In April 1999 the news media reported that Upsilon Andromedae, a star about 47 light years distant, has at least three planets the size of Jupiter or larger, and that one is in the hypothetical "life zone" corresponding to the Venus-Earth-Mars region of our solar system. This is roughly the 20th star in our stellar neighborhood to have been credited with planets; it is the first to which observers give more than one. (With the exception of Barnard's Star, which is apparently still regarded as an unproven case. Ironically, Barnard's Star was the first star other than our own to be described as having planets.)
A few decades ago, extra-solar planets were the property of science fiction writers. Astronomers deemed them unlikely around large red and blue giants (O and B class suns) on the grounds that these stars destroyed themselves in supernovas at a young age, before sensible planets would have had time to condense; unlikely around M-class dwarf stars, which were thought too small to have material for building solar systems; and improbable for dynamical reasons among the 40 % of the remaining AFG and K class suns which formed binary (or higher) systems. These days the notion seems to be surfacing that if you have enough material to form any sort of star in the first place, you have enough material to build planets as well-- perhaps several sets of planets from the primordial debris over the lifetime of a typical star.
Meanwhile, on the one planet in the one solar system known to harbor life, we've evidence that alga-like organisms existed on earth at roughly 3.8 billion years ago. This is not far removed from the "Age of Bombardment" from roughly 4.2 to 4.0 billion years ago, which saw most of the remaining planetisimals in the inner solar system swept up like dust on a carpet by the gravational pull of Mercury, Venus, Earth, Mars, the Moon, etc. With an estimated age of 4.6 billion years, it is conceivable that life existed on earth before the bombardment but that the evidence has been hidden simply because no rocks from that earlier age have survived to the present day. It is also conceivable some earlier form of life, maybe not based on DNA, existed and was eradicated by the bombardment, and that modern day organisms are the products of a second "origin of life."
Even without these speculative cases, it seems clear that life got established on earth with considerable haste. We have indications as well that life may have arisen on Mars some 3+ billion years ago, in the form of minute inclusions which might be fossilized spores in rocks blasted into space by long-ago meteorite strikes. Scientists also speculate that an "ocean" of liquid water below the frozen surface of Europa, kept warm by tidal strains imparted by Jupiter, might be warm enough for life to evolve even in the chilly "outside the life zone" regions of the solar system. All the 20+ planets now thought to orbit other suns are on the order of Jupiter's size or larger, so the implications are profound.
It would appear that we have before us a galaxy of some 400 billion stars-- one galaxy in a universe of perhaps 10 billion galaxies-- in which planets are commonplace, and life is easily generated. The odds on the existence of extrasolar lifeforms with technological civilizations, parameterized by the famous Drake equation some thirty years ago, should seem much higher today than in the early days of enthusiasm for SETI/CETI.
Against this, we've no astronomical evidence as yet of either Kardyshev II or III civilizations (K-I civilizations use energy equivalent to what might be obtained from solar radiation on a planet's surface-- our condition; K-II's utilize energy equivalent to full solar output, for example with a "Dyson sphere" of habitats arranged to absorb virtually all a star's visible radiation; K-II's would use the energy of entire galaxies). This suggests that technological progress beyond some point may be bounded for undetermined reasons-- a bit of a stumper because methods of mining the outer planets for ores and valuable industrial gases and building materials and even for stripping the outer layers of a sun away to extend its life-- K-II style achievements-- are already being discussed in engineering journals in our comparatively low-tech culture. It's a reasonable assumption that our technological capability expands at the rate of say one percent a year over historically long periods of time -- it isn't lack of ability but lack of interest that prevents us from duplicating the Pyramids-- and there's no good reason to think this is going to stop in the immediate future. Why should this be different for alien species?
Note next, we have a conspicuous shortage of visiting aliens. I am deliberately excluding from consideration aliens of the "Flying Saucer-X Files-abduction for medical examination/impregnation-Chariot of the Gods-Area 51" variety, however firmly they may be believed in by 20-30% of the American public. We can find the same sort of numbers who also firmly believe in the literal existence of angels, after all, and they're also outside the bounds of discussion within The SETI League. I've read more than my share of this literature in the past 52 years and have come to regard it not as worthy contributions to human knowledge but as the late 20th century counterpart of Millerism, Mormonism, mesmerism, Atlantis, Bacon-as-the-author-of-Shakespeare, table rapping, and countless other Victorian enthusiasms. There's always a great deal of nonsense about, which someone can be persuaded to believe.
Back to spacefaring aliens. "Where are they?" Enrico Fermi once asked his dinner guests, and fifty years on we still have no answer. Compared to exchanging radio messages, interstellar travel would be (a) dull, (b) expensive, (c) inefficient, and (d) tediously time- consuming, SETI pioneers argued in the 1960's, but spacefarers might have other goals than exchange of scientific knowledge. (Species survival, perhaps? That some of the SETI set could envison no other motive for interstellar travelers than military conquest of other species says something indeed about the aliens among us, but I am not sure what.)
About 1970, Michael Hart proposed that the absence of aliens on earth demonstrated the absence of aliens off earth. Any species with the ability to travel from star to star at rate of 0.1 C could traverse the length of a galaxy in a million years, he argued. A species which chose to build colonies which built other colonies would in time reach every sun of the Milky Way. That individual technological civilizations might not be interested in such activity, as suggested by SETI enthusiasts might well be true, but it hardly seemed possible that not one alien species would be interested in or capable of interstellar flight.
And about this time, Ron Bracewell began to argue that within a century progress in computer science would make possible robotic explorers ("von Neumann replicators") which would explore alien solar systems and exploit local resources to make duplicates of themselves which could be sent to explore other suns. Interstellar exploration could be made cheap and easy, it would seem...
So, where indeed are they? Hart has the right of it, I suspect. There aren't other advanced technological civilizations out there, or at least not anywhere close to us. Getting life started is easy, but there are countless obstacles to reaching much else. Either multicellular life is rare, or intelligent life is rare, or technology is rare.
Thus my questions: Which? Why?
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this page last updated 4 January 2003
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