Milky Way
Astronomers studying a five-planet system around a red dwarf have calculated that star-orbiting planets are the norm in our galaxy. NASA

The Milky Way is turning out to be a pretty crowded place. A new study from California Institute of Technology astronomers estimates that there are at least 100 billion planets throughout our galaxy.

"It's a staggering number, if you think about it,” Caltech researcher Jonathan Swift said in a statement on Wednesday. "Basically, there's one of these planets per star."

Swift and his colleagues were examining five planets circling a star named Kepler-32 with the Kepler telescope, which detects exoplanets with a light-measuring device -- as planets cross in front of their host star, they dim its light a little bit.

Kepler-32 is an M dwarf star, a classification that encompasses mostly red dwarfs. About three-quarters of the stars in the Milky Way are thought to be M-class stars. The Kepler-32 system was particularly serendipitously positioned, with the orbits right in line with the telescope’s view.

The planets in the Kepler-32 system have radii that range from about .8 to 2.7 times the radius of Earth and are all orbiting very close to their star, much closer than Mercury orbits our own sun. But since M dwarfs are much cooler than our yellow sun, the habitable zone where it’s possible for liquid water to persist extends closer to the star. Just one of the five planets in the system lies in this temperate zone.

Studying the Kepler-32 system will help scientists better understand how planets form. The five planets in this system seem to have been formed from a broad disc of dust and gas with the mass of about three Jupiters. However, previous research suggests that such a massive amount of material couldn’t have existed so close to the star, so Kepler-32’s planets may have been born farther away before moving in closer.

To extrapolate from one star to estimate a planet population for the rest of the galaxy, the Caltech team first figured out the probability of another M-dwarf system having the same edge-on orientation as Kepler-32. Their calculations, soon to be published in the Astrophysical Journal, estimate that there is about one planet for each of the more than 100 billion stars in our galaxy.

Even this figure is a conservative estimate, since the analysis is focused on planets that orbit close to M dwarfs. Even more planets could be orbiting further away from M class stars or around other types of stars in the galaxy.

This latest estimate jives with research announced last March, in which European researchers extrapolated from their observations of 102 red dwarfs to estimate there were tens of billions of planets that could potentially support life. They calculated that around 41 percent of red dwarfs contain at least one "super-Earth," with a mass between one and 10 times that of our planet, in the habitable zone.

The possibility of hundreds of billions of exoplanets in our celestial neighborhood is "really fundamental from an origins standpoint," Swift said. "Kepler has enabled us to look up at the sky and know that there are more planets out there than stars we can see."