Astronomers have discovered the smallest planet outside our Solar System, and the first that is undoubtedly rocky like Earth.

Measurements of unprecedented precision have shown that the planet, Kepler 10b, has a diameter 1.4 times that of Earth, and a mass 4.6 times higher.

However, because it orbits its host star so closely, the planet could not harbour life.

The discovery has been hailed as "among the most profound in human history".

The result was announced at the 217th annual meeting of the American Astronomical Society in Seattle, US, by Nasa's Kepler team.

The Kepler space telescope, designed to look for the signs of far-flung planets, first spotted the planet 560 light years away, alongside hundreds of other candidate planets.

Kepler relies on the "transiting" technique, which looks for planets that pass between their host star and Earth.

A tiny fraction of the star's light is blocked periodically, giving a hint that the star has a planet orbiting it.

The radius of the planet correlates to exactly how much light is blocked when it passes.

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.

Crucially, this meant that the star was free of the optical and magnetic activity that have introduced some uncertainty into the measurements of previous candidates for rocky exoplanets, such as Corot-7b, announced in early 2009.