Milky Way is home to at least 100 billion planets

Our Milky Way galaxy is home to at least 100bn alien planets, and possibly many more.

“It’s a staggering number, if you think about it,” lead author Jonathan Swift, of Caltech in Pasadena, California, said. “Basically, there’s one of these planets per star.”

Swift and his colleagues arrived at their estimate after studying a five-planet system called Kepler-32, which lies about 915 light years from earth.

The five worlds were detected by Nasa’s Kepler Space Telescope, which flags the tiny brightness dips caused when exoplanets cross their star’s face.

The Kepler-32 planets orbit an M dwarf, a type of star that is smaller and cooler than our sun.

M dwarfs are the most common star in the Milky Way, accounting for about 75% of the galaxy’s 100bn or so stars.

Further, the five Kepler- 32 worlds are similar in size to earth and orbit quite close to their parent star, making them typical of the planets Kepler has spotted around other M dwarfs.

So the Kepler-32 system should be representative of many of the galaxy’s planets, scientists said.

Kepler can detect planetary systems only if they’re oriented edge-on to the telescope; otherwise, the instrument won’t observe any star-dimming planetary transits.

So the researchers calculated the odds that an M dwarf system in the Milky Way would have this orientation, then combined that with the number of such systems Kepler is able to detect to come up with the estimate of 100bn exoplanets.

The team considered only planets orbiting close to M dwarfs; their analysis did not include outer planets in M dwarf systems, or any worlds circling other types of stars. So the galaxy may in fact harbour many more planets than the conservative estimate implies — perhaps 200bn, or about two per star, Swift said.

The analysis confirms three of the five Kepler-32 planets. The other two had been confirmed previously. The Kepler-32 worlds have diameters ranging from 0.8 to 2.7 times that of Earth, and all of them orbit within 16 million kilometres of their star. For comparison, Earth circles the sun at an average distance of 150 million kilometres.

Because the Kepler-32 star is smaller and less luminous than our sun, the five planets are likely not as heat-blasted as their tight orbits might imply.