Developing A Universal Religion/Life/The Probability That Life Exists Elsewhere

It is not difficult to estimate the probability that life has developed on other planets in the universe. All we need do is calculate the total number of stars, the number of these that may support habitable planets, and the likelihood that any of these planets would support life.[1]

First, the number of stars in our universe. It is estimated that our Milky Way galaxy contains between two and three hundred billion stars (2-3 x 1011), and that there are about one hundred billion galaxies (1011) in the visible universe. If the average number of stars in other galaxies is similar to ours, then there are 2-3 x 1022 stars all told. Using the smaller number we have 2 x 1022 stars to start with.

Second, there are many inhospitable zones within all galaxies (the planets of stars too close to the centre of a galaxy or to radiating black holes, for instance, are being sterilized by microwaves[2]) and stars in these regions are unlikely to support life-bearing planets, at least, not life as we think of it.[3] Let’s guess that only one tenth of each galaxy’s stars are clear of these areas, giving us 2 x 1021 stars in hospitable zones of the universe.

Third, using observations noted in The Life Of A Typical Star, we can guesstimate that some fifty percent of all stars possess planetary systems, so about 1 x 1021 (or 1021) stars are predicted to have orbiting planets.

Fourth, many exoplanets likely do not possess the conditions we consider necessary to support life (water, appropriate temperature ranges, appropriate elements and minerals, energy sources such as sunlight or planetary heat, etc.). A reasonable guess might be that of those possessing planetary systems, only one star in ten will hold a planet that is habitable. This gives us 1020 stars or 1020 habitable planets.

Fifth, we do not know if life will always arise on habitable planets.[4] If, as is turning out to be likely, the molecules from which life originates can form in space-ice, then probably all of the universe’s planets will have been inoculated by now. How much of this material then goes on to create life can only be a guess. Presumably, if the right conditions exist, eventually all will; but, to err on the conservative side, let us say that only one in a hundred habitable planets becomes a host to life.[5] Thus about 1018 (1,000,000,000,000,000,000 or one quintillion) life-bearing planets possibly exist in the visible universe. Of these, about 107, or ten million, could be in our own galaxy.

As we learn more about the nature of life and our universe, we will undoubtedly revise our estimates of the number of planets that could be home to living entities. The number may decrease or increase, even significantly, but it is very unlikely that the number will turn out to be one. Statistically, therefore, it is highly improbable that our planet is the only one to bear life; the universe contains an incredibly large number of stars, and the conditions and ingredients required to start and support life probably exist in many, many millions of places. Furthermore, these places may include intergalactic space, within gases where life’s precursors may first have formed, then evolved, to create living entities that waft through the heavens in forms vastly different from ones we might recognize.


  1. This type of formulation was first proposed in 1961 by Frank Drake, currently Chairman Emeritus of the SETI Institute. (SETI, the Search for Extra-Terrestrial Intelligence, is a project that has been running for over 25 years at University of California-Berkeley using radio telescopes.) Drake wished to guesstimate the possibility of being able to contact extra-terrestrial life, and made a calculation somewhat like the following:
    Number of technical civilizations in the Milky Way =
    Number of stars in the Milky Way (say 2x1011) x
    Fraction of stars with planetary systems (say ½) x
    Number of planets per star (say 1) x
    Number of planets favourable to life (say 1/10) x
    Fraction eventually developing life (say 1/10) x
    Fraction with intelligent life (say 1/100) x
    Fraction at our technical stage of development (say 1/10,000).
    See [[w:Drake equation|]].

    Multiplying these together we find that the number of planets with life at an “electronically-developed” stage in our galaxy could be around a thousand. Of course, the number likely to be at our stage of development, when communications over distances are carried out by AM, FM, or digitally encoded electro-magnetic waves, the kind of signals SETI’s instruments have been looking for, is quite critical. More advanced beings may well be using a different form of communication—piped-optical for example, or some other method that our current instruments would not detect. SETI has also been conducting optical searches (without success) and has just begun looking for laser beacons (which, if narrowly focused, would only be detected if we happened to pass through their beam).

    In our calculation, since we are only estimating the possibility that life exists elsewhere, we are not bothered about its intelligence or stage of development so can ignore the reduction these fractions would contribute. Moreover, we are discussing life’s presence in the entire universe, not just our own galaxy.
  2. See Guillermo Gonzalez, Donald Brownlee and Peter D. Ward, “Refuges for Life in a Hostile Universe,” Scientific American, October 2001, 60-67.
  3. Common understanding holds that, to be considered living, an entity must meet at least four criteria: consume energy, expel wastes, respond to its environment, and reproduce. But see Life's Beginning for an alternative definition.
  4. For a discussion on this topic see “Livable Planets: Calculations raise the odds for finding life in the cosmos,” by Corey S. Powell in Scientific American, February 1993, 18-20.

    The Earth may already possess a few samples of life from elsewhere in the cosmos, lying undiscovered on our ocean floors or hidden in rocks or crannies on our continents. Entities resembling a string of cells (and possibly being primitive life forms) have been discovered on a meteorite originating from Europa (one of Jupiter’s moons). However the sample is not large enough to conclude whether any of the entities were once living.

    Analyses of magnetic-field intensities along with various other measurements taken by satellites, indicate that Ganymede, Europa and Callisto (all moons of Jupiter) possess water. Some form of life may exist or have existed within this water, but this possibility remains to be explored. Probes, specifically equipped to test for water, may be sent to Europa within the next decade. Future Mars landers will be exploring areas where frozen reservoirs of water have been discovered, specifically looking for the presence of life. However, within our solar system, only our planet provides easy living; conditions on the other planets and moons are such that any life that might be found is bound to be primitive.

    Astronomers occasionally search for distant signs of life using satellites and telescopes principally designed for other purposes. This will change in 2007, provided NASA’s scheduled Kepler Mission satellite launches successfully. This mission will carry telescopes designed to locate and check the atmospheres of exoplanets for the presence of ozone. Ozone is a gas formed from free oxygen, and free oxygen can only be produced in lasting quantities by life. This is because methane, produced by bacterial decomposition of organic matter, constantly removes free oxygen by combining with it to form other compounds. If both methane and oxygen are found in exoplanet atmospheres, then life is almost certain to be producing a continuous supply of the oxygen.)
  5. This guess may be far too cautious. For reasons to be outlined in the next chapter, it is highly likely that life will always arise when circumstances permit (see also de Duve, Vital Dust, xv and 20).