By Joel Achenbach, Washington Post, July 25, 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.
As Rachel Feltman reported Thursday, NASA’s Kepler Space Telescope has discovered a bunch of new planets, including one, Kepler 452b, that scientists described as the most Earth-like planet ever found outside our solar system. It’s something like 60 percent bigger in radius than the Earth (the exact size is hard to measure because it’s 1,400 light years away and cannot be directly imaged). But it’s probably rocky, and it’s in the habitable zone of its parent star, which is like our own sun, a G-type “yellow dwarf” star. The parent star is 6 billion years old, roughly (everything’s “roughly,” unfortunately).