Our Milky Way galaxy is home to at least 100 billion alien planets, and possibly many more, a new study suggests.

"It's a staggering number, if you think about it," lead author Jonathan Swift, of Caltech in Pasadena, said in a statement. "Basically there's one of these planets per star."

With all of this in mind, let's turn to a passage in Thomas Gold's influential The Deep, Hot Biosphere:

The surface life on the Earth, based on photosynthesis for its overall energy supply, may be just one strange branch of life, an adaptation specific to a planet that happened to have such favorable circumstances on its surface as would occur only very rarely: a favorable atmosphere, a suitable distance from an illuminating star, a mix of water and rock surface, etc. The deep, chemically supplied life, however, may be very common in the universe. Astronomical considerations make it seem probable that planetary-sized, cold bodies have formed in many locations from the materials of molecular clouds, even in the absence of a central star, and such objects may be widespread and common in our and in other galaxies. It is therefore a possibility that they mostly support this or similar forms of life....

It is interesting to think of life as a kind of behavior that's possible for rocky planets in certain conditions, certain zones, certain times. What is needed is water, lots of matter, and a source of energy (internally or externally), and a planet can add to its other behaviors, the behavior of life. Sometimes (but not often) this life reaches the surface of the planet.
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