A news snippet in the Wall St Journal mentioned a new scientific finding that there are 17 billion planets in our galaxy (the Milky Way) that are similar to the Earth in size (there was no source citation). I had to look that up . . . the number seemed too large to be right.
The source, it turns out, is astrophysicist Dr Francois Fressin of the Harvard-Smithsonian Center for Astrophysics. He used data from NASA’s Kepler spacecraft, which had as its mission to find other habitable planets (as I read about this, I could almost hear William Shatner’s voice saying “to seek out new life and new civilizations”). Dr Fressin concluded that approximately one in every six stars in the Milky Way had an orbiting planet the size of Earth.
Just for emphasis, this number does not encompass all the planets in our galaxy. Rather, it represents only the subset of earth-sized planets. Our galaxy, by the way, is estimated to contain 200-400 billion stars.
Since I’ve briefly digressed with this post to the topic of astrophysics, another interesting fact is that the stars in the galaxy can’t be counted; the number can only be estimated. To do so, scientists first calculate the mass of the Milky Way. This is determined using measurements of orbital motion, which results from gravity, which derives from mass. The number of stars in the galaxy is then calculated by dividing the mass of the galaxy by the estimated mass of the ‘average’ star.
Fascinating stuff. (Maybe E.T. is out there.)