The Galactic Inquirer

The Search for Extraterrestrial Life



Martin Annis, Ph.D. (Founder – American Science and Engineering)

We have overlooked an important source of information in our search for life on Earth-like planets in the galactic Universe. There is at least one such a planet in the Universe, with information that is well known, viz. Earth. Using this planet, it is possible to establish some limits on the probability of discovering life and also of discovering intelligent life elsewhere.

The age of the Earth is about 4.5 billion years. The age of our Sun is similarly about 4.6 billion years. By comparison, the age of the Milky Way galaxy is about 12 billion years and that of the known Universe is about 13.8 billion years. Thus it is reasonable to believe that Earth is likely to be a rather common type of planet in our Galaxy and likely in other galaxies. The origin of the first single-cell life form on Earth is thought to have occurred about 3.6 billion years ago. More complex forms of life developed sequentially with finally the development of animals, apes, etc. This culminated in the origin of the branch of the tree leading to Homo sapiens about 3 million years ago. Thus the origin of intelligent life on our planet occurred only very recently compared to the age of our planet i.e. in the last 0.1% of the age of our planet).

There are two important questions concerning extraterrestrial life upon which well-known terrestrial observations can shed light.

Question 1: What is the probability that life will exist in a planet that has the same physical and chemical properties as the Sun-Earth system, e.g., geometrical distance apart, energy flux to planet, atmosphere surrounding planet, etc.?

Answer to Question 1: It is almost a certainty that life in some form has existed on such a planet. The reason is that a wide variety of life forms have originated on the planet Earth at a vast number of widely dispersed locations, in the most challenging terrestrial environments such as deep in the oceans or extremely hot environments or in difficult chemical environments, and have originated very early and over a large time period during the existence of Earth.

Timeline of Earth’s history, including the origin of microbial life 3.8 billion years ago and the evolution of multi-cellular life forms to the present day. Courtesy of Andree Valley (see )

Question 2: What is the probability that “intelligent” life will exist in a planet with the same conditions as postulated in Question 1?

Answer to Question 2: We first define the meaning of “intelligent” life. Intelligent life is defined here as life which has reached, as a minimum, the same capability as Homo sapiens to communicate with other planets in the Milky Way Galaxy. In other words, intelligent life is defined as the existence of life equivalent to Earth’s Homo sapiens. Such intelligent life originated on the planet Earth one time and only one time, very recently in the life of our planet, in one place and only one place on our planet.

Thus, the probability of intelligent life on another planet is not zero, since there is already one such planet that has intelligent life. On the other hand this probability is not close to 1, since it only happened at one time and in one place on the entire planet. Based on one data point, a zero-order estimate of intelligent life on a sister planet would be 50% +/- 49%. As stated above the time elapsed between the origin of single-cell life, and the branch in the evolutionary tree culminating in Homo sapiens, is about 3.6 billion years. Thus Homo sapiens arose once and only once on this planet and it happened very recently, about 4.5 billion years after the formation of planet Earth. If we make the reasonable assumption that evolution will occur in a similar time in planets similar to Earth, then we must conclude that all planets substantially younger than Earth will probably not have intelligent life.

Another requirement for the production of Homo sapiens is that the planet must have sustained the chemical and physical properties conducive to life, without any total interruption resulting in the destruction of all life forms, for the entire 3.6 billion years. This has been true for Earth, although there has been at least one major catastrophic impact of a good sized meteor resulting in wide-spread but not total extinction of all life forms. Also there has been at least one catastrophic volcanic eruption also causing worldwide but not total extinction of all life forms. 3.6 billion years is a significant fraction of the life of the Universe. It would not be surprising if someone calculated that there is only a small probability of a planet in our Galaxy to have 3.6 billion years of reasonable tranquility, and so we are likely a lucky planet. I suggest that it

is possible to make a useful calculation based on impact data and volcanic activity on our neighboring planets.

From the above considerations one can conclude that there is not a high probability that a particular planet meeting all of the physical and chemical requirements will have intelligent life. But given the reality that there are probably billions of such planets, and we have the data that one of us exists, we must conclude that, excluding the possibility of divine intervention, we are possibly not alone.

On the other hand, if we find even one other planet that meets many of the physical, chemical and temporal requirements for the generation of life forms, and no life forms of any kind are discovered anywhere on this other planet, then we would indeed have to reconsider our belief in a universe totally explained by science alone!

Martin Annis received his Ph.D. in Space Physics from MIT, in 1951. He founded and was the first president of American Science and Engineering. During his tenure the company discovered the first X-ray star, Scorpius X-1, for which a Nobel Prize was given to his employee, Ricardo Giaconni. He is the inventor of the X-ray system that is used to inspect passengers at airports and has 32 patents.


  1. I have begun to see an increasing convergence of opinion regarding the prospects for life beyond Earth vs. the prospects for intelligent life capable of communicating across interstellar space. In his commentary, X-ray pioneer Martin Annis considers the history of Earth as a key source of insights on the proliferation of basic life vs. intelligent life. He notes that life took hold in many places on Earth 3.6 billion years ago and has been flourishing in amazing ways ever since. By contrast, “intelligent” life emerged in only one place and at only one time — on the African savannah about 3 million years ago. We humans were illiterate hunter gatherers for most of that time and have become communicative in any interstellar sense for only the last hundred years. Given these sorts of timescales, one can readily surmise that basic life could be abundant throughout the Galaxy, but intelligent communicative life will be much rarer.

    Annis’s reasoning closely parallels my own as expressed in the last chapter of my recent book on The Milky Way – An Insider’s Guide (see Therein, I use a revised version of the Drake Equation to guesstimate lower limits on the number of habitable planets in our Milky Way Galaxy (100 million), the number of inhabited planets (10 million), the number of inhabited planets with intelligent beings (10,000), and the number of inhabited planets with creatures capable of interstellar communication (1). Already, these estimates may need to be revised upwards by factors of 100, as the Kepler mission has found enough Earth-like planets to increase the likely number of habitable planets in the Galaxy to about 10 billion. Even so, that still leaves a very small number of planets with technologically communicative life forms (100 or so).

    Annis also worries about the special circumstances of Earth’s history. Beset by very few supervolcanoes or major impacts, Earth has enjoyed a relatively benign existence that might not be so likely among its peers. The Cambrian explosion 500 million years ago, the Permian extinction 250 million years ago, and the Cretaceous-Tertiary extinction 65 million years ago pretty much delineate the key events that reset the progression of life on Earth. With more big events come more resets until evolutionary progression is squelched. Similar arguments were presented by Peter Ward and Don Brownlee in their influential book Rare Earth: Why Complex Life is Uncommon in the Universe. Again, I see a convergence of thought regarding the challenges to proliferating highly-evolved intelligent life in the Galaxy.

    Annis further notes that a full 4.5 billion years elapsed on Earth before intelligent life evolved on the planet’s surface. From this fact, he surmises that younger planets may not be sufficiently evolved to be currently hosting intelligent life. It turns out that older planets also may not be sufficiently evolved in a chemical sense to serve as hospitable hosts of intelligent life. Even with its birth dating back to 4.5 billion years, Earth is a relative newcomer on the Galactic scene. The Milky Way had already been in existence for 7.5 billion years churning out generations upon generations of stars. The massive stars, in their explosive deaths, generously enriched the interstellar medium with heavy elements of carbon, oxygen, and silicon – each stellar generation adding to the mix. Then roughly 4.6 billion years ago, the Sun and Solar System congealed from some chemically-enriched molecular cloud core. That means the Solar System came along when the Milky Way had evolved enough chemically to spawn rocky planets with watery surfaces – critical prerequisites to the life forms that we have come to know and love. So, the Solar System and Earth may in fact represent the temporal vanguard of a living and evolving wave that is sweeping over the Galaxy. Significantly older solar systems do not have the right “stuff,” while significantly younger solar systems do not have sufficient time to be currently serving as good hosts of intelligent life. These sorts of arguments were recently articulated by Dimitar Sasselov in Chapter 11 of his book The Life of Super-Earths.

    Given so much convergence of opinion, are we on the right track – or are we missing some key factor that will upend our current estimates of planets with life vs. planets with intelligent communicative life? Annis suggests that more scientific and mathematical work will lead us to a refinement of the conclusions presented herein. If we don’t find any life beyond Earth – or if we find an abundance of intelligent life everywhere we look – then some other supernatural influence would have to be operating. That unsettling prospect is yet another reason why our fledgling searches for extraterrestrial life and extraterrestrial intelligence are so important.

  2. Dr. Annis,

    I would like to take this opportunity to express my sincere appreciation for your commentary concerning the much wondered – about topic regarding the possibility of other sentient life in the universe. You seem to have deciphered the more important parameters contained within the Drake Equation in a direct and succinctly stated model and at the same time posited these contributing factors in laymen’s terms .

    One main argument which is evident throughout your presentation is the concept of time, an entity framed in vast epochs which surely tax our appreciation and understanding given the magnitudes of such durations. What does a billion years mean to homo sapiens given our ancestral appearance within the last few million years ? Current genetic research confirms that modern man is a recent phenomenon having arrived in the last 150,000 to 200,000 years. Astronomers and astrophysicists assure us that the laws of physics remain constant throughout the universe and yet we are still reluctant to consider that life might exist and have arisen outside our proprietary contextual understandings and anthropocentric frame of reference . You are quite correct in stating that fortunately we have a scientifically captured and examined history , with a life sustaining metabolic constitution and genetic framework to draw upon. We do exist, live and die, and possess a consciousness which attempts to understand it all and at the same time inspires an innate curiosity to answer the existential and metaphysical questions which arise when one ponders our own existence in a vast expanse of both time and space. In this case, are we alone or does other intelligent and consciousness – bearing life exist in the vast cosmic sea of the universe ?

    When such a cognitive awareness attempts to form a definitive conclusion despite the fact that currently we have no objective evidence, we are still confronted by the extrapolated estimates comprising billions of galaxies and trillions of stars.
    Furthermore, how many planets are there which might mimic our life – sustaining oceanic rocky planet with a gravitationally – bound satellite while orbiting in a star centered solar system at distances conducive to the origin of life ? Are the probabilities likely to occur or are they marginal at best ? It would seem that given the uncompromising numbers, the chances are quite likely that we are not alone.

    I concur with your closing statement that if all those physical and biological inputs coupled with the cosmic and geological near extinction events were perfectly congruous as to what transpired here on earth and consequently no life arose, well, what would this mean ? Such an anomaly would invite the probing questions which are exclusive to the empirically proven epistemology that we cling to as manifest truths. Does the cosmos and nature rule purely by random chance or is intelligent life the consummate expression of both time and chemistry? When we are dealing in time periods and star systems which are defined by exponents to the 9TH and 12Th power respectively , then perhaps time and time alone is the great and enduring catalyst given an adequate chemical constituency to work its magic. Any attempt at probing that ultimate question would at least begin to confront the perplexing dilemma of a universe not totally explained by science and or biology alone .

    Thank you again for a truly informative and greatly inspiring

    “ Absence of evidence is not evidence of absence “ Carl Sagan

  3. After several readings I am struck by the force of your claim that a single lifeless planet would compel us to re-examine our scientific, empirical view of the cosmos, a view grounded in centuries of science.

    You recur in your essay to the fact of billions, trillions of probable planets, but your most forceful claim is based on a single example. Why would we not be compelled instead to re-examine the epistemological, rather than the ontological question?

    It seems to me that the obvious choice would be to look closely at how we know what we think we know about the generation or persistence of life, not whether the universe is barren. It is a terrific essay, but I have to question that final turn.

  4. Dr. Annis:

    In the following passage from Question 1 –

    “It is almost a certainty that life in some form has existed on [a planet that has the same physical and chemical properties as the Sun-Earth system, e.g., geometrical distance apart, energy flux to planet, atmosphere surrounding planet, etc.]”

    – are you referring to an “Earth-like” planet such as Kepler-10b or HD 85512b or does your statement of certainty extend to astronomical objects such as say, Jupiter’s moon Europa which could potentially host non carbon-based forms of life?


    Oliver Herman
    RHS-AP Physics II

  5. Dr Annis:

    You discuss in your article how catastrophic events in Earth’s history have had an effect on its development of life. You also mention that while earth has had it’s fair share of catastrophic evens, Earth has had a relitively peaceful existence compared with other planets. At first I assumed that a relative lack of these cataclysmic events (meteor impacts, intense volcanic eruption and the like) probably helped speed up evolution. But then I began to wonder if that was really true. Could it be that these cataclysmic events in Earth’s past actually helped evolution on our planet?

    Consider some form of bacteria inside a person who is given antibiotics. The antibiotics should kill, off the bacteria unless the bacteria can evolve quickly enough to build up resistance to the antibiotic. If the bacteria does survive it is stronger, a more adapted strain of it’s past self. Could it be possible that the cataclysmic events in Earth’s past worked like antibiotics, forcing the weak species to die and speeding up the evolution of the stronger ones, eliminating species that could not survive and opening the door to new species until intelligent life was reached?

    Obviously to many catastrophic evens could set back evolution considerably, or wipe out life entirely, but I think that it is possible that some of these catastrophic events could actually aid evolution on other planets. Having constantly active volcenos covering the entire surface of a planet or a surface perpetually barraged with space debris would not be ideal, but perhaps a few cataclysmic (or life changing, ha ha) events spaced out appropriately could speed up evolution noticeably. What do you think?

    Sayles Kasten
    AP Physics Student at Rockport Public High School

  6. Dr. Annis,

    I was delighted to read your thoughts on this subject, as it is one that very much interests me, and I have always been a proponent of (excuse my simplification here) the “The universe is too vast for us to be alone” idea, which appears to be a common idea throughout your article.

    In your closing paragraph, however, you propose that should there be a planet found to be very life-friendly, yet lacking life, one might want to reconsider the way they’re looking at the subject. While it is always good to look at things from other viewpoints, I myself would not turn away due to a single data point such as this. Mr Amoroso brought up an excellent point in his comment, saying that, “If all those physical and biological inputs coupled with the cosmic and geological near extinction events were perfectly congruous as to what transpired here on earth and consequently no life arose, well, what would this mean?”. Given two similar scenarios, one would expect two similar outcomes, no? The variable being dealt with here, however, is life. Life itself is an anomaly, a miracle. Just because life can be supported, doesn’t mean life will be there to be supported. I am admittedly not an expert on the formation of life, and as a high school student, forgive me if what I’m saying is a little off, but when it comes to something as incredible and statistically improbable as life, I believe that it’s all in the numbers.

    Nathaniel Allen
    RHS AP Physics Student


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