Nasa discovered earth-sized planets in 'the Goldilocks zone'

Astronomers have found a pair of Earth-sized planets orbiting a star similar to the Sun, though neither are believed to be suitable for life, scientists on Nasa's Kepler telescope team have said.

The discovery follows confirmation earlier this month of a super-Earth sized planet, called Kepler-22b, that circles the right distance from its parent star for liquid water to exist on its surface. Water is believed to be a key ingredient for life.

"Kepler-22b has the right temperature, but it is too big. (The planets) we're announcing today are just the right size, but too hot," astronomer David Charbonneau with Harvard University, told reporters during a conference call.

"But you can bet that the hunt is on to find a planet that combines the best of both worlds, a true Earth twin," he said.

The newly discovered planets, called Kepler-20e and 20f, have at least three gas-giant siblings, one of the larger planetary systems found to date.

But the family is nothing like our solar system, where rocky worlds like Venus, Earth and Mars are grouped together closer in toward the sun while gas giants like Jupiter and Saturn are segregated in the outer regions.