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Discovery of smallest planet yet a 'milestone' in search for another Earth

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Kepler 37b, as the planet is known, is more like a mini-Mercury but on a much tighter orbit. But "the sort of precision we need from the instrument" to achieve the mission's goal "is demonstrated through this discovery."

In essence, the planet's very weak signature at some 9.3 million miles from its star is comparable to the signature researchers expect to see from an Earth-mass planet 93 million miles from its star, Earth's average distance from the sun. A planet betrays itself to Kepler by dimming its star's light repeatedly as it passes in front of it, a process astronomers call the transit method for detecting extrasolar planets.

The detection of Kepler 37b was extremely difficult, Barclay says. Many transits were needed to build up enough data to spot the dimming the planet imparts to the star. And Kepler's ability to take very precise measurements of the star's own light helped the team develop a highly accurate estimate of the star's size and mass.

Just as geophysicists can use earthquake vibrations traveling through the Earth to reconstruct the planet's interior, astrophysicists can use the same approach for determining the structure of distant stars. Kepler takes the measure of a star by recording rapid changes in its brightness from what in essence are sound waves moving through the hot gas.

In some cases, Kepler's insight into a star has forced team members to boost the size and mass estimates of its planets. These revisions mean planets that were initially thought to be comparable in size and mass to Earth, often turn out not to be rocky planets in all likelihood, Barclay explains.

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