To everything there is a season, and a time to every purpose under heaven.
The astronomer Mario Livio takes issue with the notion that the timescale for the evolution of intelligent life is completely independent of the main sequence lifetime of a star. if the two timescales were related in a particular way — if the evolutionary timescale increases as a star's lifetime increases — then we would expect to observe the two timescales as roughly equal. Carter's gloomy conclusion about the non-existence of ETCs would then not follow. But how can the lifetime of a star influence the timescale of biological evolution?182
Livio considers a simple model of how a planetary atmosphere like Earth's develops to the stage where it can support life. It is not a serious model of atmospheric development; rather, it is meant to demonstrate a possible link between stellar lifetimes and the timescale for biological evolution.
In his model, Livio identifies two key phases in the development of a life-supporting atmosphere. The first involves the release of oxygen from the photodissociation of water vapor. On Earth, this phase lasted about 2.4 billion years and resulted in an atmosphere with oxygen levels at about 0.1% of present values. The duration of this phase depends upon the intensity of radiation emitted by the star in the wavelength region of 100 to 200 nm, because only this radiation leads to the dissociation of water vapor.
The second phase involves an increase in oxygen and ozone levels to about 10% of their present values. On Earth, this phase lasted about
1.6 billion years. Once oxygen and ozone levels were high enough, Earth's surface was shielded against ultraviolet (uv) radiation in the wavelength region of 200 to 300 nm. This shield was important because it protected two key ingredients of cellular life: nucleic acids and proteins. Nucleic acids absorb radiation strongly in the wavelength region of 260 to 270 nm, while proteins absorb radiation strongly in the wavelength region of 270 to 290 nm; radiation in the region of 200 to 300 nm is therefore lethal to cell activity. It is vital — at least for land life — that an atmosphere develops a protective layer for these wavelengths. And of the likely candidates of a planet's atmosphere, only ozone absorbs efficiently in the wavelength region of 200 to 300 nm: a planet needs an ozone layer. Livio argues that, as on Earth, the timescale for developing an ozone shield against UV radiation is roughly equivalent to the timescale for the development of life.
Different types of star emit different amounts of energy in the UV region. High-mass stars are hotter than low-mass stars and emit more UV radiation, but they have shorter lifespans. So for a given planetary size and orbit, the timescale for developing an ozone layer depends upon the type of radiation emitted by the star, and thus on the star's lifetime. Following a detailed calculation, Livio argues that the time needed for intelligent life to emerge increases almost as the square of the stellar lifetime. If such a relation holds, then we are likely to observe intelligent species to emerge on a timescale comparable to the main-sequence lifetime of a star.
The purpose of Livio's model, to repeat, is simply to show whether a relationship possibly exists between the timescale for biological evolution and stellar lifetimes. Even with this proviso, one can still disagree with parts of Livio's argument. For example, his model involves a necessary condition for land life to evolve (namely, the development of an ozone layer); but this is not a sufficient condition. There are many other steps on the road to the evolution of intelligent life, so even if there is a link between stellar lifetimes and the timescale for biological evolution, it may be a minor factor. Nevertheless, encouraged by the discovery of a link between these timescales and the possibility therefore that the existence of ETCs is not ruled out, Livio asks the following question: in the history of the Universe, when is a likely time for ETCs to emerge?
If life on Earth is typical of life elsewhere, then most life-forms will be carbon-based. Livio therefore suggests that the emergence of ETCs will coincide with the peak in the cosmic production of carbon. And this is something we can calculate.
The main producers of cosmic carbon are planetary nebulae, which occur at the end of the red-giant phase of average-mass stars. Planetary nebulae shed their outer layers into the interstellar medium, and the material is recycled to form later generations of stars and planets. Since astronomers
believe they know the historical rate of star formation (it was higher in the past than it is now, with a peak about 7 billion years ago) and they know the relevant details of stellar evolution, they can calculate the rate at which planetary nebulae formed in the past — and thus the rate of cosmic carbon production. According to Livio's calculations, the rate of planetary nebula formation peaked a little less than 7 billion years ago. From this, he argues we might expect carbon-based life to have started when the Universe was about 6 billion years old. Since the time required for advanced ETCs to evolve is a significant fraction of a stellar lifetime, we would expect ETCs to develop only when the Universe was about 10 billion years old. If this is the case, then ETCs cannot be more than about 3 billion years older than us.
Livio's conclusion has been proposed by others as a resolution of the Fermi paradox. They suggest life could have emerged only relatively recently on a cosmic scale. There are presently no ETCs capable of interstellar travel or communication because, like us, they have had insufficient time to develop. Perhaps in the future the Galaxy will be aswarm with interstellar commerce and travel and gossip. For now, though, all is silence.
But even if Livio's conclusion is correct, and there are no ETCs more than 3 billion years in advance of us, I fail to see how it solves the Fermi paradox. An ETC that is 3 billion years older than us has had plenty of time to colonize the Galaxy; it has had plenty of time to announce its presence to the Universe. (In the Universal Year, ETCs could have reached our present level of technology at about October 1; they thus have 3 months to colo nize the Galaxy — a process we can measure in hours on this scale. They have had time enough to get here.) Unless it can be shown that intelligence is only coming into existence now, and thus life on Earth is among the most advanced in the Galaxy, the arguments do not really address the main thrust of the paradox.
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