An article titled “One or more bound planets per Milky Way star from microlensing observations” was published January 11, 2012 in Nature1 . Previously, planets have been detected using radial velocity methods but only those planets in orbits closer to their star could be detected. Microlensing allows detection of planets at greater distances. Their conclusion is that that there is likely at least one planet for every star. Looking at some basic numbers leads to some profound speculation.
By the numbers
Since our galaxy, the Milky Way, has approximately 100 million stars it would follow that there are at least 100 million planets. The Milky Way resides in a group of galaxies aptly named the Local Group containing 700 billion stars. 2 In turn, the Local group is contained within the Virgo Supercluster that has 200 trillion stars. There are 100 superclusters within a billion light years with a star count of 250,000 trillion or 250 quadrillion. Finally, our visible universe (14 billion light years) contains 30 billion trillion (3×1022) stars. In light of the recent article in nature that would mean there are also at least 30,000,000,000,000,000,000,000 planets.
Perspective
In a previous post I suggested it was possible to calculate there are more stars in the universe than grains of sand on Earth. A corollary to that statement can now be there are more planets in the universe than grains of sand in the world – our world, that is.
Habitable planets
But are these plants, these other worlds, really like earth? Another recent study suggests they may be more similar than we once thought. In September 2011 Wesley Traub 3,4 analyzed data from Kepler and concluded that “about one-third of FGK stars are predicted to have at least one terrestrial, habitable-zone planet.” Habitable-zone is defined as the region around a star where water could exist on the surface of a planet.
Of the 30 billion trillion planets approximately 13.5% are classified as FGK 6 meaning that within our visible universe there are 4 billion trillion planets in the habitable zone.
That’s 40,000,000,000,000,000,000,000 habitable planets!
Say “Hello”
1 http://www.nature.com/nature/journal/v481/n7380/abs/nature10684.html
2 http://www.atlasoftheuniverse.com/universe.html
3 http://arxiv.org/abs/1109.4682 (Abstract)
4 http://arxiv.org/pdf/1109.4682.pdf (Full text)
6 http://www.atlasoftheuniverse.com/startype.html
Additional reading:
http://www.nature.com/nature/journal/v481/n7380/extref/nature10684-s1.pdf
http://hubblesite.org/newscenter/archive/releases/2012/07/full/
Confronting Mediocrity 
