Future Prospects

It is a near-certainty that Earth-mass planets will eventually be found within the habitable zones of many star systems. As to whether advanced life will have developed on these planets is another question. Indeed, there is no apparent consensus among astrobiologists that the evolution of intelligence is inevitable; some say it is highly likely, while others say it is highly improbable.

Physicist Brandon Carter (Observatoire de Paris) has argued, however, that the probability of a highly intelligent, environment-manipulating species evolving increases with time. The older a planetary system is, the more likely it is that a technologically advanced civilization will eventually appear. If this is the case, then Carter argues that technologically advanced civilizations are likely to be rare within our Milky Way galaxy. This result follows because there is a critical time scale at play in all planetary systems, this time scale being the main sequence (TMS) lifetime of the parent stars. The main sequence time scale corresponds to the time that a star can generate energy through the conversion of hydrogen into helium within its deep interior.

Once this energy store is exhausted, a Sun-like star will evolve into a red giant—a bloated, low-temperature, high-luminosity star. Once a star enters its red giant phase, then the habitability zone is swept well outside of the typical planetary region, and what was once the habitable, nurturing zone for life will become sterilized.

This, then, appears to be the critical problem for the emergence of a technologically advanced civilization, in that its most likely appearance time is that corresponding to TMS, but this is also the time at which the parent star is primed to extinguish all life within the planetary system. Carter has likened the problem to that of throwing dice and requiring a string of, say, four 6's in a row to occur. If an unlimited number of throws are allowed, then the sequence of four 6's in a row will eventually appear—it's a certainty. If, on the other hand, only, say, 12 throws of the dice are allowed, then the likelihood of getting the four 6's in a row to appear is very much reduced. The main sequence lifetime limit of the parent star acts in a similar sense to the limit on the number of dice throws.

Many highly specific conditions must no doubt come into play in order for a technologically advanced civilization to emerge on a given planet within a nurturing habitable zone. On Earth, for example, the fossil record tells us that well over 99% of all species that have ever evolved are now extinct, and consequently we learn that there is no guarantee of longevity (on time scales of order, say, many hundreds of millions of years and longer) for even the best adapted of species. Humanity is no different from all the species that have gone before it, and it is certainly unclear if our current world dominance will carry on into the deep future. Indeed, it was argued in Chapter 4 that it is rather unlikely that humanity will survive into even the near-term future on the Earth if it doesn't significantly reduce its devastating environmental footprint. It is not at all unlikely that the key condition that restricts the number of civilizations that might exist at any one instant within our galaxy (and any other galaxy within the universe) is that of survival longevity. The shorter the survival time of an intelligent species, the less likely it is that it will be able to initiate space travel or transform its planetary system.

The essential upshot of Carter's argument that is of relevance to terraforming and the finding of possible new and distant worlds for humanity is that there should be many planets that are habitable, but are not inhabited, by intelligent life forms. Martyn Fogg picked up on this point a number of years ago and has suggested that terraforming might be one way in which humanity could successfully achieve interstellar colonization. Here the point is that while other Earths will be rare (that is, a one Earth-mass planet at 1 AU from a 4.56 billion year old Sun-like star), Earth-mass planets that one might successfully terraform are likely to be common. The path of colonization could therefore be delineated by a series of terraformed staging-post worlds.

The author recently reviewed, in the February 2008 issue of the Journal of the British Interplanetary Society, the possibility of inferring the existence of intelligent extraterrestrial life through the detection of terraformed planets in exoplanet systems. The key result is shown in Figure 8.12, which was constructed using the available published data on the estimated ages of exoplanetary systems along with the derived

Terraforming

Star-engineering

Orbit reorganization

Dyson sphere construction r~

Interstellar migration

Systems 'sterilized'

Age/Main sequence lifetime

Figure 8.12. Relative main sequence lifetime distribution of 123 exoplanet systems. From M. Beech, Terraformed exoplanets and SETI. Journal of the British Interplanetary Society 61 (2), 43-46 (2008).

masses for their parent stars. In this diagram, the relative spread in main sequence age of 123 exoplanetary systems is shown.

The number of systems in each bin is not the important point at this stage, other than the bins not being empty, but rather it is the point that a few of the exoplanet systems have ages very close to the main sequence lifetime limit of their parent stars that is of interest.

Of the 123 exoplanetary systems studied, 56% had ages that were greater than half of their parent stars main sequence lifetime, while 14% had ages that fell between 75% and 90% of their parent stars main sequence lifetime. An additional six systems (5% of the total studied) had an age that was within 1% of the main sequence life time of their parent stars.

The Sun is 4.56 billion years old (recall Chapter 5) and has completed about 45% of its main sequence lifetime. It has been suggested within preceding chapters that the terraforming of Mars and Venus will likely be completed within the next several to 10,000 years. Consequently, although perhaps unwisely using humanity as the standard, any planetary system with an age greater than about 50% of the main sequence lifetime of their parent star might conceivably show signs of terraforming. The important observational point here is that such systems could have habitable worlds that are located outside of the standard habitable zone. This observation, however, further provides a means of potentially verifying the existence of an extraterrestrial civilization. A habitable planet situated well outside of the canonical habitable zone can only come about through the engineering of a directed intelligence. Likewise, the detection of a Dyson sphere or evidence for planetary orbit migration would indicate the presence of an advanced civilization. In the latter case, the key test would be to show that the dynamical lifetime of the specific planet's orbit was much shorter than the actual system age.

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