[115] Where would intelligent life most likely arise? Around stars not too different from our Sun: Single, of mass 0.4 to 1.4 times the Sun's mass, and inhabiting a calm galactic neighborhood for three or more billions of years.

We live in a galaxy containing some hundred billion stars. Where would we expect to find intelligent life? It is appropriate to ask where planets may have formed. In one viable picture of the formation of our Solar System, the planets formed at roughly the same time as the Sun and represent the agglomerated dusty debris of a gigantic solar nebula. Such debris would have a temperature of a few hundred degrees and would be readily detectable in the infrared region of the spectrum. Present-day infrared investigations reveal potential planetary systems around many young stars, some quite different from the Sun in mass and temperature. How can we restrict the nature of the central star in a solar system that is to contain intelligent life?

Let us examine some crucial stellar parameters, namely, surface temperature, mass, evolutionary history, and rotational velocity. Two vital observational attributes of stars are their total energy output and their surface temperature. A plot of these two parameters against one another for stars around us in the Galaxy is not a scatter diagram, however, but reveals a broad swath through the figure. This we term the main sequence, and it represents the locus of mature stars. Stars evolve to the Main Sequence in time scales very short compared with their sojourn in middle age on the sequence Likewise, they evolve rapidly away from the sequence as they die. Time spent on the main sequence is a time of stability for stars, a period of virtual constancy of radiation output and temperature. Gone are the massive [116] flares and violent stellar winds that probed their planetary systems (if any) in early evolution. The length of this period of stability is a strong function of stellar mass. Our Sun will reside on the main sequence for 13 billion years, of which it has so far expended only 6 billion years. A star of 10 solar masses spends only 30 million years there. A star only 1/3 solar mass spends an essentially infinite time on the main sequence.

Stars more massive than the Sun are hotter, emit more strongly in the blue and ultraviolet, and would input more energy to orbiting planets at a given distance. Oceans on the planets would be very warm. The hard radiation would accelerate the rate of mutation of species in the water, hastening the emergence of life from the oceans onto the land. On land, increased hard radiation would shorten the life span of creatures and accelerate genetic alterations. This might shorten the time required to evolve intelligent life. However, a hotter star would have a stronger stellar wind and more energetic flares than the Sun has. The increased ionizing radiation (and subsequent ground penetration by ultraviolet radiation) could confine life to the oceans (or caves) where any harmful effects would be minimized. Consequently, a hot star could also hinder the emergence of intelligent species. Stars less massive than the Sun are cooler and emit much of their energy at long wavelengths (the red part of the spectrum and the near-infrared) which penetrate readily to planetary surfaces. For planets at a common distance, however, a cool star yields a much cooler planetary climate than does a hot star, with a presumably lowered rate of mutation.

There seems to be a consensus that the emergence of intelligence on a planet requires about 3 billion years of relative constancy of stellar output. Therefore, massive hot stars are unfavorable for life because of their very short main-sequence lifetimes, despite their possibly rapid induced rates of mutation. Cool stars afford unlimited time but with lowered mutation rates. We can narrow our focus to stars with mass less than about 1.4 ( denotes the mass of the Sun) and temperature about 7000 K, to ensure a minimum time of 3 billion years on the main sequence.

A habitable planet demands a specific environmental temperature range. The volume of space around a star in which this constraint is satisfied is large for hot stars but greatly diminished for cool ones. Life-bearing planets would have to be very close to a 3000 K star. This proximity leads us to another consideration-planetary rotation. Planetary spin affects many factors that bear on the likelihood of life, including surface gravity across the planet, diurnal temperature variations, global climatic patterns, and the force of winds. Consider temperature alone. A high rotation rate would smooth out day-to-night changes (although too high a spin could also promote a zero-gravity region near the equator from which matter would be lost). A lower rotation rate would enhance the differences between day and night and could strain the capacity of plants to survive. In the lower limit of [117] rotation, the planet's day and year become equal and the rotation becomes locked to that of the star, a situation inimical to life. All water and carbon dioxide could be lost to ices on the cold face, while an eternal desert would cover the hot face. There is a slowing down of rotation that comes about as a result of the tidal braking effect of the star on the body of its planet. Since tidal torque depends very steeply on an inverse power of the orbital distance, a planet very close to its star will be rapidly braked and its rotation locked. In fact, we cannot meet the incompatible constraints of minimum planetary temperature and limited tidal braking for too cool (i.e., too low mass) a star. This implies a limit of at least 0.7 (4000 K) for a star that sustains an intelligent species on a planet.

We can lower this limit on stellar mass somewhat for planets with sufficiently massive and/or close satellites. We can lock a planet to its satellite's rotation yet still have it rotate with respect to the central star. The limit on stellar mass is thereby weakened to greater than about 0.4(3000 K).

Speaking of massive satellites suggests a comment that should be made about life on massive planets. We have so far considered the star as the primary source of energy for the planetary environment. This condition can be relaxed in special circumstances. Jupiter (and to a lesser extent Saturn) radiates into space more energy than it receives from the Sun at its orbital distance, given its albedo. The excess energy is easily accounted for by postulating a minuscule, slow internal contraction of Jupiter in which gravitational energy is radiated away as infrared flux. This makes it more fruitful to regard Jupiter as a failed star than as a giant planet, with a likely central temperature of order 30,000 K, which is too low for nuclear burning. It is not impossible, then, that at levels where the Jovian atmosphere is comfortably warm for life, the gaseous mix could have the required pressure for complex chemistry. Of course, this could lead to floating life forms, which environmentally might be limited in their intelligence.

If we examine spectra of various types of stars, we can learn something about their rotational velocities from the widths of their absorption lines: the broader the lines, the more rapid the velocity. What we find is that the distribution of velocities is not random. Hot, massive stars rotate very rapidly; Sun-type stars hardly at all. In fact, there is an abrupt decrease in velocity at about 1.5(7000 K). In our Solar System, the planets contain most of the angular momentum while the Sun carries only a tiny portion. Since observational evidence is consistent with the conservation of angular momentum during stellar evolution, we can calculate the velocity at which the Sun would rotate if we could fuse it together with the planets. Its velocity would then be typical of stars more massive than 1.5. It is therefore Probable that the Sun rotates so slowly because it has transferred its original angular momentum to the planets. One might reasonably conclude that all stars that are less massive than about 1.5and also rotate slowly have shed [118] their angular momentum into planetary systems. Thus solar systems would be abundant in the Galaxy.

We have so far considered only isolated stars, although there is in fact high frequency of stars that occur in multiple systems, especially in pairs. R. S. Harrington (page 119 of this volume) discusses possible planetary orbits within such binary systems. Under suitable circumstances, there is no reason why life could not develop around such a system.

There is an increasing body of evidence that changes in solar behavior are correlated with climatic phenomena on Earth; for example, the Little Ice Age of the late 17th century occurred at a time when general solar activity was at a very low level. We are accustomed to thinking of solar activity as varying through an 11-year (strictly a 22-year) cycle. It appears, however, that there is no clear evidence, before 1700, for the existence of this regularity in sunspot frequency and other solar phenomena. This issue is controversial, but it shows how premature it would be to characterize the types of small-scale stellar variability that might be undesirable for the emergence of intelligent life. Only extreme flare activity, such as occurs during the early (pre-main-sequence) phases of stellar evolution, would be categorically inimical. There is no evidence for such flares among the restricted class of main-sequence stars with which we are now concerned.

The death of a sufficiently massive star is catastrophic: a supernova is created whose radiated luminosity may for a short period rival that of the entire galaxy in which it occurs. In our Galaxy we estimate roughly one such event each century. Their possible relevance to life is their high output of primary cosmic rays, which could have a significant influence on mutations. Short-lived organisms would double their mutation rate in response to a gross enhancement in background radiation, say 100-to 1000-fold. But long-lived forms would alter in response to only a few fold increase. It has been suggested more than once that the extinction of the dinosaurs some hundred million years ago represented the response of rather specialized, long-lived creatures to a nearby supernova explosion. This is rampant speculation, but we may presume that a planet orbiting a star in a part of the Galaxy where the supernova rate was locally much higher than that in our vicinity (e.g., much closer to the nucleus of the Galaxy than we are) would not be a satisfactory abode for intelligent life.

To summarize in what will appear to be rather chauvinistic vein, intelligent life as we define it would be likely to arise on a planet orbiting a main-sequence star of mass between about 0.4 and 1.4(between 3000 and 7000 K) in a part of the Galaxy roughly similar to our own neighborhood Such a star is likely to have shed its angular momentum into planets, to have been stable in radiative output and temperature for at least 3 billion years, to warm a nearby planet without tidally locking the planet's rotation to its own, and not to be subject to frequent local supernova explosions.

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