One in six stars has Earth-sized planet

A new analysis of Kepler spacecraft data has revealed that about 17% of stars have an Earth-sized planet in an orbit closer than Mercury. This means (since the Milky Way has about 100 billion stars), that there are at least 17 billion Earth-sized worlds out there. To detect planetary candidates, Kepler uses the transit method. This involves watching for a planet to cross its star and create a mini-eclipse that will the dim the star slightly. In the first 16 months of the survey, 2400 candidates were identified, but astronomers were concerned that Kepler may have misinterpreted some signals and missed some planets.

Francois Fressin, of the Harvard-Smithsonian Centre for Astrophysics (CfA) and his colleagues simulated the Kepler survey and were able to correct both the impurity and incompleteness of the candidate list to recover the true occurrence of Earth-sized planets orbiting other stars. Their findings will be published in The Astrophysical Journal.

“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,” said Fressin.

All-in-all, the researchers discovered that 50 per cent of stars have a planet the size of Earth or larger in nearby orbit. By adding larger planets (which have been detected in wider orbits up to the orbital distance of the Earth, this increases to 70 per cent. Extrapolating from Kepler’s currently ongoing observations and results from other detection techniques, it appears that practically all Sun-like stars have planets.

When 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-quarter of stars have a super-Earth (1.25-2 times the size of Earth) in an orbit of 150 days or less.

The team also wanted to know whether certain sizes of planet are more common around certain types of stars. They discovered that for every planet except gas giants, the type of star doesn’t matter, which contradicts previous findings.

“Earths and super-Earths aren’t picky. We’re finding them in all kind of neighbourhoods,” said co-author Guillermo Torres of the CfA.