(Credit: Serge Brunier)
For every star in the Milky Way galaxy, there is at least one exoplanet, too, according to astronomers.
So far, only 854 exoplanets — that is, planets outside of our solar system — have been catalogued, but we know that there's a lot more out there. In fact, early last year, the European Southern Observatory announced, after it had spent years on a survey, that there are likely tens of billions of planets out there.
Now it appears that the number is conservative. Very conservative.
Astronomers at the California Institute of Technology (Caltech) have been studying a star called Kepler-32. There is little remarkable about Kepler-32, a small M-dwarf star with five average planets in orbit. It is this very lack of remarkableness — this very averageness — that has led the Caltech researchers to their conclusion: that there are at least 100-400 billion planets in our galaxy.
Kepler-32, according to the researchers, is representative of around 70 per cent of the stars in the Milky Way.
"It's a staggering number, if you think about it," said Jonathan Swift, postdoc at Caltech and lead author of the paper. "Basically, there's one of these planets per star."
However, this doesn't mean that these systems are anything like our own: M-dwarfs are smaller and cooler than the sun, with the planets clustered closer to it. Our own solar system is something of a rarity in the Milky Way. In turn, though, that doesn't necessarily mean that there's zero chance for life, either; M-dwarfs still possess habitable zones; they're just a fair bit closer to the star.
The study, "Characterizing the Cool KOIs IV: Kepler-32 as a prototype for the formation of compact planetary systems throughout the Galaxy", which was accepted for publication in Astrophysical Journal, can be found online at the Cornell University Library website here (PDF); and you can read more about NASA's Kepler mission to identify and catalogue exoplanets here.