Meet Earth's billions of twins
EARTH could have billions of twins strewn across the Milky Way, a study suggests.
Astronomers estimate that at least 17 billion stars in our galaxy harbour an Earth-sized planet. This may be a small proportion of the true figure, since it only includes hot worlds that hug their parent stars closely and are easy to detect.
As more data is gathered, scientists expect to find more rocky Earth-sized planets in wider orbits.
An unknown number could lie within the habitable, or "Goldilocks", zone of their parent star – the orbital path where temperatures are just right to permit surface liquid water and, potentially, life.
Moons orbiting planets in habitable zones may increase the chances of life even further.
Of the newly discovered planets the size of Jupiter or Neptune, 15 fall into this category. While such planets would not themselves be suitable for Earth-like life, they could be circled by moons that are – like the fictitious moon Pandora in the film ' Avatar'.
The new analysis is based on data from NASA's Kepler space telescope.
Scientists presented the findings at the annual meeting of the American Astronomical Society in Long Beach, California. They are also due to appear in the 'Astrophysical Journal'.
Kepler detects candidate planets by measuring the dimming of light that occurs when they pass in front of stars. Follow-up work with ground-based telescopes is then carried out to rule out false readings. A survey of 16 months of data from Kepler has identified around 2,400 candidates.
Extrapolating the results, scientists calculated that around 17pc of stars in the Milky Way have a planet 0.8 to 1.25 times the size of Earth in a close orbit lasting 85 days or less. Like the planet Mercury, they are likely to be too hot to support life.