NASA’s Kepler space probe has discovered the most Earth-size planets yet detected orbiting another star. The two new worlds are rocky and, for the first time, remarkably similar in size to our own Earth – in fact one is slightly smaller.

They are revolving along with three bigger planets around a Sun-like star labelled Kepler-20. What’s more, the outer planet of the two, dubbed Kepler-20f, may have a thick atmosphere of water vapour, the observations reported in Nature this week suggest.

Previous discoveries thought to be rocky have been dubbed super-Earths because they were considerably larger than our home planet. One planet announced earlier this month, orbiting a different star, was around 1.42 times the Earth’s diameter but it excited astronomers because it lies in the so-called habitable zone where water, vital for life, can exist as a liquid.

The latest planets around Kepler-20 were spotted due to tiny dips in the brightness of starlight as they passed in front of their home star. Accurate measurements of these regular eclipses allowed Dr François Fressin, of the Harvard-Smithsonian Center for Astrophysics, Massachusetts, and his international team of astronomers to calcuate their sizes. Kepler-20f was found to have a radius almost identical to that of the Earth, while its neighbour, Kepler-20e, is 0.87 times our radius, making it slightly smaller than Venus.

Dr Fressin’s team believe the new planets are made of similar material to the Earth, with iron cores around 32 per cent of their diameter covered by mantles of silicate. They say further observations over the next few years should allow them to tell their make-up more precisely. However, the two worlds have “years” much shorter than our own with Kepler-20f orbiting its star in 19.6 days and Kepler-20e zipping round it in only 6.1 days. In fact all five of Kepler-20’s planets lie close to the star, orbiting within the distance of our own planet Mercury from our Sun.

Dr Fressin said: “The goal of Kepler is to find Earth-sized planets in the habitable zone. Proving the existence of Earth-sized exoplanets is a major step toward achieving that goal. These new planets are significantly smaller than any planet found up till now orbiting a Sun-like star.”

More than 700 planets have now been found outside our own Solar System since the first was spotted in 1996. The vast majority have been giant gasballs like Jupiter because their size made them easier to find. But Keper has found many smaller worlds since it was launched on its planet-hunting mission in March 2009. The probe, a space telescope, constantly watches 100,000 stars in the direction of the constellations Cygnus and Lyra to watch for blinks that give away the presence of planets.

Kepler’s first discovery of a rocky world was announced early this year. Steady advances in observational techniques are helping scientists to find ever smaller worlds in their quest to discover another planet that is just like our own Earth.

UK planetary expert Dr Emily Baldwin, of Astronomy Now magazine, told Skymania News: “It is incredibly exciting to find planets that are not only similar in size to our own Earth, but also share similar properties, as it brings us one step closer to finding a true Earth twin with running water on its surface.”

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