http://www.nasa.gov/mission_pages/kepler/overview/index.html
http://kepler.nasa.gov/about/

Kepler Mission Scientific Objective:
The scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems. This is achieved by surveying a large sample of stars to:

  1. Determine the percentage of terrestrial and larger planets there are in or near the habitable zone of a wide variety of stars;
  2. Determine the distribution of sizes and shapes of the orbits of these planets;
  3. Estimate how many planets there are in multiple-star systems;
  4. Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets;
  5. Identify additional members of each discovered planetary system using other techniques; and
  6. Determine the properties of those stars that harbor planetary systems.
Expected Results: Based on the mission described above, including conservative assumptions about detection criteria, stellar variability, taking into account only orbits with 4 transits in 3.5 years, etc., and assuming that planets are common around other stars like our Sun, then we expect to detect:

From transits of terrestrial planets in one year orbits:

  • About 50 planets if most are the same size as Earth (R~1.0 Re) and none larger,
  • About 185 planets if most have a size of R~1.3 Re,
  • About 640 planets if most have a size of R~2.2 Re,
  • About 12% with two or more planets per system.
These numbers come out substantially higher, when one takes into consideration all orbits from a few days to more than one year.



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The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of hundreds of planets orbiting other stars. There is now clear evidence for substantial numbers of three types of exoplanets; gas giants, hot-super-Earths in short period orbits, and ice giants. The challenge now is to find terrestrial planets (i.e., those one half to twice the size of the Earth), especially those in the habitable zone→ of their stars where liquid water might exist on the surface of the planet.

The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone→ and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets.

The Transit Method of Detecting Extrasolar Planets
When a planet passes in front of a star as viewed from Earth, the event is called a "transit". On Earth, we can observe an occasional Venus or Mercury transit. These events are seen as a small black dot creeping across the Sun—Venus or Mercury blocks sunlight as the planet moves between the Sun and us. Kepler finds planets by looking for tiny dips in the brightness of a star when a planet crosses in front of it—we say the planet transits the star.

Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.