Planets must have liquid water, not ice or steam {circumstellar habitable zone} (CHZ).
Galaxy centers have too much radiation. Galaxy edges have low metals and low star-formation rates. Stars must be in galactic arms {galactic habitable zone} (GHZ).
Hot water rich in sulfur, iron, hydrogen, and carbon flows from sea floor holes {hydrothermal vent}|. Bacteria-like organisms began there.
If surface had iron 3,800,000,000 years ago, reduction reactions caused methane, ammonia, and hydrogen sulfide {reducing atmosphere}.
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Date Modified: 2022.0225