21/12/2011 04:53 GMT | Updated 21/12/2011 05:44 GMT

Kepler 20f: Nasa Discovers Two Earth-Sized Planets Orbiting Star

Scientists have discovered two earth-sized rocky planets orbiting a distant star.

Kepler 20f, one of the exoplanets (planets outside the solar system) could have similar atmospheric conditions to our own planet, although researchers believe it is too hot to harbour life, boasting a temperature of more than 400C.

Its neighbour, Kepler 20e, has a temperature of 720C, however in the distant past researchers believe the temperature of the planets may have been cool enough for water to sit on the surface, which could have supported biology.

The star, Kepler 20, is 950 light years away and sits in the Lyra constellation. It is a five-planet system.

Detailed in the journal Nature, the Earth-twin and its neighbour are being heralded as the most important planets ever discovered outside our own system, with Dr Francois Fressin, of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, describing the find to the BBC as the start of a “new era”.

Kepler 20f is the same size as the earth, while its sibling is a tenth smaller. Both are believed to have started further away from the star, but have moved inwards over time.

The planets orbit Kepler 20 much quicker than the earth does the sun, with 20f taking 20 days and 20e taking just six days. The system is so named as is was discovered using the Kepler Space Telescope, which is currently scanning round 150,000 stars looking for planets in habitable zones.

An artists impression of the two planets orbiting the Kepler 20 star (AFP/Getty images).

Despite both planets uninhabitable conditions, scientists believe the discovery is of huge importance as it proves that planets similar to earth exist outside the solar system.

“It is the first time humanity has been able to discover an object similar to the Earth around a star, so maybe we will be able to find others,” said Fressin.

“This could be an important milestone. I think 10 years or maybe even 100 years from now people will look back and ask when was the first Earth-sized planet found. It is very exciting.”

Earlier in December, scientists unveiled the discovery of Kepler 22b, a planet twice the size of earth that sat in the habitable or “Goldilocks” zone, orbiting a different star. Again, scientists believe the planet does not hold life.

The Kepler telescope searches for planets by studying transition patterns, where planets pass in front of star on their orbit.

According to the researchers, the Kepler system differs from our on in that it features five planets that alternate in size. In our own solar system, the larger planets are found further away from the star.

"It means that the formation process that happened for this one could be different than with our solar system," said Jason Rowe, a research scientist at Nasa Ames Research Center. "It's a learning process. Now we're realising that there's a larger variety of solar systems out there."