– NASA estimates 1 billion ‘Earths’ in our galaxy alone (Washington Post, July 24, 2015):
There are a billion Earths in this galaxy, roughly speaking. Not a million. A billion. We’re talking 1 billion rocky planets that are approximately the size of the Earth and are orbiting familiar-looking yellow-sunshine stars in the orbital “habitable zone” where water could be liquid at the surface.
That’s a billion planets where human beings, or their genetically modified descendants, as well as their dogs and cats and tomato plants and crepe myrtle trees and ladybugs and earthworms and whatnot, could plausibly live.
The estimate comes from NASA scientist Natalie Batalha. Let’s go through some background information to see how she got to that number.