The chapters in this document are displayed in reversed order, with the last chapter appearing first.
This change ensures all chapters are included in the solutions.
OpenStax Astronomy 2e Chap 1 to 30 included - Complete Instructor Answer Guide
Chapter 30: Life in the Universe
Review Questions
1. What is the Copernican principle? Make a list of scientific discoveries that confirm it.
Answer
The Copernican principle is the idea that Earth and the Sun are in no way specially favored
bodies in the universe. Several discoveries confirm this, including (in order of discovery) the
following: Earth orbits the Sun and is not the center of our solar system, our Sun is one among
billions of other stars in the Milky Way Galaxy and is not in any central position within the
Galaxy, our Galaxy is one among billions of other galaxies in the universe, and planets are
commonly found orbiting other stars. You could also discuss that the elements that make up most
of Earth and the Sun are commonly found in other stars and other planets.
2. Where in the solar system (and beyond) have scientists found evidence of organic molecules?
Answer
Beyond our solar system, organic molecules have been found in giant clouds of dust and gas
between stars (the “interstellar medium”) and in star-forming regions. In our solar system,
besides Earth, organic molecules have been discovered on comets, in meteorites, on Saturn’s
moon Titan, in the plumes of water expelled from Saturn’s moon Enceladus, and on Neptune’s
moon Triton.
3. Give a short history of the atoms that are now in your little finger, going back to the beginning
of the universe.
Answer
All the hydrogen atoms in your little finger have been around since the universe first cooled
enough for protons and electrons to get together into atoms. Elements heavier than hydrogen in
your finger were fused in stars through a process called nucleosynthesis. Fusion of lighter
elements inside the “furnace” of stars creates new heavier elements, which are then dispersed
when stars explode or lose material more peacefully. The newly made elements eventually join
the clouds of gas and dust between the stars. Out of these, new stars form and further heavier
elements are created inside their furnace. Elements up to iron can be formed during normal
fusion inside stars, and elements heavier than that are formed during the violence of supernova
explosions. The atoms in your little finger were made available in this way to the cloud from
which the solar system formed. They then became part of the planetesimals that collided with
each other to form the proto-Earth. Then through twists and turns of planetary evolution and
life’s evolution on our own planet, the atoms found their way to your little finger. But they likely
won’t be there for long because a human lifetime is but a blink of the eye compared to the age of
the universe.
4. What is a biomarker? Give some possible examples of biomarkers we might look for beyond
the solar system.
Answer
A biomarker is a feature—a chemical substance, a structure, or a signal—that could only have
been formed by life. Beyond our solar system, we can only detect planet-scale biosignatures—
biological impacts so great that they affect the way a planet appears in reflected or emitted
electromagnetic radiation. An example of such an exoplanet biomarker would be unusual
atmospheric composition, such as the mutual presence of methane and oxygen. While this would
be a strong indication of life, it would not be unequivocal because methane and oxygen can be
produced in the absence of life under special circumstances. Another possible example might be
very short, very energetic pulses of visible light or infrared radiation or radio waves that are not
,OpenStax Astronomy 2e Instructor Answer Guide
just natural static, but are coded with information given off by huge structures in space built near
or around stars. Both would be biomarkers of technologically advanced civilizations.
5. Why are Mars and Europa the top targets for the study of astrobiology?
Answer
Five decades of observation of our neighbor world, Mars, strongly suggest that in the distant past
it had an environment (thicker atmosphere, running surface water, perhaps even lakes) that could
have sustained life on its surface. Even if such life no longer survives on Mars, its “fossils”
might still be found on the red planet. Life could also exist on modern Mars just below the
surface, where liquid water is thought to exist. Europa is a top target because of the high
likelihood of an extensive salty ocean under the thick ice shell that covers this moon of Jupiter.
This ocean, substantially deeper than Earth’s ocean, is probably in contact with a rocky seabed
and may be warmed by internal heat; thus the interaction of water and rocks could provide a
chemical energy source for life.
6. Why is traveling between the stars (by creatures like us) difficult?
Answer
Interstellar travel is difficult for many reasons. The first is certainly the vast distances between
the stars. Even at speeds very close to the speed of light, the maximum theoretical speed
achievable, it would require four years or more to travel between stars. At more realistic speeds,
trips would take far longer than a human lifetime. And the faster you go, the more expensive (in
fuel costs) the trip would be. Since we can’t depend on fuel being available at our destination,
such travel would require carrying all the fuel necessary for both the trip there and the return trip
and require accelerating all that fuel to tremendous speeds—a truly gargantuan effort, and an
extraordinarily expensive one. To be sure, those issues only come up if creatures like us are
along on the trip. Travel by machines (such as robots, computers, or smartphones) could proceed
much more slowly and less expensively. As this book went to press, a billionaire in Silicon
Valley gave $100 million to a project to find technology that could get a very tiny probe to the
nearest star using laser propulsion. See: Project Breakthrough Star-shot:
https://breakthroughinitiatives.org/News/4
7. What are the advantages to using radio waves for communication between civilizations that
live around different stars? List as many as you can.
Answer
Radio waves travel at the speed of light, are cheap to produce (they are the lowest-energy
electromagnetic waves), are not significantly absorbed by interstellar clouds, go right through
planetary atmospheres, and, most importantly, can be modulated in a way that carries
information.
8. What is the “cosmic haystack problem”? List as many of its components as you can think of.
Answer
Because so many factors go into detecting a signal from extraterrestrial intelligence, some
astronomers have compared the effort to searching for a needle in a haystack. Some of the
problem’s components include the origin and direction of the signal containing the message from
among all the possible directions one could “listen,” the frequency chosen for that signal from
among the vast range of potential frequencies in the electromagnetic spectrum, the frequency
width of that signal, the strength of that signal compared with background noise, the continuity
of that signal (whether it’s on all the time, or only sweeps over us periodically), the frequency
drift of that signal (caused by the relative motion of the sources to Earth), the system used for
,OpenStax Astronomy 2e Instructor Answer Guide
encoding any message in that signal, and whether we would recognize the nature of the message,
when it was coded by alien minds.
9. What is a habitable zone?
Answer
A habitable zone is the range of distances from a star where, if water existed on the surface of a
planet, that water would likely be liquid.
10. Why is the simultaneous detection of methane and oxygen in an atmosphere a good
indication of the existence of a biosphere on that planet?
Answer
Oxygen and methane chemically react with each other, so we would not see them together unless
there are active sources for both. At least on Earth, biology is responsible for essentially all the
oxygen and the majority of the methane in our atmosphere.
11. What are two characteristic properties of life that distinguish it from nonliving things?
Answer
Life extracts energy from its environment and has a means of encoding and replicating
information in order to make faithful copies of itself.
12. What are the three requirements that scientists believe an environment needs to supply life
with in order to be considered habitable?
Answer
The requirements are a solvent (water may be the best example), the biogenic elements
(CHNOPS) in biologically accessible form, and energy.
13. Can you name five environmental conditions that, in their extremes, microbial life been
challenged by and has learned to survive on Earth?
Answer
Five environmental conditions that microbial life has overcome are extreme temperature,
pressure, salinity, acidity, and radiation.
Thought Questions
14. Would a human have been possible during the first generation of stars that formed right after
the Big Bang? Why or why not?
Answer
No. Humans are composed primarily of the elements carbon, oxygen, hydrogen, and nitrogen,
and we live on a planet that is made mostly of silicon, iron, and oxygen. Of these elements, only
hydrogen was formed in the Big Bang. The others had to be built up via nuclear fusion in the
cores of hot stars, and subsequently ejected into space, before they were available to form a
second or third generation of stars with planets around them.
15. If we do find life on Mars, what might be some ways to check whether it formed separately
from Earth life, or whether exchanges of material between the two planets meant that the two
forms of life have a common origin?
Answer
We would need a genomic analysis (a study of its genetic structure). By comparing the genome
of the martian life with that of terrestrial life, we could see if the Mars microbes are distant
cousins or truly represent an independent origin of life on that planet.
16. What kind of evidence do you think would convince astronomers that an extraterrestrial
spacecraft has landed on Earth?
Answer
, OpenStax Astronomy 2e Instructor Answer Guide
The best evidence, of course, would be the spacecraft itself and its occupants (if any). If the alien
visitor just landed in a public place and stayed around for a few hours, the identification would
be simple. Short of this, the best evidence would be some artifact or piece of a spacecraft. Even a
tiny fragment of extraterrestrial material would surely reveal its extraterrestrial nature when
subjected to laboratory analyses (just as a lunar or martian meteorite can be identified by
differences in its composition). Another piece of convincing evidence would be a visual or radar
track of the flight path of the lander, showing it coming from beyond Earth and behaving in ways
that are different from ordinary meteorites. However, no evidence of this sort has ever been
produced. Even a fingernail clipping from an alien or some picnic garbage (of alien food) that
they inadvertently left behind could be enough, but no one has been able to come up with such
testable evidence (despite all the sensational UFO claims in popular media).
17. What are some reasons that more advanced civilizations might want to send out messages to
other star systems?
Answer
Since no one on Earth has any idea about the psychology of intelligent aliens, this is a matter of
pure speculation. Most answers assume (for lack of other information) that other intelligent
beings will have motivations similar to ours. Then their reasons could range from simple
curiosity about other life forms to a desire to help backward civilizations like ours; perhaps they
would want to convert us to their form of religion. Some astronomers (and science fiction
writers) have speculated about a “Galactic Library”—a repository of information, culture, and
beliefs from many civilizations. New civilizations may be actively sought so they can become
contributors to (and perhaps users of) this library. On the other hand, given the vast scales of
galactic time, it is possible that various users are separated by billions of years of evolution,
which would make it difficult to compare contributions. Perhaps young civilizations like ours
will be encouraged to get a card for just the children’s library for the first few million years. If
civilizations don’t survive for a long time (on the cosmic time-scale,) they may want to scan the
skies for signals to assure themselves that others have managed to survive the crises that lead
civilizations to self-destruct. If few civilizations survive, those that do may feel lonely for
company. Some negative reasons may include the desire to know about young “rising”
civilizations so they can be eliminated before they might become a threat or so they can be
quarantined so their violent tendencies don’t infect the rest of the Galaxy.
18. What are some answers to the Fermi paradox? Can you think of some that are not discussed
in this chapter?
Answer
Answers to the Fermi paradox discussed in the text (the question of why, if planets like ours and
life out there is common, no one has contacted us) are as follows: intelligence out there is rare, a
network of intelligent civilizations has not yet had time to develop, we are not aware of the
communications from advanced aliens streaming by us, advanced civilizations do not want to
interfere with the development of immature races like us, and advanced civilizations don’t last a
long time after developing technology that can destroy them or their world. Other ideas include
the notion that civilizations might exist and not want to get in touch; that intelligence does not
necessarily lead to technology; or that they may first send probes or machines to “scout” what’s
out there, and these may be much harder to find. A good book on this topic is Stephen Webb’s If
the Universe Is Teeming with Aliens . . . Where Is Everybody? Fifty Solutions to the Fermi
Paradox and the Problem of Extraterrestrial Life (2002). The Wikipedia entry on the Fermi
paradox also has a good list of suggestions.
This change ensures all chapters are included in the solutions.
OpenStax Astronomy 2e Chap 1 to 30 included - Complete Instructor Answer Guide
Chapter 30: Life in the Universe
Review Questions
1. What is the Copernican principle? Make a list of scientific discoveries that confirm it.
Answer
The Copernican principle is the idea that Earth and the Sun are in no way specially favored
bodies in the universe. Several discoveries confirm this, including (in order of discovery) the
following: Earth orbits the Sun and is not the center of our solar system, our Sun is one among
billions of other stars in the Milky Way Galaxy and is not in any central position within the
Galaxy, our Galaxy is one among billions of other galaxies in the universe, and planets are
commonly found orbiting other stars. You could also discuss that the elements that make up most
of Earth and the Sun are commonly found in other stars and other planets.
2. Where in the solar system (and beyond) have scientists found evidence of organic molecules?
Answer
Beyond our solar system, organic molecules have been found in giant clouds of dust and gas
between stars (the “interstellar medium”) and in star-forming regions. In our solar system,
besides Earth, organic molecules have been discovered on comets, in meteorites, on Saturn’s
moon Titan, in the plumes of water expelled from Saturn’s moon Enceladus, and on Neptune’s
moon Triton.
3. Give a short history of the atoms that are now in your little finger, going back to the beginning
of the universe.
Answer
All the hydrogen atoms in your little finger have been around since the universe first cooled
enough for protons and electrons to get together into atoms. Elements heavier than hydrogen in
your finger were fused in stars through a process called nucleosynthesis. Fusion of lighter
elements inside the “furnace” of stars creates new heavier elements, which are then dispersed
when stars explode or lose material more peacefully. The newly made elements eventually join
the clouds of gas and dust between the stars. Out of these, new stars form and further heavier
elements are created inside their furnace. Elements up to iron can be formed during normal
fusion inside stars, and elements heavier than that are formed during the violence of supernova
explosions. The atoms in your little finger were made available in this way to the cloud from
which the solar system formed. They then became part of the planetesimals that collided with
each other to form the proto-Earth. Then through twists and turns of planetary evolution and
life’s evolution on our own planet, the atoms found their way to your little finger. But they likely
won’t be there for long because a human lifetime is but a blink of the eye compared to the age of
the universe.
4. What is a biomarker? Give some possible examples of biomarkers we might look for beyond
the solar system.
Answer
A biomarker is a feature—a chemical substance, a structure, or a signal—that could only have
been formed by life. Beyond our solar system, we can only detect planet-scale biosignatures—
biological impacts so great that they affect the way a planet appears in reflected or emitted
electromagnetic radiation. An example of such an exoplanet biomarker would be unusual
atmospheric composition, such as the mutual presence of methane and oxygen. While this would
be a strong indication of life, it would not be unequivocal because methane and oxygen can be
produced in the absence of life under special circumstances. Another possible example might be
very short, very energetic pulses of visible light or infrared radiation or radio waves that are not
,OpenStax Astronomy 2e Instructor Answer Guide
just natural static, but are coded with information given off by huge structures in space built near
or around stars. Both would be biomarkers of technologically advanced civilizations.
5. Why are Mars and Europa the top targets for the study of astrobiology?
Answer
Five decades of observation of our neighbor world, Mars, strongly suggest that in the distant past
it had an environment (thicker atmosphere, running surface water, perhaps even lakes) that could
have sustained life on its surface. Even if such life no longer survives on Mars, its “fossils”
might still be found on the red planet. Life could also exist on modern Mars just below the
surface, where liquid water is thought to exist. Europa is a top target because of the high
likelihood of an extensive salty ocean under the thick ice shell that covers this moon of Jupiter.
This ocean, substantially deeper than Earth’s ocean, is probably in contact with a rocky seabed
and may be warmed by internal heat; thus the interaction of water and rocks could provide a
chemical energy source for life.
6. Why is traveling between the stars (by creatures like us) difficult?
Answer
Interstellar travel is difficult for many reasons. The first is certainly the vast distances between
the stars. Even at speeds very close to the speed of light, the maximum theoretical speed
achievable, it would require four years or more to travel between stars. At more realistic speeds,
trips would take far longer than a human lifetime. And the faster you go, the more expensive (in
fuel costs) the trip would be. Since we can’t depend on fuel being available at our destination,
such travel would require carrying all the fuel necessary for both the trip there and the return trip
and require accelerating all that fuel to tremendous speeds—a truly gargantuan effort, and an
extraordinarily expensive one. To be sure, those issues only come up if creatures like us are
along on the trip. Travel by machines (such as robots, computers, or smartphones) could proceed
much more slowly and less expensively. As this book went to press, a billionaire in Silicon
Valley gave $100 million to a project to find technology that could get a very tiny probe to the
nearest star using laser propulsion. See: Project Breakthrough Star-shot:
https://breakthroughinitiatives.org/News/4
7. What are the advantages to using radio waves for communication between civilizations that
live around different stars? List as many as you can.
Answer
Radio waves travel at the speed of light, are cheap to produce (they are the lowest-energy
electromagnetic waves), are not significantly absorbed by interstellar clouds, go right through
planetary atmospheres, and, most importantly, can be modulated in a way that carries
information.
8. What is the “cosmic haystack problem”? List as many of its components as you can think of.
Answer
Because so many factors go into detecting a signal from extraterrestrial intelligence, some
astronomers have compared the effort to searching for a needle in a haystack. Some of the
problem’s components include the origin and direction of the signal containing the message from
among all the possible directions one could “listen,” the frequency chosen for that signal from
among the vast range of potential frequencies in the electromagnetic spectrum, the frequency
width of that signal, the strength of that signal compared with background noise, the continuity
of that signal (whether it’s on all the time, or only sweeps over us periodically), the frequency
drift of that signal (caused by the relative motion of the sources to Earth), the system used for
,OpenStax Astronomy 2e Instructor Answer Guide
encoding any message in that signal, and whether we would recognize the nature of the message,
when it was coded by alien minds.
9. What is a habitable zone?
Answer
A habitable zone is the range of distances from a star where, if water existed on the surface of a
planet, that water would likely be liquid.
10. Why is the simultaneous detection of methane and oxygen in an atmosphere a good
indication of the existence of a biosphere on that planet?
Answer
Oxygen and methane chemically react with each other, so we would not see them together unless
there are active sources for both. At least on Earth, biology is responsible for essentially all the
oxygen and the majority of the methane in our atmosphere.
11. What are two characteristic properties of life that distinguish it from nonliving things?
Answer
Life extracts energy from its environment and has a means of encoding and replicating
information in order to make faithful copies of itself.
12. What are the three requirements that scientists believe an environment needs to supply life
with in order to be considered habitable?
Answer
The requirements are a solvent (water may be the best example), the biogenic elements
(CHNOPS) in biologically accessible form, and energy.
13. Can you name five environmental conditions that, in their extremes, microbial life been
challenged by and has learned to survive on Earth?
Answer
Five environmental conditions that microbial life has overcome are extreme temperature,
pressure, salinity, acidity, and radiation.
Thought Questions
14. Would a human have been possible during the first generation of stars that formed right after
the Big Bang? Why or why not?
Answer
No. Humans are composed primarily of the elements carbon, oxygen, hydrogen, and nitrogen,
and we live on a planet that is made mostly of silicon, iron, and oxygen. Of these elements, only
hydrogen was formed in the Big Bang. The others had to be built up via nuclear fusion in the
cores of hot stars, and subsequently ejected into space, before they were available to form a
second or third generation of stars with planets around them.
15. If we do find life on Mars, what might be some ways to check whether it formed separately
from Earth life, or whether exchanges of material between the two planets meant that the two
forms of life have a common origin?
Answer
We would need a genomic analysis (a study of its genetic structure). By comparing the genome
of the martian life with that of terrestrial life, we could see if the Mars microbes are distant
cousins or truly represent an independent origin of life on that planet.
16. What kind of evidence do you think would convince astronomers that an extraterrestrial
spacecraft has landed on Earth?
Answer
, OpenStax Astronomy 2e Instructor Answer Guide
The best evidence, of course, would be the spacecraft itself and its occupants (if any). If the alien
visitor just landed in a public place and stayed around for a few hours, the identification would
be simple. Short of this, the best evidence would be some artifact or piece of a spacecraft. Even a
tiny fragment of extraterrestrial material would surely reveal its extraterrestrial nature when
subjected to laboratory analyses (just as a lunar or martian meteorite can be identified by
differences in its composition). Another piece of convincing evidence would be a visual or radar
track of the flight path of the lander, showing it coming from beyond Earth and behaving in ways
that are different from ordinary meteorites. However, no evidence of this sort has ever been
produced. Even a fingernail clipping from an alien or some picnic garbage (of alien food) that
they inadvertently left behind could be enough, but no one has been able to come up with such
testable evidence (despite all the sensational UFO claims in popular media).
17. What are some reasons that more advanced civilizations might want to send out messages to
other star systems?
Answer
Since no one on Earth has any idea about the psychology of intelligent aliens, this is a matter of
pure speculation. Most answers assume (for lack of other information) that other intelligent
beings will have motivations similar to ours. Then their reasons could range from simple
curiosity about other life forms to a desire to help backward civilizations like ours; perhaps they
would want to convert us to their form of religion. Some astronomers (and science fiction
writers) have speculated about a “Galactic Library”—a repository of information, culture, and
beliefs from many civilizations. New civilizations may be actively sought so they can become
contributors to (and perhaps users of) this library. On the other hand, given the vast scales of
galactic time, it is possible that various users are separated by billions of years of evolution,
which would make it difficult to compare contributions. Perhaps young civilizations like ours
will be encouraged to get a card for just the children’s library for the first few million years. If
civilizations don’t survive for a long time (on the cosmic time-scale,) they may want to scan the
skies for signals to assure themselves that others have managed to survive the crises that lead
civilizations to self-destruct. If few civilizations survive, those that do may feel lonely for
company. Some negative reasons may include the desire to know about young “rising”
civilizations so they can be eliminated before they might become a threat or so they can be
quarantined so their violent tendencies don’t infect the rest of the Galaxy.
18. What are some answers to the Fermi paradox? Can you think of some that are not discussed
in this chapter?
Answer
Answers to the Fermi paradox discussed in the text (the question of why, if planets like ours and
life out there is common, no one has contacted us) are as follows: intelligence out there is rare, a
network of intelligent civilizations has not yet had time to develop, we are not aware of the
communications from advanced aliens streaming by us, advanced civilizations do not want to
interfere with the development of immature races like us, and advanced civilizations don’t last a
long time after developing technology that can destroy them or their world. Other ideas include
the notion that civilizations might exist and not want to get in touch; that intelligence does not
necessarily lead to technology; or that they may first send probes or machines to “scout” what’s
out there, and these may be much harder to find. A good book on this topic is Stephen Webb’s If
the Universe Is Teeming with Aliens . . . Where Is Everybody? Fifty Solutions to the Fermi
Paradox and the Problem of Extraterrestrial Life (2002). The Wikipedia entry on the Fermi
paradox also has a good list of suggestions.
