Washington: Astronomers using NASA’s Kepler spacecraft have discovered that one in six stars hosts an Earth-sized planet in a close orbit, suggesting there are at least 17 billion such planets in our galaxy.
A new analysis of Kepler data showed that about 17 per cent of stars have an Earth-sized planet in an orbit closer than Mercury.
Since the Milky Way has about 100 billion stars, there are at least 17 billion Earth-sized worlds out there.
Kepler detects planetary candidates using the transit method, watching for a planet to cross its star and create a mini-eclipse that dims the star slightly.
The first 16 months of the survey identified about 2,400 candidates.
By simulating the Kepler survey, Francois Fressin of the Harvard-Smithsonian Center for Astrophysics and his colleagues were able to correct both the impurity and the incompleteness of this list of candidates to recover the true occurrence of planets orbiting other stars, down to the size of Earth.
“There is a list of astrophysical configurations that can mimic planet signals, but altogether, they can only account for one-tenth of the huge number of Kepler candidates. All the other signals are bona-fide planets,” Fressin said in a statement.
Altogether, the researchers found that 50 per cent of stars have a planet of Earth-size or larger in a close orbit.
By adding larger planets, which have been detected in wider orbits up to the orbital distance of the Earth, this number reaches 70 per cent.
Extrapolating from Kepler’s currently ongoing observations and results from other detection techniques, it looks like practically all Sun-like stars have planets.
The team then grouped planets into five different sizes.
They found that 17 per cent of stars have a planet 0.8 – 1.25 times the size of Earth in an orbit of 85 days or less.
About one-fourth of stars have a super-Earth (1.25 – 2 times the size of Earth) in an orbit of 150 days or less.
(Larger planets can be detected at greater distances more easily.) The same fraction of stars has a mini-Neptune (2 – 4 times Earth) in orbits up to 250 days long.
Larger planets are much less common. Only about 3 per cent of stars have a large Neptune (4 – 6 times Earth), and only 5 per cent of stars have a gas giant (6 – 22 times Earth) in an orbit of 400 days or less.
The researchers also asked whether certain sizes of planets are more or less common around certain types of stars. They found that for every planet size except gas giants, the type of star does not matter.
Neptunes are found just as frequently around red dwarfs as they are around sun-like stars. The same is true for smaller worlds. This contradicts previous findings.
The study was presented at a meeting of the American Astronomical Society in Long Beach, California and will be published in The Astrophysical Journal.