Where are they? Astronomy and the Question of Life in the Universe

The topics today are: does life exist elsewhere in the universe? Does intelligent life exist elsewhere in the universe?

The problem in answering this is that the stars are far away. We cannot even see Earth-like planets around nearby solar analog stars, let alone see if they have life. This situation could plausibly change in the next couple of decades.

The subject of exobiology, or the question of whether life has originated on another planet (or many other planets) consists of gathering what we know from several disciplines (astronomy, biochemistry, paleontology, etc) and a lot of speculation.

Let’s go through some of the steps.

We have just seen that there are other stars like the Sun. If we say that there are roughly half a dozen solar twins within 10 parsecs (roughly correct given my table from last time) we would conclude that there are approximately 10 million solar analogs in the Milky Way galaxy! Surely some of those have planets like Earth with life on them!
Do these stars have planets around them? We cannot be sure, but everything we see about the solar system strongly suggests that planetary formation is a natural part of the process of forming stars. So we can plausible argue that every solar analog will grow a system of planets. This speculation was enormously corroborated in 1995, with the first detection of planets beyond the solar system. There are now approximately 20 star systems with planets around them (see the textbook, Chapter 15). Many of these stars are solar analogs such as 51 Pegasi. Weirdly, however, all or most of the planets found to date are in the Hot Jupiter category, which are quite different from the planets in our solar system. Such objects were completely unanticipated prior to their discovery, demonstrating that in science, “you’ve got to look”.
Would the planets that formed be like the Earth? Hard to say. The terrestrial planets seem to form in a somewhat inevitable way, but not every one would be suitable for a habitable planet. In our solar system, Venus is too close to the Sun and too hot. Mars is too miniscule to hold its atmosphere. Probably some percentage of solar analog stars would produce a terrestrial planet similar to the Earth, with water oceans, the right mass of the atmosphere, etc.
Would life arise on such planets? This is where we are really in terra incognita. We do not understand the circumstances under which life arose on Earth 3.5 billion years ago, so are not in good position to extrapolate to other (unknown) planetary systems. The standard folklore for over 50 years has been that energetic processes such as lightning in an atmosphere of gases such as methane, ammonia, and carbon dioxide, would produce complex organic molecules. This is discussed in Chapter 15 of your book. Add to this the fact that carbonaceous chrondite meteorites contain organic molecules (including amino acids), and that complex organic molecules are seen in the interstellar clouds from which stars form. All of this shows that nature knows how to make complex organic substances. However, I think it is the case that we really have no idea what initiates the final step and actually begins to make this stuff crawl. It is my opinion that until some additional science arrives from microbiology, these speculations will remain precisely that.
Would higher life arise? In the last couple of decades, it has become clearer that another potential “bottleneck” exists in the early history of a planet. The claim is that the earliest forms of life arose 500 million to a billion years after the formation of the Earth: that is 3.5 to 4.0 billion years ago. It is beginning to be clear that multicellular life did not arise until the “Cambrian Explosion” about 580 million years ago (once again, read Stephen J. Gould’s book Wonderful Life). What took so long? Stated differently, what process or processes impeded (or did not promote) the development of large organisms for a period of 3 to 3.5 billion years after the formation of the Earth? Since the lifetime of the Sun as a “Main Sequence Star” is 10 billion years, it is clear that if nature had been a bit slower in the development of things, the whole biological history of the Earth might have been of bacteria and protozoans!
Would intelligent life arise?
“I will grant you all the assumptions in the development of life on other planets up to the point of the evolution of cows. Now how many of them will build radiotelescopes” …..Dr. Richard Porcas. Given the uncertainties of the last few items, it is not surprising that the likelihood of the development of intelligent life is a complete unponderable. We simply do not know enough about what evolutionary advantages the development of big brains had for Homo Sapiens. Dr. Richard Leakey, in this book The Sixth Extinction suggests that a climatological fluke about one million years ago pressured our primate ancestors into getting smart or becoming dead. However, in the same book Leakey documents several bursts of encephalization , or increased brain size in mammals during the past fifty million years (e.g. dogs are smarter than anteaters; dolphins are smarter than dogs).
We can certainly anticipate progress in all of these areas in the next several decades. Stay tuned.