Chapter 27

Describe why, in order to be considered living, an object must contain DNA or another substance that has the same capabilities.

What properties of water are crucial to life on Earth? Discuss the advantages and disadvantages of ammonia and methyl alcohol versus water as liquids on which life might be based.

The oceans of the early Earth probably contained high concentrations of organic molecules. Where did those molecules come from?

Why have attempts to find planets beyond the solar system concentrated on giant planets like Jupiter rather than terrestrial planets?

Why does the habitable zone of a star change its location as time passes?

Why does it follow that if our attempts at interstellar communication are likely to succeed, we are very likely to be the junior partners in the conversation?

Suppose someone suggested that the "logical" frequencies at which to search for interstellar signals are precisely 1 or \(10 \mathrm{GHz}\). Present an argument that there is nothing special or logical about those frequencies.

If we discover life on other planets, how is human civilization likely to be affected? (There is no "correct" answer to this question, but it is an interesting subject for speculation.)