Why Kepler's discovery of two earth-sized planets is a 'shocker'

NASA'S Kepler space telescope has found its first earth-sized planets orbiting another star similar to our sun - but it might be a little early to send the starships.

Where are Kepler-20e and Kepler-20f?

The newly discovered exoplanets are in a star system called Kepler-20, approximately 1,000 light-years away from earth in the constellation Lyra.

Are Kepler-20e and Kepler-20f habitable?

The planets might be roughly earth-sized, but they are far too close to their sun to be considered habitable. Kepler-20e (above) orbits its sun every 6.1 earth days and has a surface temperature of over 750C - enough to melt glass. A year on, Kepler-20f lasts just 19.6 days and experiences temperatures of over 400C.

Despite this, there is a small possibility of life on these worlds. It is thought that neither of the planets spin on their axis, as the earth does, so there are areas on their surfaces that experience a perpetual twilight.

Francois Fressin, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and lead author of the paper (published in Nature) that describes the discovery of the planets, says these areas might be more suitable for life, "but I don't want to sell them as habitable".

So why are astronomers excited?

While Kepler-20e and Kepler-20f do not orbit in the 'habitable zone', their discovery suggests it is only a matter of time before scientists discover an earth-sized planet that is hospitable to life.

What is interesting about the Kepler-20 system is the arrangement of the planets. As Jonathan Fortney, an astronomer at the University of California, Santa Cruz, tells Nature, Kepler-20e and Kepler-20f are "quite strange", because they are sandwiched between three larger planets that range in size between 'super-earths' and 'mini-Neptunes'.

This, apparently, is surprising because in our own solar system there is a distinct separation between larger, gaseous planets far from the sun and smaller, rocky ones closer in.

David Charbonneau, a co-author of the paper, says different-sized planets "do not mingle in the solar system, but apparently in this system they do... These sorts of shockers have become the norm for exoplanets." ·