The search for other planets in the Milky Way has exploded in the past 25 years. Scientists have moved from theories of their existence to proof borne out by years of study of shifting starlight.
Even the most cautious scientist now takes for granted the reality of planets outside our solar system. For years, researchers were reluctant to assume that planets revolved around other stars in great numbers. They lacked the proof that the Kepler space telescope provides in monitoring distant star systems.
Two California Institute of Technology scientists have published a study that proposes there are at least 100 billion planets in the Milky Way galaxy, and concludes the actual number is probably closer to 200 billion. The scientists used Kepler-32, a five-planet system 915 light-years from Earth, for their calculations.
Kepler-32 orbits an M dwarf star, the most common star in the Milky Way. Three-quarters of the 100 billion or so stars in our galaxy are such dwarfs, which are smaller and cooler than the sun.
The Kepler-32 planets are similar to Earth in size. They orbit their sun in a region of space where water could exist on a planet without being scorched away. Using Kepler-32 as a model for what happens around other stars gives scientists a better idea about planetary formation.
The scientists are confident there is at least one planet per star in the Milky Way. That number increases greatly when stars in other galaxies are factored in.
In his book Cosmos, Carl Sagan wrote: “A galaxy is composed of gas and dust and stars — billions upon billions of stars.” We can now add billions upon billions of planets to the universe’s census.