A team of astronomers at the California Institute of Technology made the estimate while analyzing planets orbiting a star called Kepler-32, worlds they say are representative of the vast majority in the galaxy and are a perfect case study for understanding how most planets form.
Kepler-32's planets were detected by the Kepler space telescope, and the researchers have analyzed the five-planet system and compared it to other systems found by the Kepler mission, a Caltech release reported Thursday.
Kepler-32 is an M dwarf, a type that accounts for about 75 percent of all stars in the Milky Way, and its planets are typical of the class of worlds the telescope has discovered orbiting other such stars.
It's a system astronomers say will help in understanding planet formation in general.
"I usually try not to call things 'Rosetta stones,' but this is as close to a Rosetta stone as anything I've seen," planetary astronomy Professor John Johnson said. "It's like unlocking a language that we're trying to understand -- the language of planet formation."
Based on their study of Kepler-32, the astronomers have calculated that there is, on average, one planet for every one of the approximately 100 billion stars in the Milky Way.
"There's at least 100 billion planets in the galaxy -- just our galaxy," Johnson said. "That's mind-boggling."