its surface. Yet, Earth was born late enough to have the metallicity needed.
Solar System Planets. Image credit NASA
Earth has a giant moon, thought to be the result of an early collision with a Mars-sized planet11. This chance occurrence had many fortuitous consequences. It carried off a big chunk of Earth’s light upper crust, thereby allowing continents to rise above ocean basins rather than being covered by oceans. It has given Earth an enrichment of heavy metals, strong fractionalization of materials, a hot semi-liquid interior that created plate tectonics vital for climatic stability, cycling of chemicals essential to life, and the creation of mountainous continents12. It gave us a strong protective magnetic shield, life-stimulating tides, a non-wobbly23o tilt, as well as a romantic night-light for lovers. Finally, there is one other factor. Habitable zones don’tstay habitable forever. Our sun keeps getting brighter. When Earth first formed, it was only 70% as bright as it is today. Cosmologists estimate that in another billion years the sun will be so bright Earth’s animal life will be extinguished13. Which means there is only a finite window of time for intelligent life to evolve and exist on a planet.
Formation of the Moon/Giant Impactor Theory. Image Credit: NASA
Origin of Life Constraints: Now that we have suitable chemicals and a suitable habitat for life, how does it begin? How do pre biotic molecules become life? Many origin-of-life scenarios have been proposed: pre biotic soup coalescing into heterotrophicpolymers in warm ponds of seawater,1 two dimensional autotrophic film growing on pyrite crystals associated with volcanic vents,15 an RNA world of primitive self replicating strands of RNA,16 networks of autocatalytic self-replicating proteins emerging from yet-to-be proven complexity laws associated with Chaos theory17.All these ideas have weaknesses 18: still, biologists retain optimism that life will spontaneously arise if suitable conditions are available. Given the unimaginable size of the universe, it is assumed that life in some form or other likely exists other than here on Earth19.Let us accept this premise, even as an article of faith if we have to, and assume that life does arise somewhere, somehow.
Evolutionary Constraints: Now that we have started life, how do we proceed to intelligent life? Most biologists consider evolution to be non-teleological, a random process that tracks opportunistic pathways but is blind to any goal or destination other than survival of one’s progeny20. You cannot look at evolution as aiming towards the the human species21. Stephen Jay Gould speculated that if you rewound and replayed evolution’s tape, the chances of humans showing up again would be vanishingly small22. Having all the necessary geological, environmental and cosmological events repeat themselves is too unlikely.
To be sure, there does seem to be a long-term trend towards increasing levels of complexity, but many biologists hold that complexity does not inevitably lead to intelligence. Of the millions of lineages evolving and existing over millions of years, intelligence has arisen only once—on one obscure twig of one obscure branch of one particular phylum. And that took 3.8billion years. Obviously, high intellect isn’t necessary for making a living on Earth, as countless microorganisms would testify (if they could).
It is true some biologists think otherwise—that evolution proceeds along convergent pathways to ever increasing levels of complexity and that emergence of sentience is inevitable23. However, from an extraterrestrial point of view, you have to keep in mind that intelligent life (i.e. Homo sapiens) has been around for only about 100,000 years24. And high-tech intelligence for a hundred years, or so. Not even an eye blink of time in Earth’s history. Granted, it is dangerous to generalize from a sample of one, but since that is all we have to go on, astrobiology must keep Earth’s experience in mind when considering prospects for encountering extraterrestrial intelligent life. But let us take the more optimistic view that evolution eventually does lead to neurological complexity resulting in intelligence, whether of humanoid form or not.
Technological Constraints: Being smart does not necessarily mean you can make a radio telescope. Homo Sapiens has been ‘smart’ for some time, but only recently has been able to satisfy Professor Sullivan’s radio telescope criterion. High tech applications depend on intelligence, knowledge, energy sources, and available raw materials, such as accessible mineral deposits, as well as a stable land platform such as Earth’s continents. This precludes water worlds or cloud worlds where intelligent life would have a hard time forging metallic instruments, as well as acquiring the necessary knowledge of chemistry and physics. As Guillermo Gonzalez points out25, Earth is a rare classroom for acquiring knowledge of physics, astronomy, and chemistry, due to its position in thesolar system, its moon, and its favorable geological and atmospheric attributes. Intelligent beings on planets not so favored might remain in the scientific and technological dark a long time, despite bulging brains and high I.Q.’s.
Civilization Constraints: High-tech applications also require stable, cooperative, and highly organized societies. But being able to make a radio telescope also means being able to make hydrogen bombs, a variety of pollutants, and a great many surviving babies. So far (knock on wood), we’ve avoided blowing ourselves up. But we are in dire need of restraints on population. And global warming is real and accelerating. How much longer can our techonological civilization exist without doing itself in? (factor fL of the Drake Equation)? Astrobiologists consider these constraints as placing practical limitations on the lives of high techcivilizations26. The flames of high tech civilizations may flicker only briefly before burning themselves out. Constraint after constraint after constraint. Will we ever be visited by strange little men in spaceships? It looks bleak. But keep in mind we yet have much to learn about physics and reality. Some scientists suggest concepts like string and M-brane theory, parallel universes, and traversable wormhole possibilities of general relativity mean the idea of advanced civilizations shouldn’t be dismissed, and that some UFO sightings could be credible27. I remain optimistic. E. T. lives, I tell you!
1. “A What You See Is What You Beget Theory,” T. Rothman, Discovery(May 1987).
2. “Metallicity Evolution in the Early Universe,” J. Prochaska & A. Wolfe,The Astrophysical Journal, 533:L5 (2000)
3. “Formulation of Terrestrial Planets in the Universe ,” C. Lineweaver,Earth System Processes Global Meeting, Edinburgh (2001)
4. “Extrasolar Planet” New Scientist, (England, 2002)
5. “The Galactic Habitable Zone,” G. Gonzalez, D. Brownlee, & P. Ward Icarus, Vol. 152 (2001)
6. Rare Earth, p. 24, D. Brownlee & P. Ward, (Copernicus 2000)
7. Life in the Universe, p. 250, J. Bennett, S. Shostak, & B. Jakosky(Addison Wesley, 2003)
8. The Privileged Planet, p. 114, G. Gonzalez & J. Richards, (Regnery Publishing, 2004)
9. Rare Earth, p. 24, D. Brownlee & P. Ward, (Copernicus 2000)
10. The Earth System, p. 323, L. Krump, J. Kasting, & R. Crane,(Prentice-Hall, 1999)
11. “Origin of the Moon,” Hartmann et al., Conference on the Origin of the Moon, Kona, HI (1984)
12. Rare Earth, p. 200, D. Brownlee & P. Ward, (Copernicus 2000)
13. Life in the Universe, p. 240, J. Bennett, S. Shostak, & B. Jakosky(Addison Wesley, 2003)
14. Life’s Origin, J. Schopf, (Univ. of Calif. Press, 2002)
15. “Before Enzymes and Template,” G. Wachterhauser, MicrobiologicalReview (Dec. 1988)
16. “The RNA World,” W. Gilbert, Nature 319:618 (1986)
17. At Home in the Universe, S. Kauffman, (Oxford Univ. Press, 1995)
18. Life’s Solution, p. 49-50, S. C. Morris, (Cambridge Univ.Press, 2003).Also: Origins, A Skeptic’s Guide to Creation of Life on Earth, p. 186,- R. Shapero, (Bantam ed., Simon & Schuster 1986).
19. Vital Dust, p. xv, C. de Duve, (Harper Collins 1995)
20. Evolution, 3rd ed., p. 451, M. Strickberger (Jones & Bartlett, 2000)
21. Evolutionary Biology, 2nd ed., p. 369, D. Futuyama (Sinauer, 1986)
22. Wonderful Life, Chapters IV & V, S. J. Gould, - (Norton 1989)
23. Life’s Solution, p. 328, S. C. Morris, (Cambridge Univ.Press, 2003).
24. “Once We Were Not Alone,”Ian Tattersall, Scientific American, Vol13No .22 (2003).
25. The Privileged Planet, p. 114, G. Gonzalez & J. Richards, (RegneryPublishing, 2004)
26. The Earth System, p. 328-9, L. Krump, J. Kasting, & R. Crane,(Prentice- Hall, 1999) pg. 328-9
27. “Inflation-Theory Implications for Extraterrestrial Travel,”Haisch et al., Journal of the British Interplanetary Society, Vol. 58 (2005)