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About the Mission
... NASA's first mission capable of finding Earth-size planets around other stars.

"... the ways by which men arrive at knowledge of the celestial things are hardly less wonderful than the nature of these things themselves"

— Johannes Kepler

Importance of Planet Detection

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 following websites are tracking the day-by-day increase in new discoveries and are providing information on the characteristics of the planets as well as those of the stars they orbit: The Extrasolar Planets Encyclopedia, NASA Exoplanet Archive, and New Worlds Atlas.

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 and possibly life might exist.

The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets.

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.
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.

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 abundance of terrestrial and larger planets 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.

The Kepler Mission also supports the objectives of future 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:

When a planet crosses in front of its star as viewed by an observer, the event is called a transit. Transits by terrestrial planets produce a small change in a star's brightness of about 1/10,000 (100 parts per million, ppm), lasting for 1 to 16 hours. This change must be periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time, thus providing a highly repeatable signal and robust detection method.

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. Knowing the temperature of a planet is key to whether or not the planet is habitable (not necessarily inhabited). Only planets with moderate temperatures are habitable for life similar to that found on Earth.

The Kepler Mission Design

For a planet to transit, as seen from our solar system, the orbit must be lined up edgewise to us. The probability for an orbit to be properly aligned is equal to the diameter of the star divided by the diameter of the orbit. This is 0.5% for a planet in an Earth-like orbit about a Sun-like star. (For the giant planets discovered in four-day orbits, the alignment probability is more like 10%.) In order to detect many planets, one can not just look at a few stars for transits or even a few hundred. One must look at thousands of stars, even if Earth-like planets are common. If they are rare, then one needs to look at many thousands to find even a few. Kepler looks at more than 100,000 stars so that if Earths are rare, a null or near null result would still be significant. If Earth-size planets are common then Kepler should detect hundreds of them.

Considering that we want to find planets in the habitable zone of stars like the Sun, the time between transits is about one year. To reliably detect a sequence one needs four transits. Hence, the mission duration needs to be at least three and one half years. If the Kepler Mission continues for longer, it will be able to detect smaller, and more distant planets as well as a larger number of true Earth analogs.

The Kepler instrument is a specially designed 0.95-meter diameter telescope called a photometer or light meter. It has a very large field of view for an astronomical telescope 105 square degrees, which is comparable to the area of your hand held at arm's length. The fields of view of most telescopes are less than one square degree. Kepler needs the large field of view in order to observe the large number of stars. It stares at the same star field for the entire mission and continuously and simultaneously monitors the brightnesses of more than 100,000 stars for at least 3.5 years, the initial length of the mission, which can be extended.

The diameter of the telescope needs to be large enough to reduce the noise from photon counting statistics, so that it can measure the small change in brightness of an Earth-like transit. The design of the entire system is such that the combined differential photometric precision over a 6.5 hour integration is less than 20 ppm (one-sigma) for a 12th magnitude solar-like star including an assumed stellar variability of 10 ppm. This is a conservative, worse-case assumption of a grazing transit. A central transit of the Earth crossing the Sun lasts 13 hours. And about 75% of the stars older than 1 Gyr (1 billion years) are less variable than the Sun on the time scale of a transit.

The photometer must be spacebased to obtain the photometric precision needed to reliably see an Earth-like transit and to avoid interruptions caused by day-night cycles, seasonal cycles and atmospheric perturbations, such as, extinction associated with ground-based observing.

Extending the mission beyond three and one half years provides for:

  • Improving the signal to noise by combining more transits to permit detection of smaller planets
  • Finding planets in orbits with larger periods
  • Finding planets around stars that are noisier either due to being fainter or having more variability

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 12% with two or more planets per system.
Substantially higher numbers of terrestrial-sized planets may be found if one takes into consideration all orbits from a few days to more than one year.

From transits of giant planets:
  • About 135 inner-orbit planet detections,
  • Densities for 35 inner-orbit planets, and
  • About 30 outer-orbit planet detections.

Many candidates that may be short-period giant planets have already been detected. These are being studied further to eliminate false-positives.

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.

System Characteristics:

Spacebased Photometer: 0.95-m aperture

Primary mirror: 1.4 meter diameter, 85% light weighted

Detectors: 95 mega pixels (21 modules each with two 2200x1024 pixel CCDs)

Bandpass: 430-890 nm FWHM

Dynamic range: 9th to 16th magnitude stars

Fine guidance sensors: 4 CCDs located on science focal plane

Attitude stability: <9 milli-arcsec, 3 sigma over 15 minutes.

Avionics: Fully block redundant

Science data storage: >60 days

Uplink X-band: 7.8125 bps to 2 kbps

Downlink X-band: 10 bps to 16 kbps

Downlink Ka-band: Up to 4.33125 Mbps

Photometric One-Sigma Noise Designed Performance:

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

No mechanisms other than a one-time ejectable cover and three focus mechanism for the primary mirror.

Flight segment and instrument mass: 1071 kg, maximum expected (10/06)

Flight segment and instrument power: 771 W, maximum expected (10/06)

Flight Segment labeled

Kepler Spacecraft and Photometer
Kepler Spacecraft and Photometer

Mission Characteristics:

Continuously point at a single star field in Cygnus-Lyra region except during Ka-band downlink.

Roll the spacecraft 90 degrees about the line-of-sight every 3 months to maintain the sun on the solar arrays and the radiator pointed to deep space.

Monitor 100,000 main-sequence stars for planets

Mission lifetime of 3.5 years extendible to at least 6 years

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

Scientific Operations Center and Project management (operations) at Ames Research Center

Project management (development) at Jet Propulsion Laboratory

Flight segment design and fabrication at Ball Aerospace & Technologies Corp.

Mission Operations Center at Laboratory for Atmospheric and Space Physics (LASP)University of Colorado

Data Management Center at Space Telescope Science Institute

Deep Space Network for telemetry

Routine contact

X-band contact twice a week for commanding, health and status

Ka-band contact once a month for science data downlink

Kepler Mission on Facebook:

Official Kepler Mission Facebook page:
Kepler Facebook page maintained by Arif Solmaz:
Fan Page: