Astronomers have, since they could see stars a night, tried to figure out how many planets were in the sky - and the latest count from scientists is massive - 100 billion just in our own Milky Way galaxy.
A new study by astronomers at the California Institute of Technology (Caltech) made their estimate while analyzing five planets orbiting a star spotted by NASA's Kepler space telescope and called Kepler-32 -- planets that are illustrative of the majority in the galaxy and serve as a perfect case study for understanding how most planets form, the astronomers said.
"It's a staggering number, if you think about it," adds Jonathan Swift, a postdoc at Caltech and lead author of the paper on the planet count accepted by the Astrophysical Journal. "Basically there's one of these planets per star."
The planets orbit a star known as an M dwarf -- a type of system typically cooler and smaller than our own but which accounts for about three-quarters of all stars in the Milky Way, the Caltech astronomers state. The five planets, which are similar in size to Earth and orbit close to their star, are also typical of the class of planets orbiting other M dwarfs, as seen by NASA's Kepler.
In a release, the astronomers said one of the basic 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.
"To do that calculation, the Caltech team determined the probability that an M-dwarf system would provide Kepler-32's orientation. Combining that probability with the number of planetary systems Kepler is able to detect, the astronomers calculated that there is, on average, one planet for every one of the approximately 100 billion stars in the galaxy. 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 a more accurate estimate that includes data from other analyzes could lead to an average of two planets per star, according to Swift.
The implications of a galaxy chock full of planets are far-reaching, the researchers say. "It's really fundamental from an origins standpoint," says Swift, who notes 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."
NASA last year came up with some similar large numbers. "The planets, their orbits and their host stars are all vastly magnified compared to their real separations. A six-year search that surveyed millions of stars using the microlensing technique concluded that planets around stars are the rule rather than the exception. The average number of planets per star is greater than one. This means that there is likely to be a minimum of 1,500 planets within just 50 light-years of Earth," NASA said.
Microlensing takes advantage of the random motions of stars, which are generally too small to be noticed. If one star passes precisely in front of another star, the gravity of the foreground star bends the light from the background star, according to NASA. This means that the foreground star acts like a giant lens amplifying the light from the background star. A planetary companion around the foreground star can produce additional brightening of the background star. This additional brightening reveals the planet, which is otherwise too faint to be seen by telescopes. Using the microlensing technique, astronomers can determine a planet's mass. This method, however, does not reveal any clues about the world's composition, astronomers say.
The number comes from observations taken over six years by Probing Lensing Anomalies NETwork or PLANET study that found there are far more Earth-sized planets than bloated Jupiter-sized worlds. This notion is based on calibrating a planetary mass function that shows the number of planets increases for lower mass worlds. A rough estimate from this survey would point to the existence of more than 10 billion terrestrial planets across our galaxy, NASA said.
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