Our galaxy contains at least 100 billion planets - approximately one for every star - and many of them could harbour life, a new study claims.

Contrary to previous belief, the latest research by astronomers suggests star systems with planets are actually the norm across the cosmos.

Astronomers at the California Institute of Technology made their estimate while analysing planets orbiting a star called Kepler-32  - planets that are representative of the vast majority of planets in our galaxy, NASA said.

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

"It's a staggering number, if you think about it. Basically, there's one of these planets per star," added Jonathan Swift, lead author of the study.

One of the fundamental questions regarding the origin of planets is how many of them there are. Like the Caltech group, other teams of astronomers have estimated that there is roughly one planet per star, but this is the first time researchers have made such an estimate by studying M-dwarf systems, the most numerous population of planets known.

The planetary system in question, which was detected by NASA's Kepler space telescope, contains five planets. Two of the planets orbiting Kepler-32 had previously been discovered by other astronomers.

The Caltech team confirmed the remaining three, then analysed the five-planet system and compared it to other systems found by Kepler.

M-dwarf systems like Kepler-32's are quite different from our own solar system. For one, M dwarfs are cooler and much smaller than the Sun. Kepler-32, for example, has half the mass of the sun and half its radius.

The radii of its five planets range from 0.8 to 2.7 times that of Earth, and those planets orbit extremely close to their star.

The whole Kepler-32 system fits within just over a tenth of an astronomical unit (the average distance between Earth and the Sun) - a distance that is about a third of the radius of Mercury's orbit around the Sun.

The fact that M-dwarf systems vastly outnumber other kinds of systems carries a profound implication, according to Johnson, which is that our solar system is extremely rare.