Exobiology

Extraterrestrial life refers to theoretical forms of life that may exist and originate outside of the planet Earth.Forms of extraterrestrial life, or "life on other planets", range from the humanoid and monstrous beings like those from science fiction works, to life at the level of microbes and bacteria. Since little potential evidence of life on other planets exists, and none which has been confirmed by science, the notion that extraterrestrial life exists is entirely theoretical.Extraterrestrial life forms, especially intelligent ones, are often referred to as aliens.

Possible forms of extraterrestrial life.
Some theoreticists on the topic believe that life as we know it, namely carbon-based organisms, may not be the only structure upon which what we consider the concept of life (e.g. growth, consumption, reproduction, or even sentience) can be built.

The scientific study of extraterrestrial life is often called xenobiology.

Search for extraterrestrial life.
Scientists are searching for extraterrestrial life in two very different ways, directly and indirectly.
Direct search:
They are directly searching for evidence of unicellular life within the solar system: searching Mars and examining meteors which have fallen to Earth, and a proposed mission to Europa, one of Jupiter's moons with a liquid water layer under its surface, which may contain life.

There is some suggestion of the existence of microbial life on Mars. An experiment on the Viking Mars lander reported gas emissions from heated Martian soil that some argue are consistent with the presence of microbes, though the lack of corroborating evidence from other experiments on the Viking indicates that a non-biological reaction is a more likely hypothesis. Independently, in 1996 structures resembling bacteria were reportedly discovered in a meteorite known to be formed of rock ejected from Mars. Again, this report is vigorously disputed.

Indirect search:
It is theorised that any technological society in space will be transmitting information: man-made electromagnetic radiation is already detectable within an 80 light-year radius of Earth, and is constantly spreading. SETI, the Search for Extraterrestrial Intelligence, takes the data gathered by the world's largest radiotelescopes and analyses it for artificial patterns using supercomputers and one of the largest distributed computing projects in the world, SETI@home.

Some scientists believe that some UFOs are the spacecraft of intelligent extraterrestrials; however since these scientists are currently very much in the minority, work such as SETI continues in the hopes that a signal will be detected.

Astronomers also search for extrasolar planets that would be conducive to life. Current radiodetection methods have been inadequate for such a search, as the resolution afforded by recent technology is inadequate for detailed study of extrasolar planetary objects. Future telescopes should be able to image planets around nearby stars, which may reveal the presence of life (either directly or through spectrography revealing, for instance, the presence of free oxygen in a planet's atmosphere). It has been argued that one of the best candidates for the discovery of life-supporting planets may be Alpha Centauri, the closest star system to Earth.

Origins of extraterrestrial life.
Numerous hypotheses have been developed to validate the possibilty that extraterrestrial life could exist. (This is in contrast to most theories, which attempt to explain something that is commonly accepted as occurring.)

One such theory is panspermia, which holds that all life throughout the universe stems from one initial distribution of spores which consitute the seeds of life. If true, it would then follow that life is prevalent through space as these spores have traveled, and that life in various forms may exist throughout the universe.

Fermi paradox

The Fermi paradox is a paradox proposed by physicist Enrico Fermi that questions the possibilities of finding intelligent extraterrestrial life. More specifically, it deals with the attempts to answer one of the most profound questions of all time: "Are we (the earthlings) the only technologically advanced civilization in the Universe?". The Drake equation for estimating the number of extraterrestrial civilizations with which we might come in contact seems to imply that we should not expect such contact to be extremely rare. Fermi's response to this conclusion was that if there were very many advanced extraterrestrial civilizations in our galaxy, then, "Where are they? Why haven't we seen any traces of intelligent extraterrestrial life e.g. probes, spacecraft or transmissions?". Those that adhere to the premise behind the Fermi paradox often refer to that premise as the Fermi principle.

The paradox can be summed up as follows: The commonly held belief that the universe has many technologically advanced civilizations combined with our observations that suggest otherwise, is paradoxical, suggesting that either our understanding or our observations are flawed or incomplete.

Is there extraterrestrial life?
Many people believe that extraterrestrial life exists and that there are many planets in our own galaxy that harbor life. The idea that life is common everywhere and propelled from star to star by the pressure of starlight was proposed by Svante Arrhenius, who called it "panspermia," but this theory is now in disfavor.

Some believe that our current knowledge of both chemistry and of biology strongly indicates that life is an exceptionally improbable thing to arise spontaneously. "Strong life" proponents counter that because life arose on Earth as soon as the crust cooled, life itself must be intrinsically linked with terrestrial planet formation. Current data on this issue seems to support this second view or a related hypothesis that life originated elsewhere within the solar system and was transported to the Earth by a meteorite. The fact that signs of life on Earth seem to be present almost as soon as it cooled enough to support it, that life has been found in a variety of environments once thought incapable of supporting it, that planet formation seems to be fairly common, and that conditions to support bacterial life seem to exist elsewhere in our own solar system all support the position that life should be fairly common. A statistical analysis that treats the question of life arising on a planet like winning a lottery—and generalizing from the special case that, on the only terrestrial world we have seen, the lottery was won—some astrobiologists have concluded that there seems to be at least a one-in-eight chance per billion years of "appropriate" conditions that life will form.

As for the Earthly origin of life, it now seems fairly certain that it began within our solar system. The harsh radiation of interstellar space coupled with the extremely low probability that any extra-solar rocks capable of protecting life in the harsh inter-stellar environment have ever struck the Earth seem to indicate that, if terrestrial life originated elsewhere, it would almost certainly have to have been carried here on purpose. It is possible that life was brought here, but, if so, it becomes difficult to explain why the first forms of life were simple, single-celled life instead of further up the evolutionary chain, although Timothy Leary has suggested extra-terrestrial seeding of simple amino acids in his Exopsychology, relating evolution and Leary's eight circuit model of human consciousness.

A widely-accepted view is that terrestrial life originated on the Earth itself. Lately, there has been increasingly more support for an idea first mentioned by Lord Kelvin— that life first came about on Mars and was transported to Earth by a meteorite. This latter position is defended on the basis that conditions which might support Earth-compatible life existed within a relatively short distance hundreds of millions of years before the Earth cooled. The more improbable that one deems life beginning spontaneously, the more likely it becomes that life arose first on Mars.

The issue of whether intelligent life develops as readily as simpler forms is still an open question.

The Rare Earth hypothesis.
An emerging line of thought even argues that multicellular life may be exceedingly rare in the universe because of a probable rarity of Earth-like planets. This line of reasoning has been dubbed the Rare Earth hypothesis and relies on that fact that many improbable coincidences converged to make complex life on Earth possible.

Spiral arms have many novas, and the radiation from them is believed inimical to higher life. The solar system is in a very special orbit within the galaxy. It is a nearly perfect circular orbit, at a distance in which the solar system moves at the same speed as the shock waves forming the spiral arms. The Earth has been between spiral arms for hundreds of millions of years, more than thirty galactic orbits, almost all of the time there has been higher life on Earth.

Another crucial item is the Moon. Many scientists believe it was formed by a rare collision between the young Earth and a Mars-sized body 4,450 million years ago. The collision had to occur at an exact angle; too direct and Earth would have been obliterated, too shallow and the Mars-sized body would have been deflected. This giant impact sent much of the felsic rich mantle of Earth into orbit. The removal of light-rock types (felsic rock) allowed for the formation of the first ocean basins (which are heavier (mafic) rock). The impact spun the Earth. Lunar tides stabilize the Earth's axis. The axis of rotation of a sphere is unstable, and if the Earth's axis varied, the weather would vary dramatically—potentially suppressing life. Lunar tides also have helped heat the mantle. The molten mantle generates the magnetic field of the Earth. The magnetic field shields the Earth's air from the solar wind, which would otherwise accelerate light molecules away, sapping the air and water over a period of a few million years.

Furthermore, the presence of different crustal rock types allows for the existence of plate tectonics, which recycles limestone into biologically-active carbon dioxide.

This is just part of the Rare Earth hypothesis.

The Drake equation.
Those people who believe in the more optimistic assumptions used in the Drake equation proposed by Dr. Frank Drake and the even more optimistic assumptions given by Dr. Carl Sagan, add that intelligent life is also common in the Universe. Some state that by making what they feel are reasonable assumptions and arguments we can ascertain that if life is possible at all, then the universe is so vast that it should not only be possible, but almost certain that there are large numbers of extraterrestrial civilisations in the Universe. However those people who adhere to the premise of the Fermi paradox believe that, due to a lack of evidence to the contrary, in all probability, humans (as a technologically advanced species) are effectively alone in at least our part of the Milky Way Galaxy. They further say that since we cannot yet determine the variables of the Drake Equation with any real confidence, we cannot determine the numbers of extraterrestrial civilizations based solely on this equation. We must therefore, they argue, rely on data collection — which is only now beginning to be collected in a significant manner. Only then can we even begin to presume what the values of each of the variables in the Drake equation are, they say.

Current data.
Our solar system if seen from a radio telescope within a few tens of light years away would seem unusual for the huge amount of radio waves being emitted from what appears to be an otherwise unremarkable main sequence star. One can presume that similar output by a nearby star would be immediately characterized as unusual by us.

Radio and observational data have for several decades been collected and analyzed by such projects as Project Ozma, the Search for Extraterrestrial Intelligence (SETI), and the various projects searching for extrasolar planets. So far the SETI data seem to indicate that we are the only radio-transmitting species in at least that portion of our part of the Galaxy that has been surveyed; there are no known main sequence stars with unusually bright radio emissions. In addition, to date, the majority of the extrasolar planetary systems that have been found appear to be harsh environments for advanced life-forms.

Some people contend that these results probably have a significant amount of sampling error:

Other species may not use the radio frequencies we are searching in SETI or may not leak significant amounts radio waves (we leak less radio waves than a few decades ago because of the use of cable and satellite transmission).
We can more easily find planetary systems with planetary orbits and configurations that are less stable than our own.
Still other people contend that we are probably the only spacefaring species in at least our galaxy; otherwise we would be awash in their radio transmissions and be overrun by early colonization efforts.

The argument over the premise behind the Fermi paradox.

ET phone home.
Some of those who subscribe to the Fermi principle state that given enough time to develop, the radio transmissions of any sufficiently advanced civilization will begin to outshine their parent star in the radio part of the spectrum. Further, the mediocrity principle states that physical laws are the same throughout the Universe and the development of anything within the Universe has to follow these laws. Since the use of the electromagnetic spectrum for information transmission is relatively cheap and easy, one would expect any technological civilization to take advantage of at least a part of this spectrum during their development. We have been actively searching for extraterrestrial signals for almost 30 years with projects such as SETI and have been passively listening to radio static for nearly 100 years. During this entire period we have yet to hear any confirmed alien broadcasts nor have we observed any main sequence stars with unusual electromagnetic radio signatures that might indicate a technological civilization.

Those that believe the galaxy has many technologically advanced civilizations counter that the extraterrestrials may simply be using a medium other than radio or they eventually choose to hide their transmissions for some unknown reason. They also point out we are leaking progressively less radio as we transmit more TV via cable and satellite. This could very well be so, proponents of the Fermi principle say, but only if there are very few such civilizations in both space and time and they very quickly abandon radio as a means of data transmission. Either way, they say, if there were many of these civilizations their transmissions would make a large impact on at least some part of the electromagnetic spectrum for at least a small part of their development. They further state, that if there are as many advanced extraterrestrial civilizations as Drake and Sagan have estimated, then their presence would be made obvious by their transmissions. The fact that we have been able to receive and produce these transmissions for only a tiny fraction of our history may be limiting radio SETI in this regard.

The anthropic principle.
Those that believe the Fermi principle also state that from the Anthropic Principle one can see as a logical fallacy the following statement: "With billions of galaxies and countless trillions of planets in the Universe, intelligent life must exist somewhere besides Earth. After all intelligent life happened here, so why not on many of the trillions of other worlds? It is illogical to think that we are the only one." With the Anthropic Principle, Fermi principle adherents say, one can quickly point out that if a particular planet is the only planet out of the trillions that has intelligent life on it, it would be certain that the people there would assume that they could not be the only planet with intelligent life. They would think that, given the sheer numbers of other worlds, there must be others like themselves in the Universe. However, the Anthropic Principle makes it necessary to gather additional information before such an assumption could be made.

Freeman Dyson's contribution.
Popularized by Dr. Freeman Dyson, a Dyson Sphere is an opaque shell around a star. Such a shell would be created by advanced alien civilizations that wished to harness as much of the radiant energy of their sun as possible. The exact design of the Dyson sphere was not specified; it could consist of billions of independent solar collectors and space habitats or be a single unified structure, but in any case it would be made of solid matter and would intercept most of the star's emitted light to re-radiate as waste heat. A star surrounded by a Dyson sphere would thus emit a distinctive black body spectrum without the strong emission lines that incandescent stellar plasma exhibits, probably with its peak unusually far into the infrared for a star of its size. With this speculation, he advised astronomers to search the night sky for unusually colored stars, which, he postulated, could only signify highly advanced and intelligent life. No such stars have yet been found.

Some adherents to the Fermi principle state that it is highly unlikely that all advanced civilizations would not eventually take full advantage of the power source of their home star, and in doing so changing the electromagnetic signature of their sun.

Dr. Dyson also proposed a type of invention which he deemed likely to appear within the life-span of an intelligent civilization, the absence of which tends to support the Fermi principle. He said that he thought that it would soon be possible for us to create an explorer-device which drew power from its surroundings to propel itself through the universe in search of intelligent life forms. Moreover, it would be possible to create versions of this device which could create and launch vast numbers of copies of itself by the process of machine reproduction. Even allowing for the realities of vast distances between stars and the relativistic speed-limit, if intelligent life were common, stars in our own galaxy much older than our own would be within a range to have built and launched fleets of these automated exploration devices.

Extraterrestrial colonization.
Adherents to the Fermi principle furthermore argue that, from what we know about life's ability to overcome scarcity and colonize new habitats on our own planet, we can reasonably assume that life elsewhere will follow similar principles. Given this, Fermi principle adherents state that any advanced civilization will almost certainly try to seek out new resources and colonize first their solar system, and then surrounding solar systems.

Several possible explanations
Solutions to the Fermi paradox can be grouped in three categories:

They are here
Those who attribute UFOs to alien spacecraft have a ready answer to the paradox. Alternatively, it is perfectly reasonable to believe that a lifeform intelligent enough to travel to our planet is intelligent enough to exist here undetected. Alternatively, we may have been detected at a distance and either a return message, or an alien emissary itself, is currently en-route. The fact that they haven't detected us sooner or we didn't detect them first may be simple coincidence.

They exist but have not communicated with us
One possibility is that advanced civilizations either deliberately or accidentally hide evidence of their existence from humanity. They might do so out of ethical concerns for primitive beings or a desire to encourage cultural diversity. Civilizations might also be deliberately hiding themselves in order to avoid destruction from more advanced civilizations. Another possible explanation is that advanced civilizations would construct multiple concentric Dyson Spheres around their stars, each one radiating less energy than the next smallest one, with the outermost sphere radiating energy at close to the background radiation.

It has also been proposed that a fundamental information theoretical axiom might be behind the lack of recognized signals. Information theory states that a message which is compressed maximally is indistiguishable from white noise. The counterargument to this would be that even though as bandwidth becomes a bottleneck to communication, there ought still be some niche technologies which would not or could not strive to maximal data compression.

Yet another idea is that all intelligent life inevitably evolves towards a technological singularity and quickly becomes unrecognizable to humanity in our present state.

Another hypothesis is that the whole existence of human life on earth (even if our species survives for some hundred thousand years) is but a brief moment compared to the age of the universe. Seen from afar, many planets could follow the cycle of life arising and being extinct during the billion year span. The chances are that civilizations would be too far apart in either time or space to actually meet.

A more recent idea (sometimes called the fiber optic objection), observes that the use of broadcast technologies like radio for the transmission of information are fundamentally wasteful of energy and that advanced technological civilizations may not use them at all for that reason. Because broadcasts are radiated in all directions evenly, a large amount of power is needed for a transmitter to send messages any significant distance. Adherents of this concept observe that human technology is currently moving away from broadcast for long-distance communication and replacing it with wires, optical fibers, and focussed electromagnetic technologies like aimed narrow-beam radio, microwave or laser transmission. Most recent technologies that employ broadcasting, such as mobile phones and Wi-Fi networks, use very short-range transmitters to communicate with fixed stations that are themselves connected by wires or narrow beams. It is argued that this trend may make Earth itself nearly undetectable from space within a few decades, and that therefore most civilizations would only be detectable for a short period of time between the discovery of radio and the switch to more efficient technologies.

A similar but slighly modified theory suggests that another civilization may in fact be attempting to communicate with us, but for any of a number of reasons we are unable to detect their signals. This could be because we haven't yet pointed receivers in their direction, or they are using an esoteric or highly advanced method of communication we are not able to detect or interpret at all.

They do not last long enough to communicate with us.
Science fiction authors have proposed another possible explanation — that someone, or something, is destroying intelligent life in the universe as fast as it is created. This theme can be found in novels such as Frederik Pohl's Heechee novels, Fred Saberhagen's Berserker novels and Greg Bear's The Forge of God.

Another alternative is simply that they destroy themselves. Life on Earth, and intelligent life on Earth, evolved as a result of the competition for scarce resources. The evolutionary psychology that developed during this struggle has left its mark on our characters, and left human beings subject to involuntary, instinctual drives to consume resources and to breed. It seems likely that intelligent life on other planets evolved subject to similar constraints, and as such pessimism about their long term viability is a justifiable position. Technological civilizations may usually or invariably destroy themselves (via nuclear war, biological warfare, grey goo or in a Malthusian catastrophe after destroying their planet's ecosphere) before or shortly after developing radio or spaceflight technology. Larry Niven and Jerry Pournelle's The Mote in God's Eye has as its central premise a civilisation that taxes its resource base and cyclically self-destructs, but which tries to preserve its culture from one cycle to the next.

They do not exist.
Others argue that the conditions for life, or at least complex life, are rare. For instance, some hypotheses say that complex life required the stimulation of tides from Earth's Moon to evolve, and the Moon is the result of freak occurrence, a body of a certain mass striking Earth at just the right angle to carve off the material and put it in a stable orbit.

Another possibility is that ice ages, comet or meteor impacts, supernovae, gamma ray bursters or other catastrophic planetary or galactic events are so common that complex life rarely has the time to evolve. Alternately, these events may not be frequent enough on other planets and evolution is slowed because there aren't enough mass extinctions to encourage diversity.

Even if the conditions for life are common, the evolution of human-like intelligence, the invention of radio technology or interest in the exploration of outer space may be vanishingly rare.

A common concept used in the scientific method to test the validity of certain ideas is Occam's Razor. To paraphrase, Occam's Razor states that the explanation for a given phenomenon that has the fewest assumptions should be preferred over more complicated ones. The simplest explanation, say adherents to the premise behind the Fermi paradox, is that as a technologically advanced species, we are alone in our part of the Cosmos.

Drake equation

 

The Drake equation (also known as the Green Bank equation) is a famous result in the speculative fields of xenobiology, astrosociobiology and the search for extraterrestrial intelligence.

This equation was devised by Dr. Frank Drake in the 1960s in an attempt to estimate the number of extraterrestrial civilizations in our galaxy with which we might come in contact. The main purpose of the equation is to allow scientists to quantify the uncertainty of the factors which determine the number of extraterrestrial civilizations.

The Drake equation is closely related to the Fermi paradox.

The Drake equation states that

N = R* × fp × ne × fl × fi × fc × L
where:

N is the number of extraterrestrial civilizations in our galaxy with which we might expect to be able to communicate
and

R* is the rate of star formation in our galaxy
fp is the fraction of those stars which have planets
ne is average number of planets which can potentially support life per star that has planets
fl is the fraction of the above which actually go on to develop life
fi is the fraction of the above which actually go on to develop intelligent life
fc is the fraction of the above which are willing and able to communicate
L is the expected lifetime of such a civilisation

Historical estimates of the Drake equation parameters.
Considerable disagreement on the values of most of these parameters exists, but the values used by Drake and his colleagues in 1961 are: R* = 10/year, fp = 0.5, ne = 2, fl = 1, fi = fc = 0.01, and L = 10 years. The value of R* is the least disputed. fp is more uncertain, but is still much firmer than the values following. Confidence in ne was once higher, but the discovery of numerous gas giants in close orbit with their stars has introduced doubt that life-supporting planets commonly survive the creation of their stellar systems. In addition, most stars in our galaxy are red dwarfs, which have little of the ultraviolet radiation that has contributed to the evolution of life on Earth. Instead they flare violently, mostly in X-rays - a property not conducive to life as we know it (simulations also suggest that these bursts erode planetary atmospheres). The possibility of life on moons of gas giants (e.g. Jupiter's satellite Europa) adds further uncertainty to this figure.

What evidence is currently visible to humanity suggests that fl is very high; life on Earth appears to have begun almost immediately after conditions arrived in which it was possible, suggesting that abiogenesis is relatively "easy" once conditions are right. But this evidence is limited in scope, and so this term remains in considerable dispute. One piece of data which would have major impact on this term is the controversy over whether there is evidence of life on Mars. The conclusion that life on Mars developed independently from life on Earth would argue for a high value for this term.

fi, fc, and L are obviously little more than guesses. fi has been impacted by discoveries that the solar system's orbit is circular in the galaxy, at such a distance that it remains out of the spiral arms for hundreds of millions of years (evading radiation from novae). Also, Earth's very large, unusual moon appears to aid retention of hydrogen by breaking up the crust, inducing a magnetosphere by tidal heating and stirring, and stabilizing the planet's axis of rotation. In addition while it appears that life developed soon after the formation of Earth, the Cambrian explosion in which a large variety of multicelluar life forms came into being occurred considerable amounts of time after the formation of Earth, which suggests the possibility that special conditions were necessary for this to occur. In addition some scenarios such as the Snowball Earth or research into the extinction events have raised the possibility that life on Earth is relatively fragile. Again, the controversy over life on Mars is relevant since the finding that life did form on Mars but cease to exist would affect estimates of these terms.

The well-known astronomer Carl Sagan speculated that all of the terms except for the lifetime of a civilization are relatively high, and the determining factor in whether there are large or small numbers of civilizations in the universe is the civilization lifetime, or in other words the ability of technological civilizations to avoid self-destruction. In Sagan's case, the Drake equation has been a strong motivating factor for his interest in environmental issues and his efforts to warn against the dangers of nuclear warfare.

(Note, however, that in the year 2001 a value of 50 for L can be used with exactly the same degree of confidence that Drake had in using 10 in the year 1961.)

The remarkable thing about the Drake equation is that by plugging in apparently fairly plausible values for each of the parameters above, the resultant expectant value of N is generally often >> 1. This has provided considerable motivation for the SETI movement. However, this conflicts with the currently observed value of N<<1 - one observed humanity in entire universe. Other assumptions give values of N that are << 1, in accord with the observable evidence.

This conflict is often called the Fermi paradox, after Enrico Fermi who first publicised the subject, and suggests that our understanding of what is a "conservative" value for some of the parameters may be overly optimistic or that some other factor is involved to suppress the development of intelligent space-faring life.

Other assumptions give values of N that are << 1, but some observers believe this is still compatible with observations due to the anthropic principle: no matter how low the probability that any given galaxy will have intelligent life in it, the galaxy that we are in must have at least one intelligent species by definition. There could be hundreds of galaxies in our galactic cluster with no intelligent life whatsoever, but of course we would not be present in those galaxies to observe this fact.

Some computations of the Drake equation, given different assumptions:

R* = 10/year, fp = 0.5, ne = 2, fl = 1, fi = fc = 0.01, and L = 50 years
N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 50 = 0.05
Alternatively, making some more optimistic assumptions, and assuming that 10% of civilizations become willing and able to communicate, and then spread through their local star systems for 100,000 years (a very short period in geologic time):

R* = 20/year, fp = 0.1, ne = 0.5, fl = 1, fi = 0.5, fc = 0.1, and L = 100,000 years
N = 20 × 0.1 × 0.5 × 1 × 0.5 × 0.1 × 100000 = 5000
[edit]
Current estimates of the Drake equation parameters
This section attempts to list best current estimates for the parameters of the Drake equation. Please list new estimates for these values here, giving the rationale behind the estimate and a citation to their source.

R*, the rate of star creation in our galaxy

Estimated by Drake as 10/year
fp, the fraction of those stars which have planets

Estimated by Drake as 0.5
ne, the average number of planets which can potentially support life per star that has planets

Estimated by Drake as 2
fl, the fraction of the above which actually go on to develop life

Estimated by Drake as 1
In 2002, Charles H. Lineweaver and Tamara M. Davis (at the University of New South Wales and the Australian Centre for Astrobiology) estimated fl as > 0.33 using a statistical argument based on the length of time life took to evolve on Earth. Lineweaver has also determined that about 10% of star systems in the Galaxy are hospitable to life, by having heavy elements, being far from supernovas and being stable themselves for sufficient time. [1] (http://www.newscientist.com/news/news.jsp?id=ns99994525)
fi, the fraction of the above which actually go on to develop intelligent life

Estimated by Drake as 0.01. Solar systems in galactic orbits with radiation exposure as low as Earth's solar system are more than 100,000 times rarer, however.
fc, the fraction of the above which are willing and able to communicate

Estimated by Drake as 0.01
L, the expected lifetime of such a civilisation

Estimated by Drake as 10 years.
A lower bound on L can be estimated from the lifetime of our current civilization from the advent of radio astronomy in 1938 (dated from Grote Reber's parabolic dish radio telescope) to the current date. In 2004, this gives a lower bound on L of 66 years.
In an article in Scientific American, Michael Shermer estimated L as 420 years, based on compiling the durations of sixty historical civilizations. Using twenty-eight civilizations more recent than the Roman Empire he calculates a figure of 304 years for "modern" civilizations. Note, however, that the fall of most of these civilizations did not destroy their technology, and they were succeeded by later civilizations which carried on those technologies, so Shermer's estimates should be regarded as pessimistic.

Panspermia

 

Panspermia is a theory (more directly described as a hypothesis, as there is no compelling evidence yet available to support or contradict it) that suggests that the seeds of life are prevalent throughout the universe and life on Earth began by such seeds landing on Earth and propagating. The theory has origins in the ideas of Anaxagoras, a Greek philosopher.

An important proponent of the theory was the British astronomer Sir Fred Hoyle. Hoyle's advocacy is both a blessing and a curse; although he was a highly original thinker and won top scientific accolades, some of his principal ideas such as steady state theory have been largely shown to be false. His science fiction writing also makes it easy for critics to discredit theories of extraterrestrial life.

Evidence and mechanisms.
There is some evidence to suggest that bacteria may be able to survive for very long periods of time even in deep space (and may therefore be the underlying mechanism behind Panspermia). Recent studies out of India have found bacteria at heights greater than 40 km in Earth's atmosphere where mixing from the lower atmosphere is unexpected, while Streptococcus mitus bacteria that had accidentally been taken to the moon on the Surveyor 3 spacecraft in 1967, could easily be revived after being taken back to earth 31 months later. However, a consequence of panspermia is that life throughout the universe would have a surprisingly similar biochemistry, being derived from the same ancestral stock. So the high-altitude bacteria might be expected, whether of earth or extra-terrestrial origin, to have a biochemistry similar to terrestrial forms. This is not resolvable until life on another planet can have its chemistry analysed.

Another objection to Panspermia is that bacteria would not survive the immense heat and forces of an impact on earth; no conclusions (whether positive or negative) have yet been reached on this point.

Suggestive evidence in favour of panspermia are

The remarkably rapid appearance of life on Earth in the fossil record. The earliest evidence is of fossilised stromatolites or bacterial aggregates, which are dated at only 3.8 billion years old -- only 500 million years after the oldest dated rocks. On some models of planet formation this is almost too soon for the Earth to have cooled sufficiently to allow liquid water and support life.
Bacteria and more complex organisms have been found in more extreme environments than thought possible, such as black smokers or oceanic volcanic vents. Some extremophile bacteria have been found living at temperatures above 100C, others in strongly caustic environments.
Bacteria which don't rely on photosynthesis for energy. In particular, endolithic bacteria using chemosynthesis found inside rocks and in subterranean lakes.
Semi-dormant bacteria found in ice cores over a mile beneath the antarctic - this lends credibility to the concept of sustaining the components of life on the surface of icy comets.
Inconclusive results from the Viking program biological tests. Tests were performed to detect the metabolism of radioactive elements by soil microbes on Mars, and then similar tests performed after the sample was raised to very high temperatures to kill any life. The tests were consistent with the presence of life, but the official NASA stance is that the effect was chemical rather than biological.

Some have taken the theory as an answer to those arguing the improbability of the origin of life, in that wherever life first began, it spread throughout the universe by panspermia. However, panspermia doesn't alleviate the need for life to have started somewhere at some time, it merely extends the time frame and environments available for life to originate.

Some believers in panspermia, however, believe that life never evolved from inorganic molecules, but that it has existed as long as all other forms of matter. This is an extension of panspermia called cosmic ancestry.

Directed Panspermia.
A second prominent proponent of panspermia is Nobel prize winner Francis Crick, along with Leslie Orgel who proposed the theory of directed panspermia in 1973. This suggests that the seeds of life may have been purposely spread by an advanced extraterrestrial civilisation. Crick argues that small grains containing DNA, or the building blocks of life, fired randomly in all directions is the best, most cost effective strategy for seeding life on a compatible planet at some time in the future. The strategy might have been pursued by a civilisation facing catastrophic annihilation, or hoping to terraform planets for later colonisation.