Overview of the Kepler Mission


...a NASA mission to discover
Earth-sized habitable planets
around other stars.

The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of giant planets like Jupiter orbiting stars beyond our Solar System. But almost all of the extrasolar planets detected so far are gas giants that are 30 to 600 times more massive than Earth and have thick atmospheres probably not particularly conducive to life.

The Kepler Mission is designed to survey our area of the Milky Way Galaxy to detect and characterize hundreds of Earth-size and larger planets in or near the habitable zone. The habitable zone of a star is where liquid water can exist on a planet's surface.

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Results from this mission will allow us to place our Solar System within the continuum of planetary systems in the Galaxy.

The Extended Solar Neighborhood

The figure shows what we believe to be the local structure of our Galaxy, the Milky Way. The stars sampled are similar to the immediate solar neighborhood. Young stellar clusters, ionized hydrogen (HII) regions and the neutral hydrogen (HI) distribution define the arms of the Galaxy.

See also: Importance of Planet Detection

Kepler Mission Science Objectives:

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The scientific goal 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 how many terrestrial and larger planets there are in or near the habitable zone of a wide variety of spectral types of stars;
  2. Determine the range of sizes and shapes of the orbits of these planets;
  3. Estimate the how many planets there are in multiple-star systems and their orbit characteristics;
  4. Determine the range of orbit size, brightness, size, mass and density 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.

The Kepler Mission is designed to test the hypotheses that:

  • Most main-sequence stars have terrestrial planets in or near the habitable zone;
  • On an average two Earth-size planets form in the region between 0.5 and 1.5 AU, based on our Solar System and the accretion model of Wetherill (1996).

The Kepler Mission also supports the objectives of the NASA Origins theme missions Space Interferometry Mission (SIM) and Terrestrial Planet Finder (TPF),

  • By identifying the common stellar characteristics of host stars for future planet searches,
  • By defining the volume of space needed for the search and
  • By allowing SIM to target systems already known to have terrestrial planets.

The Transit Method of Detecting Extrasolar Planets:

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When a planet passes infront of its star, the event is call a transit. Transits by terrestrial planets produce a tiny change in star brightness by a factor of about 1/10,000, lasting for 2 to 16 hours. The orbit size and planet size can be calculated from the period (how long it takes the planet to orbit once around the star) and depth of the transit (how much the brightness of the star drops). From orbit size, planet temperature can be estimated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.

The Kepler instrument is a 0.95-meter diameter light sensing telescope (photometer) with a huge field of view—105 deg2 or about the area of both your hands held at arm's length. It monitors brightnesses of 100,000 stars continuously and simultaneously for the life of the mission—4 years. It watches only stars brighter than 14th magnitude.

For a planet to transit, as seen from our solar system, the orbit must be lined up edgewise to us. The alignment is less critical for planets cloer in to their stars. The probability for the orbits to be properly aligned is about 0.5% near the habitable zone (HZ).

Detection of two transits suggest a candidate planet—an orbital period can be calculated and a third transit predicted. A third transit and subsequent transits of a star, all with a the same period, depth, and duration provides a rigorous method of confirming planet discovery.

The photometer must be spacebased to obtain this precision and to avoid interruptions caused by day-night, seasonal cycles and atmospheric perturbations, such as, extinction associated with ground-based observing.

Expected Results:

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Based on the assumptions described above, we expect to perform a census of planets with periods from days to a few years and to detect:

Transits of terrestrial planets:

  • About 50 planets if most have size about the size of Earth (R~1.0 Re),
  • 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.

Modulation of the reflected light from giant inner planets:

  • About 870 planets with periods less than one week.

Transits of giant planets:

  • About 135 inner-orbit planet detections,
  • Densities for 35 inner-orbit planets, and
  • About 30 outer-orbit planet detections.

Detection of the short-period giant planets should occur within the first several months of the mission. The sample size of stars for this mission is large enough to capture the richness of the unexpected. Should no detection be made, a null result would still be very significant.

Flight System Characteristics:

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Spacebased Photometer: 0.95-m aperture

Primary mirror: 85% light weighted

Detectors: 42 - 2200x1024 CCDs

Photometric One-Sigma Noise Performance:

Total noise with solar-like stellar variability and photon
shot noise for an mv=12 star: < 2x10-5

No mechanisms besides antenna gimbals and one-time ejectable cover.

Spacecraft and instrument characteristics:

Mass: 903 kg (including reserve) plus 34% margin

Power: 613 W (including reserve) plus 32% margin

Kepler Photometer and Spacecraft

Mission Characteristics:

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D2925-10L (Delta II) launch into an Earth-trailing heliocentric orbit.

Scientific Operations Center at NASA Ames.

Mission Operations Center at Honeywell

Data Management Center at Space Telescope Science Institute

Deep Space Network for telemetry.

Continuously point at a single star field in Cygnus.

Monitor 100,000 main-sequence stars for planets.

Mission lifetime of 4 years.


see Mission News—Status and Schedule



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