Our galaxy contains 100BILLION planets, new study claims (that means there’s one for every star in the sky)
Contrary to previous belief, the latest research by astronomers suggests star systems with planets are actually the norm across the cosmos.
And the analysis only includes planets in close orbits around certain kinds of stars, meaning the massive estimate they have offered could yet be doubled.
John Johnson, assistant professor of planetary astronomy at the California Institute of Technology (Caltech), called the estimate ‘mind boggling’.
‘There’s at least 100 billion planets in the galaxy — just our galaxy,’ he said.
Jonathan Swift, a postdoc student at Caltech and lead author of the paper due for publication in Astrophysical Journal, added: ‘It’s a staggering number, if you think about it. Basically there’s one of these planets per star.’
Astronomers at Caltech made their estimate by analysing planets orbiting a sun called Kepler-32 which, they say, is representative of the vast majority of stars in our galaxy.
That system, which was detected by Nasa’s planet-hunting Kepler space telescope, contains five planets, the existence of two of which had already been confirmed by other astronomers.
The Caltech team confirmed the remaining three, then analysed the five-planet system and compared it to other systems found by the Kepler mission.
The planets orbit an M dwarf star — also known as red dwarfs, this type accounts for about three-quarters of all stars in the Milky Way.
The five planets, which are similar in size to Earth and orbit close to their star, are also typical of the class of planets that the telescope has discovered orbiting other M dwarfs, Swift says.
Therefore, the researchers concluded, the majority of planets in the galaxy probably have characteristics comparable to those of the five planets.
Other teams of astronomers have already 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.
But 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.
As a result, they say, their estimate is conservative. In fact, says Mr Swift, a more accurate estimate that includes data from other analyses could lead to an average of two planets per star.
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 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 Professor Johnson, which is that our solar system is something of a cosmic ‘weirdo’.
But although the planets in M-dwarf systems orbit close to their stars, it does not necessarily mean that they’re fiery, hellish worlds unsuitable for life, the astronomers say.
Indeed, because M dwarfs are small and cool, their temperate zone — also known as the ‘habitable zone,’ the region where liquid water might exist — is also further inward.
Even though only the outermost of Kepler-32’s five planets lies in its temperate zone, many other M dwarf systems have more planets that sit right in their temperate zones.
The implications of a galaxy chock full of planets are far-reaching, the researchers say.
‘It’s really fundamental from an origins standpoint,’ said Mr Swift, who noted that because M dwarfs shine mainly in infrared light, the stars are invisible to the naked eye.
‘Kepler has enabled us to look up at the sky and know that there are more planets out there than stars we can see.’