Kepler is a space observatory launched by NASA to discover Earth-like planets orbiting other stars. The spacecraft, named in honor of the 17th-century German astronomerJohannes Kepler, was launched on 7 March 2009, and has been active for 3 years, 7 months and 28 days as of 4 November 2012.
The Kepler mission 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.” Kepler‘s only instrument is a photometer that continually monitors the brightness of over 145,000main sequence stars in a fixed field of view. This data is transmitted to Earth, thenanalyzed to detect periodic dimming caused by extrasolar planets that cross in front of their host star.
Kepler is a project under NASA’s Discovery Program of relatively low-cost, focused science missions. Construction and initial operation were managed by NASA’s Jet Propulsion Laboratory, with Ball Aerospace responsible for developing the Kepler flight system. The Ames Research Center is responsible for the ground system development, mission operations (from December 2009), and science data analysis.
The Kepler observatory is currently in active operation, with the first main resultsannounced on 4 January 2010. As expected, the initial discoveries were all short-period planets. As the mission continued, additional longer-period candidates were found – as of December 2011, there are a total of 2,326 candidates. Of these, 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars. The Kepler team estimated that 5.4% of all stars host Earth-size planet candidates, and that 17% of all stars have multiple planets. In December 2011, two of the Earth-sized candidates, Kepler-20e and Kepler-20f, were confirmed as planets orbiting a Sun-like star, Kepler-20.
The Kepler mission began with a planned mission lifetime of at least 3.5 years. In 2012, the mission was extended to 2016, partly due to difficulties in processing and analyzing the huge volume of data collected by the spacecraft.
The spacecraft has a mass of 1,039 kilograms (2,290 lb), has a 0.95-meter (37.4 in) aperture, and a 1.4-meter (55 in) primary mirror – at the time of its launch, this was the largest mirror on any telescope outside of Earth orbit.The spacecraft has a 115 deg2 (about 12 degree diameter) field of view (FOV), roughly equivalent to the size of one’s fist held at arm’s length. Of this, 105 deg2 is of science quality, with less than 11% vignetting. The photometer has a soft focus to provide excellent photometry, rather than sharp images. The mission goal is a combined differential photometric precision (CDPP) of 20 ppm for a m(V)=12 solar-like star for a 6.5 hour integration, though the observations so far fall short of this objective (see mission status). 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 of the spacecraft’s camera is made up of 42 CCDs at 2200 × 1024 pixels, which made it at the time the largest camera yet launched into space, possessing a total resolution of 95 megapixels. The array is cooled by heat pipes connected to an external radiator. The CCDs are read out every six seconds (to limit saturation) and co-added on board for 30 minutes. However, even though at launch Kepler had the highest data rate of any NASA mission, the 30-minute sums of all 95 million pixels constitute 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.
In January 2006, the project’s launch was delayed eight months because of budget cuts and consolidation at NASA. It was delayed again by four months in March 2006 due to fiscal problems. At this time, the high-gain antenna was changed from a gimballed design to one fixed to the frame of the spacecraft to reduce cost and complexity, at the cost of one observation day per month.
The Kepler 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. 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.
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. 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.
On June 19, 2009, the spacecraft successfully sent its first science data to Earth. It was discovered that Kepler had entered safe modeon 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 that had been collected since June 19 was downlinked that day. On October 14, 2009, the cause of these safing events was determined to be a low voltage power supply which provides power to theRAD750 processor. 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 October 2010, the module was described as “failed”, but the coverage still exceeded the science goals.
On July 14th, 2012, one of the four reaction wheels used for fine pointing of the spacecraft failed. Although the spacecraft can perform all duties using only the three remaining reaction wheels, another failure would leave the spacecraft unable to continue in its mission. This is a potential threat to the extended mission.
In terms of photometric performance, Kepler is working well, much better than any Earth-bound telescope, but still short of the design goals. The objective was a combined differential photometric precision (CDPP) of 20 parts per million (PPM) on a magnitude 12 star for a 6.5 hour integration. This estimate was developed allowing 10 ppm for stellar variability, roughly the value for the Sun. The obtained accuracy for this observation has a wide range, depending on the star and position on the focal plane, with a median of 29 ppm. Most of the additional noise appears due to a larger-than-expected variability in the stars themselves (19.5 ppm as opposed to the assumed 10.0 ppm), with the rest due to instrumental noise sources slightly larger than predicted. Work to better understand, and perhaps calibrate out, instrument noise is ongoing.
Since the signal from an Earth size planet is so close to the noise level (only 80 ppm), the increased noise means each individual transit is only a 2.7 σ event, instead of the intended 4 σ. This, in turn, means more transits must be observed to be sure of a detection. Scientific estimates indicated that a 7–8 year mission, as opposed to the originally planned 3.5 years, would be needed to find all transiting Earth-sized planets. On April 4, 2012, the Kepler mission was approved for extension through the fiscal year 2016.
Spacecraft orbit and orientation
The Kepler space observatory is in a heliocentric orbit, so that Earth does not occult the stars, which are observed continuously, and so the photometer is not influenced by stray light from Earth. This orbit avoids the gravitationalperturbations and torques inherent in an Earth orbit, allowing for a more stable viewing platform. The photometer points to a field in the northernconstellations of Cygnus, Lyra and Draco, which is well out of the ecliptic plane, so that sunlight never enters the photometer as the spacecraft orbits the Sun. Cygnus is also a good choice to observe because it will never be obscured byKuiper belt objects or the asteroid belt.
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 byKepler are roughly the same distance from the galactic 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.
Kepler’s orbit has been described by NASA as Earth-trailing. With an orbital period of 372.5 days, Kepler slowly falls further behind Earth.
Kepler is operated out of Boulder, Colorado, by the Laboratory for Atmospheric and Space Physics (LASP). The spacecraft’s solar array is rotated to face the Sun at the solstices andequinoxes, so as to optimize the amount of sunlight falling on the solar array and to keep the heat radiator pointing towards deep space. Together, LASP and the spacecraft’s builders, Ball Aerospace & Technologies Corp., control the spacecraft from a mission operations center located on the research campus of the University of Colorado. LASP performs essential mission planning and the initial collection and distribution of the science data. The mission’s initial life-cycle cost was estimated at US$600 million, including funding for 3.5 years of operation. In 2012, NASA announced that the Kepler mission would be funded until 2016.
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 Kaband link at a maximum data transfer rate of approximately 550 KBps. 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.
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 KeplerScience Office (SO). Accordingly, the SOC develops the pipeline data processing software based on scientific algorithms developed by the SO. During operations, the SOC:
- Receives calibrated pixel data from the DMC;
- Applies the analysis algorithms to produce light curves for each star;
- Performs transit searches for detection of planets (threshold-crossing events, or TCEs); and
- 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).
Field of view
Kepler has a fixed field of view (FOV) against the sky. The diagram to the right shows thecelestial 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. The mission website has acalculator 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. Data on extrasolar planet candidates is submitted to the Kepler Follow-up Program, or KFOP, to conduct follow-up observations.
- Kepler’s field of view covers 115 square degrees, around 0.28 percent of the sky, or “about two scoops of the Big Dipper.” It means that it would take around 400 Kepler like telescopes to cover whole sky.
Objectives and methods
- To determine how many Earth-size and larger planets there are in or near the habitable zone (often called “Goldilocks planets“) of a wide variety of spectral types of stars.
- To determine the range of size and shape of the orbits of these planets.
- To estimate how many planets there are in multiple-star systems.
- To determine the range of orbit size, brightness, size, mass and density of short-period giant planets.
- To 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 previously detected by other projects were 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 (Jupiter is 318 times more massive than Earth). 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-size 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.
The probability of a random planetary orbit being along the line-of-sight to a star is the diameter of the star divided by the diameter of the orbit. For an Earth-like planet at 1 AU transiting a Sol-like star the probability is 0.465%, or about 1 in 215. At 0.72 AU (the orbital distance of Venus) the probability is slightly larger, at 0.65%; such planets could be Earth-like if the host star is a late G-type star such as Tau Ceti. In addition, because planets in a given system tend to orbit in similar planes, the possibility of multiple detections around a single star is actually rather high. For instance, if a Kepler-like mission conducted by aliens observed Earth transiting the Sun, there is a 12% chance that it would also see Venus transiting.
Kepler‘s 115-deg2 field of view gives it a much higher probability of detecting Earth-like planets than the Hubble Space Telescope, which has a field of view of only 10 sq. arc-minutes. Moreover, Kepler is dedicated to detecting planetary transits, while the Hubble Space Telescope is used to address a wide range of scientific 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 for observation, and they are observed simultaneously, with the spacecraft 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.
Since Kepler must see at least three transits to confirm that the dimming of a star was caused by a transiting planet, and since larger planets give a signal that is easier to check, scientists expected the first reported results to be larger Jupiter-size planets in tight orbits. The first of these were reported after only a few months of operation. Smaller planets, and planets farther from their sun will take longer, and discovering planets comparable to Earth is expected to take three years or longer.
Once Kepler has detected a transit-like signature, it is necessary to rule out false positives with follow-up tests 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.
In addition to transits, planets orbiting around their stars undergo reflected light variations changes – like the Moon, they go throughphases from full to new and back again. Since Kepler cannot resolve the planet from the star, it sees only the combined light, and the brightness of the host star seems to change over each orbit in a periodic manner. Although the effect is small – the photometric precision required to see a close-in giant planet is about the same as to detect an Earth-sized planet in transit across a solar-type star – Jupiter-sized planets are detectable by sensitive space telescopes such as Kepler. In the long run, this method may help find more planets than the transit method, because the reflected light variation with orbital phase is largely independent of the planet’s orbital inclination, and does not require the planet to pass in front of the disk of the star. In addition, the phase function of a giant planet is also a function of its thermal properties and atmosphere, if any. Therefore, the phase curve may constrain other planetary properties, such as the particle size distribution of the atmospheric particles.
Mission results to date
A photo taken by Kepler with two points of interest outlined. Celestial north is towards the lower left corner.
Detail of Kepler’s image of the investigated area. The location of TrES-2b within this image is shown. Celestial north is towards the lower left corner.
NASA held a press conference to discuss early science results of the Kepler mission on August 6, 2009. At this press conference, it was revealed that Kepler had confirmed the existence of the previously known transiting exoplanet HAT-P-7b, and was functioning well enough to discover Earth-size planets.
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. From the first few months of data, Kepler scientists have determined that about 7,500 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.
The first six weeks of data revealed five previously unknown planets, all very close to their stars. Among the notable results are one of the least dense planets yet found, two low-mass white dwarf stars that were initially reported as being members of a new class of stellar objects, and a well-characterized planet orbiting a binary star.
On 15 June 2010, the Kepler mission released data on all but 400 of the ~156,000 planetary target stars to the public. 706 targets from this first data set have viable exoplanet candidates, with sizes ranging from as small as the Earth to larger than Jupiter. The identity and characteristics of 306 of the 706 targets were given. The released targets included 5 candidate multi-planet systems. Data for the remaining 400 targets with planetary candidates was to be released in February 2011. (For details about this later data release, see the Kepler results for 2011 below.) Nonetheless, the Kepler results, based on the candidates in the list released in 2010, imply that most candidate planets have radii less than half that of Jupiter. The Kepler results also imply that small candidate planets with periods less than 30 days are much more common than large candidate planets with periods less than 30 days and that the ground-based discoveries are sampling the large-size tail of the size distribution. This contradicted older theories which had suggested small and Earth-like planets would be relatively infrequent. Based on the Kepler data, an estimate of around 100 million habitable planets in our galaxy may be realistic. However, some media reports of the TED talk have led to misunderstandings, apparently partly due to confusion concerning the term “Earth-like”. By way of clarification, a letter to the Director of the NASA Ames Research Center, for the Kepler Science Council dated August 2, 2010 states, “Analysis of the current Kepler data does not support the assertion that Kepler has found any Earth-like planets.”
In 2010, Kepler identified two systems containing objects which are smaller and hotter than their parent stars: KOI 74 and KOI 81.These objects are probably low-mass white dwarf stars produced by previous episodes of mass transfer in their systems.
In 2010, the Kepler team released a paper which had data for 312 extrasolar planet candidates from 306 separate stars. Only 33.5 days of data were available for most of the candidates. NASA also announced data for another 400 candidates were being withheld to allow members of the Kepler team to perform follow-up observations. The data for these candidates were made public on February 2, 2011.
On February 2, 2011, the Kepler team announced the results of analysis of the data taken between 2 May and 16 September 2009. They found 1235 planetary candidates circling 997 host stars. (The numbers that follow assume the candidates are really planets, though the official papers called them only candidates. Independent analysis indicated that at least 90% of them are real planets and not false positives). 68 planets were approximately Earth-size, 288 super-Earth-size, 662 Neptune-size, 165 Jupiter-size, and 19 up to twice the size of Jupiter. 54 planets were within the habitable zone, including 5 less than twice the size of the Earth. In contrast to previous work, roughly 74% of the planets are smaller than Neptune, most likely as a result of previous work finding large planets more easily than smaller ones.
That February 2, 2011 release of 1235 extrasolar planet candidates, included 54 that may be in the “habitable zone.” There were previously only two planets thought to be in the “habitable zone,” so these new findings represent an enormous expansion of the potential number of “Goldilocks planets” (planets of the right temperature to support liquid water). All of the habitable zone candidates found thus far orbit stars significantly smaller and cooler than the Sun (habitable candidates around Sun-like stars will take several additional years to accumulate the three transits required for detection). Of all the new planet candidates, 68 are 125% ofEarth‘s size or smaller, or smaller than all previously discovered exoplanets.“Earth-size” and “super-Earth-size” is defined as “less than or equal to 2 Earth radii (Re)” [(or, Rp ≤ 2.0 Re) – Table 5]. Six such planet candidates [namely: KOI 326.01 (Rp=0.85), KOI 701.03 (Rp=1.73), KOI 268.01 (Rp=1.75), KOI 1026.01 (Rp=1.77), KOI 854.01 (Rp=1.91), KOI 70.03 (Rp=1.96) – Table 6] are in the “habitable zone.” A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported.
The frequency of planet observations was highest for exoplanets two to three times Earth-size, and then declined in inverse proportionality to the area of the planet. The best estimate (as of March, 2011), after accounting for observational biases, was: 5.4% of stars host Earth-size candidates, 6.8% host super-Earth-size candidates, 19.3% host Neptune-size candidates, and 2.55% host Jupiter-size or larger candidates. Multi-planet systems are common; 17% of the host stars have multi-candidate systems, and 33.9% of all the planets are in multiple planet systems.
BY December 5, 2011, the Kepler team announced that they had discovered 2,326 planetary candidates, of which 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Compared to the February 2011 figures, the number of Earth-size and super-Earth-size planets increased by 200% and 140% respectively. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars, marking a decrease from the February figure; this was due to the more stringent criteria in use in the December data.
Based on Kepler’s findings, astronomer Seth Shostak estimated in 2011 that “within a thousand light-years of Earth,” there are “at least 30,000” habitable planets. Also based on the findings, the Kepler team has estimated that there are “at least 50 billion planets in the Milky Way”, of which “at least 500 million” are in the habitable zone. In March 2011, astronomers at NASA’s Jet Propulsion Laboratory (JPL) reported that about “1.4 to 2.7 percent” of all sunlike stars are expected to have earthlike planets “within the habitable zones of their stars”. This means there are “two billion” of these “Earth analogs” in our own Milky Way galaxy alone. The JPL astronomers also noted that there are “50 billion other galaxies,” potentially yielding more than one sextillion “Earth analog” planets if all galaxies have similar numbers of planets to the Milky Way.
In January 2012, an international team of astronomers reported that each star in the Milky Way Galaxy may host ” on average…at least 1.6 planets“, suggesting that over 160 billion star-bound planets may exist in our galaxy alone. Kepler also recorded distantstellar super-flares, some of which are 10,000 times more powerful than the superlative 1859 Carrington event. The superflares may be triggered by close-orbiting Jupiter-sized planets.
The Kepler team originally promised to release data within one year of observations. However, this plan was changed after launch, with data being scheduled for release up to three years after its collection. This resulted in considerable criticism,leading the Kepler science team to release the third quarter of their data one year and nine months after collection. The data through September 2010 (quarters 4, 5 and 6) was made public in January 2012.
Follow-ups by others
Periodically, the Kepler team releases a list of candidates (Kepler Objects of Interest, or KOIs) to the public. Using this information, a team of astronomers collected radial velocity data using the SOPHIE échelle spectrograph to confirm the existence of the candidateKOI-428b in 2010. In 2011, the same team confirmed candidate KOI-423b.
Citizen scientist participation
Since December 2010, Kepler mission data has been used for the Zooniverse project “Planethunters.org”, which allows volunteers to look for transit events in the light curves of Kepler images to identify planets that computer algorithms might miss. By June 2011, users had found 69 potential candidates that were previously unrecognized by the Kepler mission team. The team has plans to publicly credit amateurs who spot such planets.
In January 2012, the British Broadcasting Corporation (BBC) program Stargazing Live aired a public appeal for volunteers to analyse Planethunters.org data for potential new exoplanets. This led to the discovery of a new Neptune-sized exoplanet by two amateur astronomers – one in Peterborough, England – to be named Threapleton Holmes B. 100,000 other volunteers were reportedly engaged in the search by late January, analysing over 1 million Kepler images.
In April 2012, an independent panel of senior NASA scientists recommended that the Kepler mission be continued through 2016. According to the senior review, Kepler observations needed to continue until at least 2015 to achieve all the stated scientific goals.
In addition to discovering hundreds of exoplanet candidates, the Kepler spacecraft has also reported 26 exoplanets in 11 systems which have not yet been added to the Extrasolar Planet Database. Exoplanets discovered using Kepler‘s data, but confirmed by outside researchers, include KOI-423b, KOI-428b, KOI-196b, KOI-135b, KOI-204b, KOI-254b, KOI-730, and Kepler-42 (KOI-961). The “KOI” acronym indicates that the star is a Kepler Object of Interest.
Both Corot and Kepler measured the reflected light from planets. However, these planets were already known, since they transit their host star. Kepler’s data allowed the first discovery of planets by this method, KOI 55.01 and 55.02.
Kepler Input Catalog
The Kepler Input Catalog (or KIC) is a publicly searchable database of roughly 13.2 million targets used for the Kepler Spectral Classification Program and Kepler Mission. The catalog alone is not used for finding Kepler targets, because only a portion of the listed stars (about one-third of the catalog) can be observed by the spacecraft itself.