Review

#01

Our only examples are those on Earth.

#08

Language allows for the exchange of ideas, and ideas guide our behavior (in part).

#09

Interstellar space.

#10

Europa and Titan.

#14

Stability of the star and planet. Whether the planet's orbit is in the habitable zone.

#15

Longer lifetimes imply more existing civilizations today and additionally that they are on average closer together and so easier to find.

#18

The galaxy is fairly transparent to radio waves, and background ``noise'' of astrophysical sources is fairly small.