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The Kepler Mission uses a NASA space observatory designed to discover Earth-like planets orbiting other stars.^[1] The spacecraft was launched on March 7, 2009 at 03:49:57 UTC (March 6, 10:49:57 p.m. EST).^[2][3] The mission is named in honor of German astronomer Johannes Kepler.^[4] With a planned mission lifetime of at least 3.5 years, Kepler uses a photometer developed by NASA to continuously monitor the brightness of over 145,000^[5] main sequence stars in a fixed field of view. The data collected from these observations will be analyzed to detect periodic fluctuations that indicate the presence of extrasolar planets (planets outside our solar system) that are in the process of crossing the face of other stars. Kepler is a mission under NASA's Discovery Program of low-cost, focused science missions. NASA's Ames Research Center is the home organization of the science principal investigator and is responsible for the ground system development, mission operations and science data analysis. Kepler mission development was managed by NASA's Jet Propulsion Laboratory until December 2009 but was then transferred to NASA Ames Research Center. Ball Aerospace & Technologies Corp. was responsible for developing the Kepler flight system.

The Kepler Mission is currently in active operation, with the first main results announced on 4 January 2010. As expected, the initial discoveries were all short-period planets, with longer period planets expected later. The first six weeks of data revealed five previously unknown planets, all very close to their stars^[6][7]. Among the notable results are one of the least dense planets yet found^[8], and two mystery objects that are planet sized but hotter than the stars they circle.^[9]

Objectives and methods

Components of the Kepler telescope

The scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems.^[10] This is achieved by surveying a large sample of stars to achieve several goals:

• Determine how many Earth-sized and larger planets there are in or near the habitable zone (often called "Goldilocks planets")^[11] of a wide variety of spectral types of stars.
• Determine the range of size and shape of the orbits of these planets.
• Estimate how many planets there are in multiple-star systems.
• Determine the range of orbit size, brightness, size, mass and density of short-period giant planets.
• Identify additional members of each discovered planetary system using other techniques.
• Determine the properties of those stars that harbor planetary systems.

Most of the extrasolar planets detected so far by other projects are giant planets, mostly the size of Jupiter and bigger. Kepler is designed to look for planets 30 to 600 times less massive, closer to the order of Earth's mass. The method used, the transit method, involves observing repeated transit of planets in front of their stars, which causes a slight reduction in the star's apparent magnitude, on the order of 0.01% for an Earth-sized planet. The degree of this reduction in brightness can be used to deduce the diameter of the planet, and the interval between transits can be used to deduce the planet's orbital period, from which estimates of its orbital semi-major axis (using Kepler's laws) and its temperature (using models of stellar radiation) can be calculated.

Exoplanets in Kepler's FOV, in context of all discovered exoplanets (as of 2010-03-13), with some transit probabilities for example scenarios indicated.

The Kepler Mission has a much higher probability of detecting Earth-like planets than the Hubble Space Telescope, since it has a much larger field of view (approximately 10 degrees square), and will be dedicated for detecting planetary transits. The Hubble Space Telescope, in contrast, is used to address a wide range of questions and rarely looks continuously at just one starfield. Of the approximately half million stars in Kepler's field of view, around 150,000 stars were selected^[13] for observation and they are observed simultaneously, measuring variations in their brightness every 30 minutes. This provides a better chance for seeing a transit. In addition, the 1-in-215 probability means that if 100% of stars observed had the same diameter as the Sun, and each had one Earth-like terrestrial planet in an orbit identical to that of the Earth, Kepler would find about 465; but if only 10% of stars observed were such, then it would find about 46. The mission is well suited to determine the frequency of Earth-like planets orbiting other stars.^[14][15]

Since Kepler must see at least three transits to be sure the dimming was caused by a planet, and since larger planets give a signal that is easier to check, scientists expect the first reported results will be larger Jupiter sized planets in tight orbits. The first of these were reported after only a few months of operation. Smaller planets, and planets further from their sun will take longer, and discovering planets comparable to Earth is expected to take three years or longer.^[16]

Data from the mission will also be used for studying variable stars of various types and performing asteroseismology^[17], particularly on stars showing solar-like oscillations.^[18]

Kepler Input Catalog

The Kepler Input Catalog (or KIC) is a publicly searchable database of the roughly 13.2 million targets under study by the Kepler Mission.^[19][20]

Status

Kepler mission launch, March 6, 2009.

The observatory was launched on March 7, 2009 at 03:49:57 UTC (March 6, 10:49:57 p.m. EST) aboard a Delta II rocket from Cape Canaveral Air Force Station, Florida.^[22][2] The launch was a complete success and all three stages were completed by 04:55 UTC. The cover of the telescope was jettisoned on April 7, 2009 and the first light images were taken on the next day.^[23][24]

On April 20, 2009 it was announced that the Kepler science team had concluded that further refinement of the focus would dramatically increase the scientific return.^[25] On April 23, 2009 it was announced that the focus had been successfully optimized by moving the primary mirror 40 micrometers (1.6 thousandths of an inch) towards the focal plane and tilting the primary mirror 0.0072 degree.^[26] On May 12, 2009 at 5:01 p.m. Pacific Time (17:01 UTC-8) Kepler successfully completed its commissioning phase and began its search for planets around other stars.^[27][28]

On June 19, 2009, the spacecraft successfully sent its first science data to Earth. It was discovered that Kepler had entered safe mode on June 15. A second safe mode event occurred on July 2. In both cases the event was triggered by a processor reset. The spacecraft resumed normal operation on July 3 and the science data collected since June 19 was downlinked that day.^[29] On October 14, 2009, the cause of these safing events was determined to be a low voltage power supply which provides power to the RAD750 processor.^[30] On January 12, 2010, one portion of the focal plane transmitted anomalous data, suggesting a problem with focal plane MOD-3 module, covering 2 out of Kepler's 42 CCDs. As of Jan. 19, MOD-3 remains offline.^[31]

Kepler downloads roughly 90-100 gigabits of science data^[32] about once per month^[33] - the latest such download was on 2?5 February 2010.^[34]

Once Kepler has detected a transit-like signature, it is necessary to rule out false positives with follow-up tests^[35] such as doppler spectroscopy. Although Kepler was designed for photometry it turns out that it is capable of astrometry and such measurements can help confirm or rule out planet candidates.^[36]

Mission Results to Date

NASA held a press conference to discuss early science results of the Kepler Mission on August 6, 2009.^[37] At this press conference, it was revealed that Kepler has confirmed the existence of the previously-known transiting exoplanet HAT-P-7b, and is functioning well enough to discover Earth-size planets.^[38][39]

Since Kepler's detection of planets depends on seeing very small changes in brightness, stars that vary in brightness all by themselves (variable stars) are not useful in this search.^[33] From the first few months of data, Kepler scientists have determined that about 7500 stars from the initial target list are such variable stars. These were dropped from the target list, and will be replaced by new candidates. On November 4, 2009, the Kepler project publicly released the light curves of the dropped stars.^[40]

Ground-based follow-up studies of the first six weeks of data, reveal five previously unknown planets, all very close to their stars, one (Kepler-4b) slightly larger than Neptune and four (Kepler-5b, 6b, 7b, and 8b) larger than Jupiter^[6], including one (Kepler-7b), that is one of the least dense planets found yet.^[8] Another discovery, not yet understood, are at least two objects that are the size of planets, but hotter than their stars.^[9] One analysis suggests these objects are white dwarfs.^[41] The Kepler team also announced it has selected 177 targets for follow-up observations and expects between 24% and 62% of these to be planets.^[42]

Mission details

Kepler Mission search in context of Milky Way galaxy.

The photometer's field of view in the constellations Cygnus, Lyra and Draco.

An additional benefit of that choice is that Kepler is pointing in the direction of the Solar System's motion around the center of the galaxy. Thus, the stars which are observed by Kepler are roughly the same distance from the galaxy center as the Solar System, and also close to the galactic plane. This fact is important if position in the galaxy is related to habitability, as suggested by the Rare Earth hypothesis.

The spacecraft has a mass of , has a aperture, and a primary mirror (the largest on any telescope outside of Earth orbit).^[44] The spacecraft has a 105 deg˛ (about 12 degree diameter) field of view (FOV), roughly equivalent to the size of one's fist held at arm's length. The photometer has a soft focus to provide excellent photometry, rather than sharp images. The combined differential photometric precision (CDPP) for a m(V)=12 solar-like star for a 6.5 hour integration will be 20 ppm, including an expected stellar variability of 10 ppm. An Earth-like transit produces a brightness change of 84 ppm and lasts for 13 hours when it crosses the center of the star. The focal plane is made up of 42 CCDs at 2200 × 1024 pixels which makes it the largest camera launched into space with a resolution of 95 megapixels.^[45][46] The array is cooled by heat pipes connected to an external radiator.^[15] The CCDs are read out every six seconds (to limit saturation) and co-added on board for 30 minutes. However, even though Kepler has the highest data rate of any NASA mission, the 30 minute sums of all 95 million pixels are more data than can be stored and sent back to Earth. Therefore the science team has pre-selected the relevant pixels associated with each star of interest, amounting to about 5 percent of the pixels. The data from these pixels is then requantized, compressed and stored, along with other auxiliary data, in the on-board 16 gigabyte solid-state recorder. Data that is stored and downlinked includes science stars, p-mode stars, smear, black level, background and full field-of-view images.^[15]

The mission's life-cycle cost is estimated at US$600 million, including funding for 3.5 years of operation.^[15]

Mission operations

Kepler's orbit ? solar array adjusted at solstices and equinoxes

NASA contacts the spacecraft using the X band communication link twice a week for command and status updates. Scientific data are downloaded once a month using the K_a band link at a maximum data transfer rate of 4.33 Mb/s. The Kepler spacecraft conducts its own partial analysis on board and only transmits scientific data deemed necessary to the mission in order to conserve bandwidth.^[47]

Science data telemetry collected during mission operations at LASP is sent on for processing at the Kepler Data Management Center (DMC), located at the Space Telescope Science Institute on the campus of The Johns Hopkins University in Baltimore, Maryland. The science data telemetry is decoded and processed into uncalibrated FITS-format science data products by the DMC, which are then passed along to the Science Operations Center (SOC) at NASA Ames Research Center, for calibration and final processing. The SOC at NASA Ames Research Center (ARC) develops and operates the tools needed to process scientific data for use by the Kepler Science Office (SO). Accordingly, the SOC develops the pipeline data processing software based on scientific algorithms developed by the SO. During operations, the SOC (1) receives calibrated pixel data from the DMC, (2) applies the analysis algorithms to produce light curves for each star, (3) performs transit searches for detection of planets (threshold-crossing events, or TCEs) and (4) performs data validation of candidate planets by evaluating various data products for consistency as a way to eliminate false positive detections. The SOC also evaluates the photometric performance on an on-going basis and provides the performance metrics to the SO and Mission Management Office. Finally, the SOC develops and maintains the project?s scientific databases, including catalogs and processed data. The SOC finally returns calibrated data products and scientific results back to the DMC for long-term archiving, and distribution to astronomers around the world through the Multimission Archive at STScI (MAST).

Known extrasolar planets in Kepler's field of view

Celestial north]] is towards the lower left corner.

The image, shown to the right, is magnified detail from the full field of view image shown above. This image also identifies the TrES-2b system and clearly shows the parent binary star, GSC 03549-02811, in the constellation Draco.

Diagram of Kepler's investigated area with celestial coordinates.

Kepler has a fixed field of view (FOV) against the sky. The diagram to the left shows the celestial coordinates and where the detector fields are located, along with the locations of a few bright stars with celestial north at the top left corner. Click on the diagram to see a detailed view.

The mission website has a calculator that will determine if a given object falls in the FOV and if so where it will appear in the photo detector output data stream.

Below is a table of known extrasolar planetary systems from the list of extrasolar planets that fall within the FOV of Kepler.

Kepler has also identified two hot companions which cannot currently be classified as planets.

See also

Operational space missions

• COROT is a French space mission, launched in December 2006, to discover planets by transits.
• Spitzer Space Telescope is a NASA infrared telescope sometimes used for exoplanet research.
• MOST is an asteroseismology satellite built by Canada and launched in June 2003.

Proposed space missions

• Darwin Mission
• New Worlds Mission NWM
• Space Interferometry Mission SIM
• Terrestrial Planet Finder TPF

Other

• PlanetQuest
• Trans-Atlantic Exoplanet Survey or TrES
• HATNet Project or HAT
• SuperWASP
• List of extrasolar planets
• List of space telescopes

References

External links

• Summary Table of Kepler Discoveries
• Kepler Mission website on www.nasa.gov
• Kepler Mission Profile by NASA's Solar System Exploration
• Kepler Asteroseismic Science Consortium (KASC)
• Spherical panorama of Kepler in the clean room prior to fueling

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