Milky Way holds over 100 billion planets

An artist's illustration of the alien solar system Kepler-47, a twin star system that is home to two planets. The planets have two suns like the fictional planet Tatooine in the "Star Wars" universe.

The stars in the night sky stretch out to unimaginable distances. It's hard for human minds to comprehend the vastness of our galaxy, particularly when astronomers throw out phrases like "light-years", "astronomical units," and - of course - "billions." A new study from the California Institute of Technology isn't making it any easier. The most comprehensive survey of the Milky Way to date has concluded that our galaxy holds over 100 billion planets.

"There are at least 100 billion planets in the galaxy, just our galaxy," John Johnson, an astronomy professor at Caltech and co-author of the study, said in a press release. "That's mind-boggling."

The study reached this conclusion after analyzing planets orbiting a star called Kepler-32. The planets observed are representative of the majority in the galaxy and, according to the study, serve as a perfect example of how planets form.

The planetary system around Kepler-32 contains five planets in all - three of which were previously undiscovered. The star itself is an M dwarf, a type of star that accounts for roughly three-quarters of all the stars in our galaxy. The study concludes that the majority of planets in the Milky Way most likely resemble those found around Kepler-32.

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

Scientists analyzed small changes in the star's brightness to determine the characteristics of the five planets, such as their size and orbits. The orientation of the planetary system was such that astronomers could observe each planet crossing the face of the star, a fortunate occurrence which allowed to precise measurements.

The Caltech team calculated the probability that an M-dwarf star system would have similar orientations to Kepler-32 and combined that with the number of planetary systems the team is able to detect. The result is the conclusion that there is roughly one planet for every one of the approximately 100 billion stars in the Milky Way.

But this analysis only accounts for planets that are in close orbit to M-dwarf stars, not planets that may be orbiting more distantly or orbit different types of stars. Thus, the actual number of planets could be far, far higher.

"It's a staggering number, if you think about it," said Jonathan Swift, lead author of the study.