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Astronomers using the Kepler space telescope have discovered the smallest planet outside our Solar System, and the first that is rocky like Earth.

The discovery was announced at the 217th annual meeting of the American Astronomical Society in Seattle, USA, by Nasa's Kepler team. The measurements which have been made indicate that the planet - named Kepler 10b - has a diameter 1.4 times that of Earth, and a mass 4.6 times higher, and is 560 light years away from Earth. The high level of accurate detail provided in the measurements is a remarkable achievement, and one that shows just how highly developed astronomical instruments have now become.

Follow-up measurements by a telescope at the Keck observatory in Hawaii confirmed the find of Kepler 10b by measuring how the planet pulls to and fro on its parent star as it orbits. These measurements also bore out the fact that the parent star was about eight billion years old - a grandfather among stars of its type.

However, it is unlikely that Kepler 10b would host life because its daytime temperature exceeds 1,300C due to the planet's close proximity to its host star. However, the discovery is still a significant step in Kepler's mission to discover whether planets that could potentially harbour life are common.

Geoffrey Marcy, a pioneer of the hunt for exoplanets, from the University of California Berkeley described how Kepler 10b represented "a planetary missing link, a bridge between the gas giant planets we've been finding and the Earth itself, a transition...between what we've been finding and what we're hoping to find".

"This report...will be marked as among the most profound scientific discoveries in human history," he said.

Visit the NASA website to discover more about the Kepler mission.

Or visit the eLibrary to find further teaching resources related to Exoplanets.