The scientific objective of the Kepler Mission is to explore the structure and diversity of extrasolar planetary systems. More specifically, this is achieved by observing a large sample of stars to:

Goal 1: Determine the frequency of terrestrial and larger planets in or near the habitable zone of a wide variety of spectral types of stars.
The frequency of planets is derived from the number and size of planets found and from the number and spectral type of stars monitored. Even a null result would be highly meaningful because of the large number of stars searched and the low false alarm rate.

Goal 2: Determine the distributions of sizes and orbital semi-major axes of these planets.
The planet's area is found from the fractional brightness decrease and the stellar area. For a detection with a statistical significance of >8 sigma, the uncertainty of the planetary area is about 14% and the planetary radius of 7%.

The planet's semi-major axis is derived from the measured period and stellar mass, using Kepler's Third Law. An uncertainty in the semi-major axis of about 1% results from a 3% uncertainty in the mass of the central star, derived from ground-based spectroscopic observations and stellar modeling.

Goal 3: Estimate the frequency of planets and orbital distribution of planets in multiple-stellar systems.
This goal is achieved by comparing the number of planetary systems found in single versus multi-stellar systems. Multiple-stellar systems are identified from ground-based spectroscopic measurements if they are tightly bound or from high angular resolution observations if they are widely spaced systems.

Goal 4: Determine the distributions of semi-major axis, albedo, size, mass and density of short-period giant planets
Short-period giant planets are also detected from variations in their reflected light. As above, the semi-major axis is derived from the orbital period and the stellar mass.

Transits should also be seen in about 10% of the cases and the size of the planet determined. These planets are found in the first few months of the mission. From the planet size, semi-major axis and the amplitude of reflected light modulation, the albedo is determined. The density is calculated when the planet is seen both in transit (to yield its size) and when Doppler spectroscopy is used (to determine the planet's mass for stars with m_v<13 and cooler than F5) as was done for the case of HD209458b.

Goal 5: Identify additional members of each photometrically discovered planetary system using complementary techniques.
Observations using both the Space Interferometry Mission (SIM) and ground-based Doppler spectroscopy are used to search for additional massive companions which do not transit, thereby providing greater details of each planetary system discovered.

Goal 6: Determine the properties of those stars that harbor planetary systems
The spectral type, luminosity class, and metalicity for each star showing transits are obtained from ground-based observations. Additionally, rotation rates, surface brightness inhomogeneities and stellar activity are obtained directly from the photometric data. Stellar age and mass is determined from Kepler p-mode measurements (asteroseismology).

Support for Origins Theme Missions:

Further, the results of the above stated Goals support the Origins theme missions, the Space Interferometry Mission (SIM) and the Terrestrial Planet Finder (TPF), by:

• Identifying the common stellar characteristics of host stars for future planet searches,
• Defining the volume of space needed to search and
• Providing a list of targets for SIM where systems are already known to have terrestrial planets.

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