Our galaxy contains 100 billion planets

There are at least 100 billion planets in the Milky Way alone, say Caltech scientists, based on extrapolations from the Kepler-32 system.

Kepler-32 contains five planets, two of which had already been confirmed by other astronomers. The Caltech team confirmed the remaining three, then analyzed the system and compared it to others found by the Kepler mission.

Like about three-quarters of the stars in the Milky Way, Kepler-32’s star is an M dwarf: and its planets are typical of those found orbiting other similar stars, says the team, implying that most planets in the galaxy are probably broadly similar.

But it has a big advantage for astronomers: its orientation is such that Kepler views the system edge-on, allowing them to study changes in the star’s brightness to determine the planets’ sizes and orbital periods.

“I usually try not to call things ‘Rosetta stones,’ but this is as close to a Rosetta stone as anything I’ve seen,” says John Johnson, assistant professor of planetary astronomy at Caltech. “It’s like unlocking a language that we’re trying to understand – the language of planet formation.”

And the study of Kepler-32 has supported the theory that there are roughly 100 billion planets in our galaxy, or one per star.

To make the calculation, the team determined the probability that an M-dwarf system would provide Kepler-32’s edge-on orientation, and then combined that probability with the number of planetary systems Kepler can detect.

However, their analysis only considers planets that are in close orbits around M dwarfs – not the outer planets of an M-dwarf system, or those orbiting other kinds of stars – meaning there could be twice as many planets as they calculate.

“Kepler has enabled us to look up at the sky and know that there are more planets out there than stars we can see.” says Caltech postdoc Jonathan Swift.