Characteristics and Origins of the Solar System

Lecture 35

December 12 ,2001

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

 

 

Announcements

 

Exobiology

 

Here we really  have a case of trying to piece together a story that is (in its crucial parts) 4.5 Gyr old.  Unlike the formation of the solar system, there is probably more going on that just chemistry and physics. 

 

            We deal with this in an astronomy course, because a big piece of the question of the existence of life outside of the solar system is contributed by astronomy. 

            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. 

 

            I’ll state the traditional viewpoints form the outset.  This is a stated or unstated credo that has been adhered to by most people interested in life in the universe.  This view is that life should be common because we live on an (apparently normal) terrestrial planet  orbiting an average sort of star.  If those aspects of the whole picture are common, why shouldn’t the origin of life as well? 

 

            That has been the traditional viewpoint, but it might not be right.  We can come up with a long “laundry list” of attributes of planet Earth that appear to be crucial for life, but don’t seem to be a necessary part of the terrestrial planet formation process. 

 

Preview:  Significance of the “where are they” question, originally formulated by the physicist Enrico Fermi.  Where are the signs of alien civilizations millions of years more advanced than us?  Fermi’s question has been made even more intriguing by the speculations of Linewater on the evolutionary history of terrestrial planets. 

 

            Let’s go through a series of questions related to the existence of life in the universe.  The order of these steps will be from the ones we are most confident of, to those where we really have no idea. 

 

  1. 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!  In any case, the abundance of stellar homes for other life forms is well established. 
  2. 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.  However, the joker in the deck is whether a solar system like ours would form, or Hot Jupiters.  It seems almost certain that Hot Jupiters would mean the end of the trail for habitable terrestrial planets.  At the moment we don’t know how these two classes of planetary systems stack up.
  3. 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.  An important concept here is that of the habitable zone around stars.  This is the annulus within which liquid water could exist on a terrestrial planet. 
  4. 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 about 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.  The basis of this belief is the Miller-Urey experiment, which was first performed  1953.  A sealed laboratory flask was filled with gases which would have been plentiful in the early atmosphere of Earth, such as carbon dioxide, methane, ammonia, etc,  and electrical sparks were passed through the flask.  Gunk-like organic materials precipitated to the bottom of the flask.  Look at Figure 15.5 of your textbook.  The relevance of this experiment is attested to by the carbonaceous material which appears to be so omnipresent in the solar system.  Nonetheless, 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. Until some additional science arrives from microbiology,  we will be stuck at this point in the narrative.