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Missione Kepler
Immagine del veicolo
Immagine d'artista del telescopio spaziale Kepler
Dati della missione
OperatoreNASA
NSSDC ID2009-011A
DestinazioneOsservazione astronomica
Esitoin corso
Nome veicoloKepler
VettoreDelta II (7925-10L)
Lancio7 marzo 2009
Luogo lancioCape Canaveral Air Force Station, PAD 17B
Durata≥ 7,5 anni
Proprietà del veicolo spaziale
Massa1039 kg
CostruttoreBall Aerospace
StrumentazioneSpecchio principale con apertura di 0,95 m
Parametri orbitali
OrbitaEliocentrica
Periodo372,5 giorni
Eccentricità0,03188
Semiasse maggiore1,01319 UA
Sito ufficiale
Programma Discovery
Missione precedenteMissione successiva
Dawn GRAIL
Elemento Wikidata assente · Manuale

La missione Kepler è una missione spaziale della NASA parte del programma Discovery, il cui scopo è la ricerca e conferma di pianeti simili alla Terra in orbita attorno a stelle diverse dal Sole, tramite l'utlizzo del telescopio spaziale Kepler.[1] Il veicolo spaziale, chiamato in onore dell'astronomo tedesco del diciassettesimo secolo Johannes Kepler,[2] è stato lanciato con successo il 7 marzo 2009.[3]

Il telescopio Kepler è stato "specificatamente progettato per monitare una porzione della nostra regione della Via Lattea e scoprire dozzine di pianeti simili alla Terra vicino o nella zona abitabile, e determinare quante delle miliardi di stelle della nostra galassia posseggano pianeti".[4] Per fare ciò, un fotometro monitora costantemente la luminosità di più di 145.000 stelle di sequenza principale nel suo campo di vista fissato, presso le costellazioni del Cigno, della Lira e del Drago.[5]I dati sono trasmessi a Terra, dove vengono analizzati in cerca di periodiche diminuzioni di luminosità delle stelle causate da pianeti extrasolari che transitano di fronte alla loro stella. Nel marzo 2013 il team di Kepler aveva individuato 2.740 candidati pianeti e confermato altri 115.[6][7] Nel gennaio 2013 un gruppo di astronomi dell'Harvard-Smithsonian Center for Astrophysics ha stimato dai dati di Kepler che nella Via Lattea risiedano "almeno 17 miliardi" di esopianeti simili alla Terra.[8]

Il programma Discovery, di cui Kepler è parte, consiste in missioni scientifiche di basso costo focalizzate su obiettivi precisi. La costruzione del telescopio e le operazioni iniziali sono state gestite dal Jet Propulsion Laboratory, insieme alla Ball Aerospace, responsabile dello sviluppo del sistema di volo. L'Ames Research Center è invece responsabile dello sviluppo dei sistemi a Terra, delle operazioni di missione dal dicembre 2009 e dell'analisi dei dati scientifici. Il tempo previsto per la missione è stato inizialmente di 3,5 anni, ma nel 2012 è stata estesa al 2016[9][10], in parte per difficoltà dovute all'analisi dell'enorme volume di dati raccolti dal telescopio.[11]

Il telescopio spaziale Kepler

 
Diagramma del telescopio che evidenzia i suoi principali componenti.

Il telescopio possiede una massa di 1039 kg, ed è costituito da uno specchio primario di 1,4 m di diametro e con un'apertura di 0,95 m, all'epoca del lancio il più grande mai mandato in orbita.[12] Lo strumento ha un campo di vista di 115 gradi quadrati (circa 12° in diametro), equivalente all'area sottesa da un pugno a braccio teso, 105 dei quali utili per dati di qualità scientifica e con meno dell'11% di vignettatura. Il fotometro ha un effetto di soft focus, per ottenere così misurazioni fotometriche eccellenti piuttosto che immagini nitide. L'obiettivo della missione è una precisione fotometrica differenziale combinata (combined differential photometric precision o CDPP) di 20 ppm per una stella di magnitudine 12 di tipo solare e per un periodo di integrazione di 6,5 ore, anche se le osservazioni finora non hanno raggiunto questo obiettivo. Il transito di un pianeta terrestre produce una variazione di luminosità di 84 ppm e dura circa 13 ore.

Fotocamera

 
La schiera di sensori di Kepler: sono montati su una supericie curva per compensare la curvatura di campo di Petzval.

Il piano focale della fotocamera del telescopio è costituito da una matrice di 42 sensori CCD, ciascuno con una dimensione di 2200 × 1024 pixel. La fotocamera possiede quindi una risoluzione totale di 95 megapixel, il che la rende la più grande all'epoca mai lanciata nello spazio.[13][14] I sensori sono raffreddati da dei condotti termici (Heat pipe) connessi a un radiatore esterno.[15]

I CCD sono letti ogni 6 secondi, per limitarne la saturazione, e le immagini sono generate a bordo dello strumento sommando per 30 minuti tali letture. Nonostante al lancio Kepler possedesse il più alto rateo di produzione di dati di qualsiasi altra missione NASA, l’immagine somma dei 95 milioni di pixel per 30 minuti costituisce più informazione di quanta possa essere memorizzata e trasmessa a Terra. Pertanto il team ha preselezionato i pixel associati a ciascuna stella di interesse, vale a dire il 5% del totale. I dati da questi pixel sono in seguito riquantificati, compressi e memorizzati, insieme a dati ausiliari, nell’unità di memoria a stato solido da 16 GB di bordo. I dati memorizzati e scaricati a Terra comprendono le immagini delle stelle del progetto, lo striscio, il livello di nero, il fondo e il pieno campo.[15]

Storia della missione

 
Kepler's launch on 7 March 2009.

Nel gennaio 2006 il lancio del telescopio è stato ritardato di otto mesi per via di tagli al bilancio della NASA,[16] e di altri quattro mesi nel marzo dello stesso anno per problemi fiscali.[16] In questo periodo è stato cambiato il design dell’antenna ad alto guadagno, rinunciando alla sospensione cardanica e collegandola al direttamente al telaio del veicolo spaziale, così da ridurre costi e complessità, al costo di un giorno di osservazione al mese perso.[16]

Il telescopio spaziale Kepler è stato lanciato il 7 marzo 2009 alle 03:49:57 UTC (6 marzo ore 10:49:57 pm EST) a bordo di un lanciatore Delta II dalla Cape Canaveral Air Force Station, Florida.[17][3] Il lancio è stato un successo completo e tutte e tre le fasi erano complete alle 04:55 UTC. La copertura del telescopio è stata espulsa il 7 aprile e le immagini di prima luce sono state scattate il giorno successivo.[18][19]

Il 20 aprile 2009 il team scientifico di Kepler ha annunciato che ulteriori rifiniture del fuoco avrebbero incrementato notevolmente la qualità dei dati di ritorno.[20] Il 23 aprile seguente è stato annunciato come il fuoco fosse stato ottimizzato con successo, muovendo lo specchio primario di 40 micrometri verso il piano focale e inclinandolo di 0,0072 gradi.[21]

Il 13 maggio 2009 alle ore 01:01 UTC Kepler ha completato con successo la fase di commissioning e ha cominciato la sua ricerca di pianeti extrasolari.[22][23]

Il 19 giugno 2009 il veicolo spaziale ha trasmesso con successo i suoi primi dati scientifici a Terra. Si è scoperto che il 15 giugno Kepler è entrato in "safe mode" (modalità di sicurezza), e una seconda volta il 2 luglio. Entrambi gli eventi sono stati innescati da un "reset del processore". Il telescopio è ritornato ad operare normalmente il 3 luglio, e i dati raccolti dal 19 giugno sono stati trasmessi a Terra quel giorno.[24] Il 14 ottobre 2009 si è determinato che la causa di questi eventi di “messa in sicurezza” del telescopio era un generatore di potenza a bassa tensione, che alimenta il processore RAD750.[25] IL 12 gennaio 2010 una porzione del piano focale ha trasmesso dati anomali, indicando un problema con il piano focale del modulo MOD-3, che controlla 2 dei 42 CCD di Kepler. Il modulo tuttora è dichiarato come non funzionante, ma la copertura eccede ancora gli obiettivi scientifici.[26]

Kepler scarica circa 12 gigabyte di dati[27] all’incirca una volta al mese[28], un esempio di tale download è quello del 22-23 novembre.[29]

Il 14 luglio 2012 una delle quattro ruote di reazione utilizzata per il puntamento fine del telescopio si è guastata.[30] Seppur Kepler richieda solo tre delle ruote di reazione per posizionarsi accuratamente, il guasto ad un altra di queste renderebbe la sonda incapace di continuare la sua missione, e questa è una minaccia potenziale alla missione estesa.[31]

Il 17 gennaio 2013 la NASA ha annunciato che una delle tre ruote di reazione rimanenti mostrava segni di frizione in aumento, e che Kepler avrebbe operato in modo discontinuo per 10 giorni come possibile soluzione al problema. Se anche la seconda ruota dovesse guastarsi la missione Kepler terminerebbe.[32][33] Il 29 gennaio la NASA ha annunciato il ritorno con successo alla normale modalità di raccolta dati.[34]

Prestazioni

In termini di prestazioni fotometriche Kepler sta lavorando bene, meglio di qualsiasi telescopio terrestre, seppure inferiormente all’obiettivo di progetto. Tale obiettivo era di una precisione fotometrica differenziale combinata (CDPP) di 20 ppm per una stella di magnitudine 12 in 6,5 ore di integrazione: questa stima era stata calcolata considerando una variabilità delle stelle di 10 ppm, all’incirca il valore di variabilità solare. L’accuratezza ottenuta invece per questo tipo di osservazione ha una vasta gamma di valori, in dipendenza dalla stella e dalla sua posizione sul piano focale, con una mediana di 29 ppm. Molto del rumore addizionale sembra essere dovuto a una maggiore variabilità delle stelle stesse, circa 19,5 ppm, mentre il resto è dovuto a fonti di rumore strumentale maggiori di quanto previsto.[35] Si sta lavorando per comprendere meglio il rumore strumentale ed eliminarlo.[36]

Siccome il segnale di un pianeta terrestre transitante è molto vicino al livello di rumore (circa 80 ppm), l’aumento di questo implica per ogni singolo evento di transito un livello di significatività di 2,7 σ, invece del 4 σ di progetto. Questo a sua volta significa che più transiti devono essere necessariamente osservati per essere sicuri di una rivelazione di pianeta. Le stime scientifiche hanno indicato in 7-8 anni il tempo necessario alla missione per trovare tutti i pianeti terrestri transitanti, contro i 3,5 originalmente pianificati.[37] Il 4 aprile 2012 è stata approvata la missione estesa di Kepler fino al'anno fiscale 2016.[38][10]

Orientameno e orbita

 
Volume di ricerca di Kepler nella Via Lattea.
 
Il campo visivo di Kepler nelle costellazioni del Cigno, della Lira e del Dragone.

Kepler è su un’orbita eliocentrica,[39][40] che evita occultazioni da parte della Terra, luce diffusa, perturbazioni gravitazionali e momenti torcenti associati alle orbite terrestri. Il fotometro punta il suo campo visivo verso le costellazioni del Cigno, della Lira e del Dragone, ben lontane dal piano dell’eclittica, così che la luce del Sole non entra mai nel fotometro durante la sua orbita. Il campo inoltre non è oscurato né dalla fascia di Kuiper né da quella degli asteroidi.[15]

Questa è inoltra la stessa direzione del moto del Sistema Solare attorno al centro della Galassia. Di conseguenza le stelle osservate da Kepler sono all’incirca alla stessa distanza dal centro galattico del Sole e altrettanto vicine al piano galattico. Questa condizione è importante se l’abitabilità dipendesse dalla posizione nella Galassia, come suggerito dall’ipotesi della rarità della Terra.

L’orbita di Kepler è stata definita dalla NASA come di trascinamento terrestre[41], poiché il periodo di rivoluzione di 372,5 giorni, più lungo di quello terrestre, fa sì che lentamente il telescopio rimanga indietro rispetto alla Terra.

Spacecraft operations

 
Kepler's orbit. The spacecraft's solar array is adjusted at solstices and equinoxes.

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 and equinoxes, so as to optimize the amount of sunlight falling on the solar array and to keep the heat radiator pointing towards deep space.[15] 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.[15] In 2012, NASA announced that the Kepler mission would be funded until 2016.[10]

Communications

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 Ka band 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.[42]

Data management

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).

Field of view

 
Diagram of Kepler's investigated area with celestial coordinates.

Kepler has a fixed field of view (FOV) against the sky. The diagram to the right 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. 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. 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". Thus, it would require around 400 Kepler-like telescopes to cover the whole sky relative to Earth.[43]

Objectives and methods

The scientific objective of Kepler is to explore the structure and diversity of planetary systems.[44] This spacecraft observes a large sample of stars to achieve several key goals:

  • To determine how many Earth-size and larger planets there are in or near the habitable zone (often called "Goldilocks planets")[45] 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.[46] 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,[47] 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.[15][48]

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.[39]

Once Kepler has detected a transit-like signature, it is necessary to rule out false positives with follow-up tests[49] 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.[50]

In addition to transits, planets orbiting around their stars undergo reflected light variations changes – like the Moon, they go through phases 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.[51]

Data collected by Kepler is also being used for studying variable stars of various types and performing asteroseismology,[52] particularly on stars showing solar-like oscillations.[53]

Mission results to date

 
A photo taken by Kepler with two points of interest outlined. Celestial north is towards the lower left corner.
File:Kepler329150main NGC6791.jpg
Detail of Kepler's image of the investigated area showing open star cluster NGC 6791. 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.

The Kepler observatory 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. As the mission continued, additional longer-period candidates were found.

2009

NASA held a press conference to discuss early science results of the Kepler mission on 6 August 2009.[54] 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.[55][56]

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.[28] 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 4 November 2009, the Kepler project publicly released the light curves of the dropped stars.[57]

The first six weeks of data revealed five previously unknown planets, all very close to their stars.[58][59] Among the notable results are one of the least dense planets yet found,[60] two low-mass white dwarf stars[61] that were initially reported as being members of a new class of stellar objects,[62] and a well-characterized planet orbiting a binary star.

2010

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.[63] This contradicted older theories which had suggested small and Earth-like planets would be relatively infrequent.[64][65] Based on the Kepler data, an estimate of around 100 million habitable planets in our galaxy may be realistic.[66] 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 2 August 2010 states, "Analysis of the current Kepler data does not support the assertion that Kepler has found any Earth-like planets."[67][68][69]

In 2010, Kepler identified two systems containing objects which are smaller and hotter than their parent stars: KOI 74 and KOI 81.[70] These objects are probably low-mass white dwarf stars produced by previous episodes of mass transfer in their systems.[61]

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.[63] NASA also announced data for another 400 candidates were being withheld to allow members of the Kepler team to perform follow-up observations.[71] The data for these candidates were made public on 2 February 2011.[72]

2011

 
A chart showing exoplanets in Kepler's FOV, in context of all discovered exoplanets (as of 2010-10-03), with some transit probabilities indicated for example scenarios.

On 2 February 2011, the Kepler team announced the results of analysis of the data taken between 2 May and 16 September 2009.[72] 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).[73] 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. 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 2 February 2011 release of 1235 extrasolar planet candidates, included 54 that may be in the "habitable zone", including 5 less than twice the size of the Earth.[74][75] 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).[76] 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).[77] Of all the new planet candidates, 68 are 125% of Earth's size or smaller, or smaller than all previously discovered exoplanets.[75] "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].[72] 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][72] are in the "habitable zone."[74] A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported.[78]

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.[79]

 
A size comparison of the exoplanets Kepler-20e[80] and Kepler-20f[81] with Venus and Earth.

By 5 December 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.[82]

On 20 December 2011, the Kepler team announced the discovery of the first Earth-size exoplanets, Kepler-20e[80] and Kepler-20f,[81] orbiting a Sun-like star, Kepler-20.[83]

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.[84] 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.[85] 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.[86]

2012

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.[87][88] Kepler also recorded distant stellar super-flares, some of which are 10,000 times more powerful than the superlative 1859 Carrington event.[89] The superflares may be triggered by close-orbiting Jupiter-sized planets.[89] The Transit Timing Variation (TTV) technique, which was used to discover Kepler-9d, gained popularity for confirming exoplanet discoveries.[90] A planet in a system with four stars was also confirmed, the first time such a system had been discovered.[91]

File:717582main NewCandidatesbySize-07Jan13 full.jpg
Sizes of Kepler Planet Candidates - based on 2,740 candidates orbiting 2,036 stars Template:As of (NASA).

Template:As of, there were a total of 2,321 candidates.[82][92][93] 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[80] and Kepler-20f,[81] were confirmed as planets orbiting a Sun-like star, Kepler-20.[83][94][95]

2013

According to a study by Caltech astronomers published in January 2013, the Milky Way Galaxy contains at least one planet per star, resulting in 100 - 400 billion exoplanets.[96][97] The study, based on planets orbiting the star Kepler-32, suggests that planetary systems may be common around stars in our galaxy. The discovery of 461 more planets was announced on 7 January 2013.[7] The longer Kepler watches, the more planets with long periods it can detect.[7]

«Since the last Kepler catalog was released in February 2012, the number of candidates discovered in the Kepler data has increased by 20 percent and now totals 2,740 potential planets orbiting 2,036 stars.»

(NASA[7])

A new candidate, announced on 7 January 2013, is KOI-172.02, an Earth-like exoplanet orbiting a star similar to our Sun in the habitable zone and possibly a "prime candidate to host alien life".[98]

Data releases

The Kepler team originally promised to release data within one year of observations.[99] However, this plan was changed after launch, with data being scheduled for release up to three years after its collection.[100] This resulted in considerable criticism,[101][102][103][104][105] leading the Kepler science team to release the third quarter of their data one year and nine months after collection.[106] The data through September 2010 (quarters 4, 5, and 6) was made public in January 2012.[107]

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 candidate KOI-428b in 2010.[108] In 2011, the same team confirmed candidate KOI-423b.[109]

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.[110] By June 2011, users had found 69 potential candidates that were previously unrecognized by the Kepler mission team.[111] 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.[112] 100,000 other volunteers were reportedly engaged in the search by late January, analysing over 1 million Kepler images.[113]

Mission status

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.[114] On 14 November 2012, NASA announced the completion of Kepler's primary mission, and the beginning of its extended mission, which may last as long as four years.[115]

Confirmed exoplanets

 
Kepler's first five exoplanets to scale.
  Lo stesso argomento in dettaglio: List of planets discovered by the Kepler spacecraft, List of exoplanetary host stars e List of planetary systems.

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.[116] Exoplanets discovered using Kepler's data, but confirmed by outside researchers, include KOI-423b,[109] KOI-428b,[108] KOI-196b,[117] KOI-135b,[118] KOI-204b,[119] KOI-254b,[120] KOI-730,[121] and Kepler-42 (KOI-961).[122] The "KOI" acronym indicates that the star is a Kepler Object of Interest.

Both Corot[123] and Kepler[124] 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.[125]

Kepler Input Catalog

  Lo stesso argomento in dettaglio: 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.[126][127] 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.[126]

Voci correlate

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Collegamenti esterni

Cataloghi and database di pianeti extrasolari

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