17 billion planets 'like Earth'

The Milky Way galaxy is home to at least 17 billion planets similar in size to Earth, a new estimate suggests.

Not all are potentially habitable, but the huge number is a starting point in the search for worlds like our own.

Scientists have yet to find a twin Earth - one that is not only the right size but also located in a zone that is not too hot and not too cold where water might exist in liquid form.

Two independent groups came up with the new estimate after a fresh analysis of data gathered by Nasa's Kepler spacecraft, launched in 2009 to track down other Earths. The craft spots a planet when it passes in front of the planet's star.

One team led by Francois Fressin of the Harvard-Smithsonian Centre for Astrophysics estimated at least one in six stars has an Earth-size planet orbiting it.

Using a different method, researchers at the University of California, Berkeley and University of Hawaii determined that 17% of stars host planets that are one to two times the diameter of Earth.

The findings were presented at the American Astronomical Society.