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Would a human have been possible during the first generation of stars that formed right after the Big Bang? Why or why not?
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?
What kind of evidence do you think would convince astronomers that an extraterrestrial spacecraft has landed on Earth?
What are some reasons that more advanced civilizations might want to send out messages to other star systems?
What are some answers to the Fermi paradox? Can you think of some that are not discussed in this chapter?
Why is there so little evidence of Earth’s earliest history and therefore the period when life first began on our planet?
Why was the development of photosynthesis a major milestone in the evolution of life?
Does all life on Earth require sunshine?
Why is life unlikely to be found on the surface of Mars today?
In this chapter, we identify these characteristic properties of life: life extracts energy from its environment, and has a means of encoding and replicating information in order to make faithful copies of itself. Does this definition fully capture what we think of as “life”? How might our definition be biased by our terrestrial environment?
Given that no sunlight can penetrate Europa’s ice shell, what would be the type of energy that could make some form of europan life possible?
Why is Saturn’s moon Enceladus such an exciting place to send a mission?
In addition to an atmosphere dominated by nitrogen, how else is Saturn’s moon Titan similar to Earth?
How can a planet’s atmosphere affect the width of the habitable zone in its planetary system?
Why are we limited to finding life on planets orbiting other stars to situations where the biosphere has created planet-scale changes?
Suppose astronomers discover a radio message from a civilization whose planet orbits a star 35 light-years away. Their message encourages us to send a radio answer, which we decide to do. Suppose our governing bodies take 2 years to decide whether and how to answer. When our answer arrives there, their governing bodies also take two of our years to frame an answer to us. How long after we get their first message can we hope to get their reply to ours? (A question for further thinking: Once communication gets going, should we continue to wait for a reply before we send the next message?)
The light a planet receives from the Sun (per square meter of planet surface) decreases with the square of the distance from the Sun. So a planet that is twice as far from the Sun as Earth receives (1/2) 2 = 0.25 times (25%) as much light and a planet that is three times as far from the Sun receives (1/3) 2 = 0.11 times (11%) as much light. How much light is received by the moons of Jupiter and Saturn (compared to Earth), worlds which orbit 5.2 and 9.5 times farther from the Sun than Earth?
Think of our Milky Way Galaxy as a flat disk of diameter 100,000 light-years. Suppose we are one of 1000 civilizations, randomly distributed through the disk, interested in communicating via radio waves. How far away would the nearest such civilization be from us (on average)?
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