More than 100 Earth-sized planets found

Nasa's Kepler telescope has discovered more than 100 Earth-sized planets orbiting alien stars.

It has also detected nine small planets within the so-called habitable zone, where conditions are favourable for liquid water - and potentially life.

The finds are contained within a catalogue of 1,284 new planets detected by Kepler - which more than doubles the previous tally.

Nasa said it was the biggest single announcement of new exoplanets.

Space agency scientists discussed the new findings in a teleconference on Tuesday.

Statistical analyses of the expanding sample of worlds helps astronomers understand how common planets like our own might be.

Dr Natalie Batalha said the calculations so far suggested there could be tens of billions of potentially habitable planets in the Milky Way.

"If you ask yourself where is the next habitable planet likely to be, it's within about 11 light-years, which is very close," said Dr Batalha.

Astronomers said they consider planets that are 1.6 times Earth's radius or smaller are likely to be rocky, and may therefore be potential targets in the hunt for life.

Future telescopes such as the James Webb Space Telescope could analyse the light from the atmospheres of exoplanets for potential markers of biology.

Lead author Timothy Morton said the overwhelming majority of exoplanets found by Kepler fell into the super-Earth (1.2-1.9 times bigger than the radius of Earth) and sub-Neptune sized (1.9-3.1 times bigger than Earth's radius).

He noted that planets in this size range had no known analogues in our Solar System.

Kepler employed the transit method to detect planets orbiting other stars. This involves measuring the slight dimming of a star's light when an orbiting planet passes between it and the Earth.

The same orbital phenomenon was involved when Mercury passed across the face of the Sun on Monday 9 May.