Planetary boom: Scientists now say Earth has countless cousins
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The search for other planets in the Milky Way has exploded in the last 25 years. Scientists have moved from mere theories about their existence to proof borne out by years of study of shifting starlight.
Now even the most cautious scientist takes the reality of planets outside our solar system for granted. For years, many researchers were reluctant to assume that planets revolved around other stars in great numbers. What was lacking was the kind of proof that the Kepler Space Telescope now provides when it comes to monitoring distant star systems.
Scientists Jonathan Swift and John Johnson, both of the California Institute of Technology, have published a study that proposes there are at least 100 billion planets in the Milky Way galaxy and that the actual number is probably closer to 200 billion.
The Caltech scientists used Kepler-32, a five-planet system 915 light-years from Earth as a representative template for their calculations. Kepler-32 orbits an M dwarf star -- the most common kind of star in the Milky Way. Three-quarters of the estimated 100 billion stars in our galaxy are M dwarf stars, which are smaller and cooler than the sun.
The Kepler-32 planets are similar to Earth in size and orbit their sun within the "habitable zone," a region of space where liquid water could exist on a planet without being scorched away. Using Kepler-32 as a model for what happens around other stars has given scientists a better idea about planetary formation and how common it is.
Mr. Swift and Mr. Johnson are confident that there is at least one planet per star just in the Milky Way. That number increases exponentially when the stars in other galaxies are factored in, too.
In his book "Cosmos," the late, great 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.
First Published January 7, 2013 12:00 am