100 Billion Planets Populate the Milky Way Galaxy, Say Astronomers


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For years now, NASA's Kepler mission has been confirming the existence of planets outside our solar system. Now, a new review of Kepler data suggests that there are billions upon billions of planets just in the Milky Way galaxy.

"There are at least 100 billion planets in the galaxy, just our galaxy," said John Johnson, assistant professor of planetary astronomy at Caltech and coauthor of the new study. "That's mind-boggling."

The new study, set to be published in The Astrophysical Journal, looked at the planets orbiting a star named Kepler-32, then compared the system to others discovered by the Kepler space telescope. Astronomers stated that the Kepler-32 planets are representative of a majority of planets in the Milky Way, and serve as a case study for how planets form. Systems similar to Kepler-32 comprise around three-quarters of all the stars in our galaxy, leading researchers to their 100 billion-planet estimate.

"I usually try not to call things 'Rosetta stones,' but this is as close to a Rosetta stone as anything I've seen," said Johnson. "It's like unlocking a language that we're trying to understand—the language of planet formation."

The prevalence of Kepler-32-type stars, however, suggests that our own solar system may be quite rare. "It's just a weirdo," said Johnson.

Kepler-32 is an M dwarf star that is much cooler than our sun, with around half its mass and radius. The five planets orbiting Kepler-32 also orbit much closer to the star than the planets in our solar system. All of the Kepler-32 planets orbit their star within one-tenth of the distance from the Earth to the sun, or just one-third the distance from Mercury to the sun.

That doesn't mean Kepler-32's planets are inhospitable, though. The star's small size also means its habitable zone, where liquid water can exist, is smaller, and the outermost Kepler-32 planet lies within that zone.

(Image courtesy NASA/JPL-Caltech)