Thanks to the Kepler space telescope, a myriad of planets orbiting other stars have either been discovered or indicated by initial data. A study just concluded by Cal Tech suggests that there are hundreds of billions of planets in the Milky Way Galaxy.
262 potentially habitable worlds discovered
According to a story in Space Daily, the telescope may have discovered 262 habitable worlds during its operational life. Further study will be needed to confirm the existence of these planets. But thus far scientists are looking at four Mars-size, 23 Earth-size, and 235 super Earth-size planets that may be capable of sustaining life orbiting other stars. Using what scientists call an "Earth Similarity Index" (ESI), defined by such factors as surface temperature, mean radius, bulk density and escape velocity, 24 of these potential planets have an ESI of 90 percent or more. One planet, an Earth-size planet in a 231-day orbit around the star KIC-6210395, is considered the best candidate for another Earth.
100 billion worlds in our galaxy
In the meantime, according to NASA's JPL, a recently released study by the California Institute of Technology suggests that there are at least 100 billion planets in the Milky Way Galaxy alone. The researchers at Cal Tech arrived at this conclusion by extrapolating data the Kepler space telescope received from a star called Kepler-32. Kepler-32 is a M-dwarf type star, smaller than the Sun, with five planets detected, ranging from .8 to 2.7 times the size of Earth, all of them orbiting the star at a distance of one tenth the distance Earth orbits the Sun. M-dwarfs are among the most numerous types of stars in the galaxy. The Cal Tech researchers also suggest that solar systems such as exist around our own sun are exceedingly rare.
2013 -- The year we discover another Earth
A recent story in Slate suggests that 2013 may be the year that scientists will be able to definitely state that another Earth has been discovered orbiting another star. Such a planet, with a high ESI, would be an Earth-sized, wet, rocky world that it orbiting its star within its habitable zone. That zone, also called the "Goldilocks zone," is where conditions for liquid water and a tolerable temperature are "just right" to sustain life. Such a discovery, long-anticipated in both science and science fiction, will have profound implications about how human beings regard their place in the universe.
Mark R. Whittington is the author of Children of Apollo and The Last Moonwalker. He has written on space subjects for a variety of periodicals, including The Houston Chronicle, The Washington Post, USA Today, the L.A. Times, and The Weekly Standard.