The host star lies about 600 light-years away from us toward the constellations of Lyra and Cygnus. The star, a G5 star, has a mass and a radius only slightly smaller than that of our Sun, a G2 star. As a result, it is about 25% less luminous than the Sun. The planet orbits the G5 star with an orbital period of 290 days, compared to 365 days for the Earth, at a distance about 15% closer to its star than the Earth from the Sun. This results in the planet's balmy temperature of around 72 degrees Fahrenheit. It orbits in the middle of the star's habitable zone, where liquid water is expected to be able to exist on the surface of the planet. Liquid water is necessary for life as we know it so speculation will be rampant that this new planet might well be not only habitable, perhaps even inhabited - especially during funding talks.
The discovery team, led by William Borucki of the NASA Ames Research Center, used photometric data from the NASA Kepler space telescope, which monitors the brightness of 155,000 stars. Earth-size planets whose orbital planes are aligned such that they periodically pass in front of their stars result in tiny dimming of their host star's light; dimming that can only be measured by a specialized space telescope like Kepler.
Numerous large, massive gas giant planets have been detected previously in habitable-zone orbits around solar-type stars, but gas giants are not thought to be capable of supporting life. This new exoplanet is the smallest-radius planet discovered in the habitable zone of any star to date. It is about 2.4 times larger than that of the Earth, putting it in the class of exoplanets known as super-Earths.
While the mass of this new planet is not known, it must be less than about 36 times that of the Earth, based on the absence of a measurable Doppler (radial velocity) wobble in the host star. The masses of several other super-Earths have been measured with the Doppler technique and determined to lie in the range of about 5 to 10 times that of the Earth: Some appear to be rocky, while others probably contain major fractions of ice and water.
The finding will be published in the Astrophysical Journal.