Kepler is a Schmidt-Cassegrain optical design with 95 cm aperture. The on-board 'camera' is not used to take photographic images, but is adjusted for photometric purposes. The 95-megapixel photometer consists of an array of 42 CCDs, each composed of 2200 x 1024 pixels on an area of 50 x 25 mm. So equipped, Kepler can 'see' and scan 105 square degrees worth of sky, 12 degrees in diameter.
The distance to most of the stars for which Earth-size planets can be detected by Kepler is from 600 to 3,000 light years. Less than 1% of the stars that Kepler will be looking at are closer than 600 light years. Stars farther than 3,000 light years are too faint for Kepler to observe the transits needed to detect Earth-size planets.
Kepler's field of view encompasses approximately 4.5 million stars out of which 2326 were selected as candidates for Kepler's planets hunt, as of December 2011. Many of them are multiple stars. The number of planets assumed to be of approximately Earth size is 207, but only 10 of them lie in the habitable zone of their parent stars. The habitable zone is a region around a star where water can exist in liquid state.
Still there isn't a second Earth, but there may be interesting worlds among those 10 top candidates. The smallest Kepler planet detected to date in a habitable zone is Kepler-22b, a body 2.4 times the size of Earth orbiting a sun-like star in 290 days in a distance of 0.85 AUs, which is roughly between Venus and Earth in our solar system. Since the star is slightly less luminous its habitable zone lies closer to it than our sun's. Planet b lies pretty close to the zone's inner boundary in that a balanced temperature of 22°C is assumed.
Although the planet's mass is not yet deduced (probably between Earth and Uranus), it is a feast for Kepler scientists. A rocky world with water and 22°C warm would fit a paradise planet whatever it may look like in a distance of over 600 light-years - out of reach for detailed exploration. In fact none of Kepler's current 28 confirmed planets can be imaged optically. Their existance is based on measurements of subtle changes in the light intensity of a star as an object such as a planet transits across its surface. From so obtained data, a planet's orbital period, mass, radius, star distance, etc. can be mathematically deduced.
The smallest known planet is Kepler-21b with 1.6 Earth radii and a surface temperature of over 1600°C as a result of its tight 6 million km orbit which takes the planet 2.8 days to complete. In other words, one year on Kepler-21b lasts a mere 2.8 days. In our solar system, Mercury lies 10 times further away from the sun. Possibility of existence of liquid water and life on this molten rock an be entirely ruled out, also considering that the star Kepler-21 is almost twice as large and over 5 times as luminous than our sun.
At the left a diagram that shows the distribution of confirmed Kepler planets on the Kepler telescope's field of view - a total of 28 as of December 12th, 2011.
Planets around the stars Kepler-10 to Kepler-22 have been detected in 2011, the others in 2010.
The space telescope is currently 32 million kilometers from Earth.
At the top right an illustration depicting what astronomers and geologists alike are looking for. An earth-size warm and habitable planet with continents and oceans plus a bonus -- a habitable moon nearby.