Earth could have billions of twins

Earth could have many 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 could increase the chances of life even further.

Fifteen newly discovered candidate planets the size of Jupiter or Neptune 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 the American space agency 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 minute dimming of light that occurs when they pass in front of their stars. Follow-up work using ground-based telescopes is then carried out to rule out false readings.

A survey of 16 months of data from Kepler has identified around 2400 candidates.


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Extrapolating the results, scientists calculated that around 17 per cent 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.

Larger planets are easier to detect at greater distances using Kepler's "transit" method.

The analysis suggests around a quarter of stars have a "super-Earth" up to twice the size of Earth in orbits of up to 150 days.

Around five per cent of stars are predicted to have Jupiter-like gas giants in orbits of 400 days or less.

In comparison, the Earth takes 365 days to orbit the Sun, the length of a year.

Planets were found to be orbiting all kinds of stars, not just those similar to the Sun. They included cool red dwarfs with habitable zones closer in than the Sun's.

Dr Guillermo Torres, one of the scientists from the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, said: "Earths and super-Earths aren't picky. We're finding them in all kinds of neighbourhoods."

Yesterday astronomers working on Kepler data announced the discovery of 15 new Neptune and Jupiter-size candidate planets in the habitable zones of their stars.

One, known as PH2 b, was confirmed as a genuine exoplanet with 99.9 per cent certainty.

The discovery, made with the help of volunteers taking part in the Planet Hunters project processing data on their home computers, opens up the possibility of life-supporting moons.

Planet Hunters astronomer Dr Chris Lintott, from Oxford University, said: "Jupiter has several large water-rich moons. Imagine dragging that system into the comfortably warm region where the Earth is.

"If such a planet had Earth-size moons, we'd see not Europa and Callisto but worlds with rivers, lakes and all sorts of habitats: a surprising scenario that might just be common."