Kepler Mission

Contents

Importance of Planet Detection

The "Kepler Mission: A search for Habitable Planets" is NASA's first mission capable of finding Earth-size and smaller 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. With the exception of the pulsar planets, all of the extrasolar planets detected so far are gas giants, approximately 150 as of 2005. The challenge now is to find terrestrial planets (habitable planets like Earth), which are 30 to 600 times less massive than Jupiter.

The Kepler Mission, a NASA Discovery mission, is specifically designed to survey our region of the Milky Way galaxy to detect and characterize hundreds of Earth-size and smaller planets in or near the habitable zone. The habitable zone encompasses the distances from a star where liquid water can exist on a planet's surface.

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

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.

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 call 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 2 to 16 hours. This change must be absolutely 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. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.

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 solar-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 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, 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.

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— or about the area of both your hands held at arm's length, in order to observe the necessary 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 the life of the mission—3.5 years.

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 combine differential photometric precision over a 6.5 hour integration is less than 20 ppm (one-sigma) for a 12^th 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 are less variable then 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:

1. Improving the signal to noise by combining more transits to permit detection of smaller planets
2. Finding planets in orbits with larger periods
3. 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 R_e) and none larger,
• About 185 planets if most have a size of R~1.3 R_e,
• About 640 planets if most have a size of R~2.2 R_e,
• 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.

From modulation of the reflected light from giant inner planets

• About 870 planets with periods less than one week.

From 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.

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.

External Links

• Kepler Education Projects - Kepler Mission, NASA.
• Kepler Mission Homepage - Kepler Mission, NASA.
• "Johannes Kepler: His Life, His Laws and Times" - Kepler Mission, NASA.
• "International Year of Astronomy and Johannes Kepler" - Kepler Mission, NASA.
• StarDate Radio Programs - The Kepler Mission EPO at SETI Institute works closely with the McDonald Observatory at the University of Texas, Austin, to coordinate a series of StarDate radio programs in both English and Spanish featuring Kepler and the search for extrasolar terrestrial planets.

Preview Image

"Kepler with distant solar system." "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. (Source: NASA-Kepler Mission.)

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Original content retrieved from ""http://kepler.nasa.gov/""