A recent refinement of the probabilities for life elsewhere uses a new kind of heuristic called the "Astrobiology Matrix". Its many terms draw on rapid advances in chemistry, biology, geology and astronomy--but the core of the argument remains: how to define a frequency for life elsewhere, when very large [infinite] number of cases all get a multiplicative probability that individually can seem infinitesimally small?
|In a universe brimming with stars, it is difficult to imagine that life exists nowhere else. The infinite multiplied by 'near-zero' is still finite.
Astrobiology Magazine has previously examined many of the small probabilities that might define a frequency for life elsewhere, including the probability for intelligent life, the probability for compatible host stars, and the probability of earth-like conditions on any stably orbitting exoplanets.
As a difficult and multi-dimensional question, this frequency is also about a matrix.
The Copernicans: 'Nothing So Wonderful is So Special'
As Carl Sagan's collaborator on the series Cosmos--Steven Soter, of the American Museum of Natural History--, told Astrobiology Magazine:"You've got, it now looks like, something on the order of a trillion planets in our Milky Way galaxy alone, and a hundred billion other galaxies. Those numbers are staggering. My own opinion-and it's, I stress, still only an opinion-is that the universe is full of life, that we're not alone"
The sixteenth century Italian philosopher Giordano Bruno is often credited as the first to proclaim publicly of an infinite universe that harbors countless inhabited worlds. For that proclamation, Bruno was burned naked and upside down at the stake. By the early 1900s, the most famous scientist of his time, Simon Newcomb, could write what today seems increasingly self-evident: "We all know that this earth on which we dwell is only one of countless millions of globes scattered through the wilds of infinite space...If so, the probabilities are that millions of them are essentially similar to our own globe. Have we any reason to believe that life exists on these other worlds?"
|Frank Drake (above) believes that there could be a million intelligent civilizations in the Milky Way galaxy, and probably billions of such civilizations throughout the universe. Model calculations can be simulated with the simple Drake Equation calculator at the end of this article.
Image Credit: SETI Institute
Billions and billions...
Answering this perplexing and important question of life elsewhere, however, is a task for one of the only cross-disciplinary studies that must speculate without alot of statistical data. Indeed, an inhabited Earth is a statistical sample of one. As Dr. Michael Meyer, astrobiology senior scientist at NASA Headquarters, Washington, put the problem: "if [the] question is about intelligent life, the unknowns are tremendous. Out of approximately 10, 000, 000, 000, 000, 000, 000, 000 stars, it seems reasonable that at least one other star is capable of harboring a planet sustaining complex life. If complex life is rare, space is so vast and inimical to life that we may never learn about our nearest neighbor. But, if there are many planetary experiments that did not fail, then it is just a matter of time before we learn that we are not alone."
The Drake Matrix
With increasing precision astrobiologists can guess at how many intelligent civilizations may be out there--a guess that sensitively depends on the multiplication of small probabilities. Frank Drake made a stab at guessing the number in 1961, when he formulated the "Drake Equation." According to this equation, there could be a million intelligent civilizations in the Milky Way galaxy, and probably billions of such civilizations throughout the universe.
The Drake Equation is based, in part, on an estimate of the number of planets in the galaxy that might harbor life. Such planets would have to exist in "habitable zones" - those regions around stars that would best support life as we know it. These planets would be the most likely places where life capable of achieving intelligence is fostered and sustained.
In detail, the Drake equation illustrates a method for estimating the number of communicating civilizations in the galaxy: as the number of stars in the Milky Way galaxy [N], times the fraction of stars with planets around them [fp], times the number of planets per star ecologically able to sustain life [ne], times the fraction of those planets where life actually evolves [fl], times those that evolve intelligent life [fi], times the number that communicate [fc], times the fraction of a planet's life during which the communication civilization survives [fL].
The Mysteries of Intelligence: fi
Intelligent life may be special. At least that is one contention of co-author of the book "Rare Earth", Peter Ward, a biopaleontolgist at the University of Washington. Ward described to Astrobiology Magazine, that there are very different probabilities for life in the simple sense of single cells versus intelligent life. His thesis can be thought to draw into doubt the probability of intelligent life, or fi:
"In my view, life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake and Sagan envisioned. However, complex life - animals and higher plants - is likely to be far more rare than commonly assumed. Life on Earth evolved from single celled organisms to multi-cellular creatures with tissues and organs, climaxing in animals and higher plants. But is Earth's particular history of life - one of increasing complexity to an animal grade of evolution - an inevitable result of evolution, or even a common one? Perhaps life is common, but complex life - anything that is multi-cellular - is not."
|The Drake Matrix: How to compare the astrobiological potential of different planetary and stellar bodies? More than just considering chemistry, energy and time, it tries to offer a method for addressing the questions of exactly how long, and what limits might exist for metals and water. Different energy sources are considered as solar [electromagnetic], tidal [gravity], or radioactive [nuclear]. To take a concrete example, Jupiter's moon Europa is thought to have an ice-covered ocean which is heated by tidal and radioactive energy from its parent planet. In that sense, Earth is unlike Europa, since Earth uses electromagnetic energy predominantly and depends intimately on liquid water and its iron core.
Image Credit: Hungarian Astronomical Association, A. Mizser and A. Kereszturi
Based on the Drake equation, what are the individual probabilities for intelligent life? Frank Drake notes that the history of science has often been sidetracked by overemphasizing the importance of humans' unique place in the universe: "We look at the Earth, and with regards to that origin, as best we know, no special or freak circumstances were required. It took water, organics, a source of energy, and a long time."
Indeed, somewhat ironically, one of the least well-understood factors in the development of intelligent life may be the one factor that humans themselves directly can control: the lifetime of their technological civilization. The Drake equation requires a long enough technology opportunity for radio communication to mature. There is no sampling data available to crunch numerically there, at least not unless the only known civilization now capable of doing the calculation is lost.
So as a connected train of small probabilities that began with the seemingly vast number of favorable stars, one of the least well-known factors may well be the last one: what is the lifetime of our technological civilization?
The Astrobiology Matrix
Recent college courses at the Eötvös Loránd University of Sciences in Budapest have been organized by two members of the Hungarian Astronomical Association, A. Mizser and A. Kereszturi. Because astrobiology doesn't fall into a single university department, but instead crosses many different disciplines, the team has devised a matrix to visualize how an answer to the question of life elsewhere depends on cross-cutting traditional academic boundaries. They call their table, the Astrobiology Matrix.
|The Astrobiology Matrix
Image Credit: Hungarian Astronomical Association, A. Mizser and A. Kereszturi
About the Astrobiology Matrix, the co-authors wrote: "We had found this table useful in the explanation 'what is astrobiology?'."
The concept of time is emphasized in the table, and plays a different role than in the Drake Equation. Time here represents a flow from the top of the matrix down to the bottom. As life unfolded in time, biology comes after chemistry and physics, just as the biosphere had to wait for an exoplanet to form first. Similarly evolution of cells must follow the ascension of prebiotic molecules. This direction is emphasized from top to bottom.
The concept of complexity is also different from the probabilities in the Drake Equation. It has been a long-held impression in science that the more complex the natural system, the more unlikely it might be. A complex system is not easy to make, or it wouldn't be considered complex.
This same argument is common to the evolution debate, and is often summarized as 'an eye is too complex to arise as a series of random mutations'. But increasingly, the development of complexity from rather simple processes has attracted serious mathematical and biological study.
The Astrobiology Matrix tries to illustrate the ascension of complexity directionally, from the top of the table to the bottom. From simpler starting materials at the top to life at the bottom--and in between, what must arise first are the heavy atomic nuclei, molecules, prebiotic systems, living organisms and finally ecological systems.
In this way, the Astrobiology Matrix is as much about the future of life as its past. To summarize the question 'What is astrobiology?' : Astrobiology is the study of the origin, evolution, dispersion and future of life in the universe. That is its task, its probabilistic equation and also its multi-dimensional matrix.
Despite all the rationale behind various heuristics for illustration, the question of how many intelligent civilizations are out there can only be answered if we discover alien life. NASA is planning to launch the Terrestrial Planet Finder in 2012. This satellite will operate for 6 years, searching for Earth-sized planets around distant stars.
In the meantime, scientists with the Search for Extra Terrestrial Intelligence (SETI) continue to explore the electromagnetic spectrum for alien transmissions. The SETI Institute recently published "SETI 2020," a book detailing the focus of SETI strategies between now and the year 2020.
Related Web Pages
Drake Matrix [PDF]: Mizser, A.; Kereszturi, A. The Astrobiology Matrix and the "Drake Matrix" in Education [ 34th Annual Lunar and Planetary Science Conference, March 17-21, 2003, League City, Texas, no.1114 ]
The Great Debate: Is Complex Life Common in the Universe?
Cosmic Imperative for Life: Ann Druyan Interview
Search for Life in the Universe: Neil deGrasse Tyson Interview