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Chapter 27 - Life in the Universe
CHAPTER 27
LIFE IN THE UNIVERSE
CHAPTER OUTLINE AND LECTURE NOTES
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Life
One of the problems in organizing an astronomy textbook is where to put the subject of life in the Universe.
In the first edition of this book I chose to combine a discussion of what we know about the origin of life on
Earth with the treatment of life elsewhere. This material was part of the chapter on Earth. My reason for
having such a limited discussion of life in the Universe was that I thought the subject was still very
speculative. For example, at that time, only a little more than 10 years ago, we didn’t yet have any evidence
for planetary systems orbiting ordinary stars other than the Sun.
A lot has happened in the last decade, including the discovery of many planets and planetary systems
beyond the solar system, measurements that suggest there may once have been life on Mars, and the
strengthening of the possibility that conditions might be favorable for life on Jupiter’s satellite Europa.
The Origin and Evolution of Life on Earth
Life Elsewhere in the Solar System
Those who thought that the issue of whether the meteorite shown in Figure 27.3 really shows evidence of past
life on Mars would be quickly resolved were wrong. In the years since the original announcement of the
discovery of chemical and structural evidence for life in the meteorite there have been a number of additional
studies. Some seem to confirm that the material in the comet has a biological origin. Other studies seem to
confirm that a non-biological explanation is preferred.
Life in Other Planetary Systems
The number of stars known to be orbited by planets has grown steadily. There are now more than 200 stars
that have been found to have planets. Many other stars are known to be orbited by dark bodies (probably
brown dwarfs) with masses between 17 and 60 times the mass of Jupiter. The list is likely to have grown by
the time you read this. An up-to-date list of known and suspected planets and brown dwarfs can be found on
the World Wide Web at http://www.obspm.fr/department/darc/planets/encycl.html.
Interstellar Communication
A measure of how interesting this topic is to the public is the long list of television shows and movies devoted
to the topic.
KEY TERMS
amino acid — A carbon-based molecule from which protein molecules are assembled.
cell — A small, highly organized collection of molecules enclosed in a wall-like membrane.
Cells are the basic units of life.
Drake Equation — An equation that, if we knew the values of all of its terms, could be used to
calculate the number of extraterrestrial civilizations in the Milky Way with which we might
potentially establish communications.
habitable zone — The range of distances from a star within which liquid water can exist on the
surface of an Earth-like planet.
life — The property of a body that (1) has organization, (2) metabolism, (3) regeneration and
growth, (4) response to stimulation, and (5) reproduction and evolution.
nucleic acid — A long chain of nucleotides. DNA and RNA are nucleic acids.
nucleotide — The class of organic molecules of which nucleic acids are composed.
protein — A large molecule, consisting of a chain of amino acids, that makes up the bodies
of organisms.
SETI — The search for extraterrestrial intelligence.
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Chapter 27 - Life in the Universe
transit — An event that occurs when a smaller body passes between a larger body and an
observer causing the apparent brightness of the larger body to drop.
ANSWERS TO QUESTIONS AND PROBLEMS
Conceptual Questions
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DNA or a similar substance is necessary so that the offspring of an organism can inherit the
organism’s characteristics.
2. Water is an excellent solvent and can bring many substances together to react with each other. The large heat
capacity of water protects organisms from getting too hot or too cold. The large range of temperatures for
which water is a liquid helps prevent organisms from freezing or boiling. The high heat of vaporization of
water makes it easy for evaporation to cool organisms. Ammonia and methyl alcohol are both liquids at
temperatures where water is frozen. Ammonia, however, is a liquid over a much smaller range of temperature
than is water. Methyl alcohol has a lower heat capacity and heat of vaporization than water does.
3. Two possibilities are that the organic molecules fell to Earth in primordial solar system bodies or were formed
from the gases in Earth’s atmosphere by lightning, sunlight, heat, or shock waves.
4. The nebular theory predicts that planets should be numerous in the galaxy. Catastrophic theories predict that
planets should be very rare.
5. It is much easier to detect Jupiter-like planets than terrestrial planets.
6. The observations were periodic variations in the Doppler shifts of spectral lines of the stars as they orbited
their common centers of mass with their planets.
7. They are similar to the solar system in that they contain one or more giant planets. They are different in that
in many planetary systems the giant planets are much closer to their stars than Jupiter is to the Sun and that
many of the giant planets have highly elliptical orbits.
8. The luminosity of the star changes as the star evolves.
9. The longer life endures on a typical planet, the greater the number of inhabited planets in the galaxy.
10. If many planets are inhabited by technological civilizations, then the average technological civilization must
last much longer than ours has thus far.
11. 1 and 10 GHz only sound like special frequencies because they depend on the definition of the Hz which, in
turn, depends on the definition of the second. The length of the second depends on the length of the day and
Mesopotamian practice of dividing the hour into 60 minutes and the minute into 60 seconds.
12. Feel free to speculate.
Problems
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1 second of arc
3 × 10-6 seconds of arc
0.1 m/s
2.2 years
There is no correct or incorrect answer.
500,000 years
90
4 hours, 4 years
Figure-based Questions
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0.5 AU
5.5 billion years, 1 billion years from now
2.5 billion years from now
2.8 billion years
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