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
- Don’t forget the final exam. Friday, December 21, 4:30PM. Twenty five multiple choice questions. Sample will be on the web this weekend.
- Look at the material in Chapter 15 of the book. Especially look at the cover picture
for this chapter.
Exobiology
- Brief remarks on the state of my knowledge of the
subject.
- Nobody else is much better. Exobiology is a
scientific discipline without subject matter.
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.
- 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.
- 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.
- 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.
- 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.