SPECIAL FEATURE: PERSPECTIVE PERSPECTIVE SPECIAL FEATURE: Exploring exoplanet populations with NASA’s Kepler Mission Natalie M. Batalha1 National Aeronautics and Space Administration Ames Research Center, Moffett Field, 94035 CA Edited by Adam S. Burrows, Princeton University, Princeton, NJ, and accepted by the Editorial Board June 3, 2014 (received for review January 15, 2014) The Kepler Mission is exploring the diversity of planets and planetary systems. Its legacy will be a catalog of discoveries sufficient for computing planet occurrence rates as a function of size, orbital period, star type, and insolation flux.The mission has made significant progress toward achieving that goal. Over 3,500 transiting exoplanets have been identified from the analysis of the first 3 y of data, 100 planets of which are in the habitable zone. The catalog has a high reliability rate (85–90% averaged over the period/radius plane), which is improving as follow-up observations continue. Dynamical (e.g., velocimetry and transit timing) and statistical methods have confirmed and characterized hundreds of planets over a large range of sizes and compositions for both single- and multiple-star systems. Population studies suggest that planets abound in our galaxy and that small planets are particularly frequent. Here, I report on the progress Kepler has made measuring the prevalence of exoplanets orbiting within one astronomical unit of their host stars in support of the National Aeronautics and Space Admin- istration’s long-term goal of finding habitable environments beyond the solar system. planet detection | transit photometry Searching for evidence of life beyond Earth is the Sun would produce an 84-ppm signal Translating Kepler’s discovery catalog into one of the primary goals of science agencies lasting ∼13 h. population statistics requires corrections for in the United States and abroad. The goal Kepler’s pixel and flux measurements (2) observation and detection biases. This is a looms closer as a result of exoplanet discov- are publicly available at the Mikulski Archive work in progress. However, occurrence rate eries made by the National Aeronautics and for Space Telescopes (MAST) (http://archive. calculations based on subsets of the data al- Space Administration’s(NASA)10thDiscov- stsci.edu/kepler). Transit searches have been ready indicate that nature produces small ery Mission, Kepler. Launched in March performed on successively larger data volumes planets relatively efficiently in the warmer 2009, the Kepler spacecraft is exploring the yielding incremental planet candidate cat- environs of a planetary system. Giant planets diversity of planets and planetary systems alogs that are hosted at NASA’sExoplanet in such orbits are orders of magnitude less within one astronomical unit (AU). The pri- Archive (NEA) (http://exoplanetarchive.ipac. frequent than their sub-Neptunian counter- mary mission objective is to determine the parts. Ironically, the hot Jupiters that com- caltech.edu). To date, approximately three- prevalence of potentially habitable, Earth-size prised the very first Doppler and transiting quarters of the data have been thoroughly planets in the galaxy. Discovering exoterrans exoplanet discoveries are actually quite rare. in the habitable zone (HZ), characterizing searched. As of this writing (April 2014), the Current results for habitable-zone planets tell those that have habitable environments, and archive is host to over 3,500 viable planet us that we may not have to look very far be- then focusing on the signatures of biological candidates (with radii smaller than twice fore happening upon a planet similar to Earth. chemistry is a path of exploration that Jupiter). All have been subjected to a series A comprehensive review of Kepler exo- stretches decades into the future. It begins of statistical tests (based on the Kepler data planet science is beyond the scope of this by determining if planets like Earth are itself) that ensure a low rate of instrumental contribution. Here, I focus on the science abundant. and astrophysical false positives (3). leading to the determination of planet oc- Kepler has a follow-up observation pro- currence rates, from the discovery catalogs ’ NASA s 10th Discovery Mission gram to increase the reliability of the catalog to the first calculations of the prevalence of From 2009 to 2013, Kepler monitored a 115- even further by (i) improving the accuracy Earth-like planets. square-degree field in the constellations of the host star properties which in turn Kepler Transforms the Discovery Space Cygnus and Lyra, collecting ultrahigh pre- improves the accuracy of the planet proper- Exoplanet discoveries trickled in at a steady cision photometry of over 190,000 stars si- ties (or changes the interpretation altogether) multaneously at a 30-min cadence. Nearly rate in the latter half of the 1990s. Approxi- and (ii) identifying bound stellar companions uninterrupted photometry is possible due mately 30 were reported with sizes ranging and line-of-sight neighbors that might in- to a heliocentric orbit and off-ecliptic point- from 0.4 to 8 Jupiter masses and orbital ing. The observations yield an evenly sam- dicate an astrophysical false positive. Ground- periods ranging from 3 to 3,800 d. Heralding pled, minimally gapped flux time series that and space-based telescopes with apertures in the new millennium, the first transiting can be searched for periodic diminutions of ranging from 1.5 to 10 m are being used to acquire high-resolution spectroscopy and light due to the transit of an exoplanet Author contributions: N.M.B. designed research, performed re- across the stellar disk in an aligned ge- high-contrast/high-spatial resolution images. search, analyzed data, and wrote the paper. ometry. The photometer was engineered to Strategic high-precision Doppler measure- The author declares no conflict of interest. achieve 20-ppm relative precision in 6.5 h ments are providing planet masses in an This article is a PNAS Direct Submission. A.S.B. is a guest editor for a 12th magnitude G-type main-sequence effort to delineate the transition between invited by the Editorial Board. star (1). For reference, the Earth orbiting terrestrial and giant planets. 1Email: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1304196111 PNAS | September 2, 2014 | vol. 111 | no. 35 | 12647–12654 Downloaded by guest on September 25, 2021 exoplanet was discovered (4, 5). The timing detections), they are estimated using a poly- 2,658 stars based on 1.5, 13, 16, 22, and 34.5 was a boon for Kepler as it was proposing to nomial fit to solar system planets (R = M0.4854) of the ∼48 mo of data acquired during the use this detection technique from space. In (6). Shown here are 3,553 Kepler discoveries primary mission (7–11). Kepler data in the 2000, Kepler was one of the three Discovery associated with 2,658 stars. Approximately prime mission were downlinked monthly but Mission proposals invited to submit a Con- 22% of the Kepler host stars are known to processed on a quarterly basis. Transit searches cept Study Report. It was selected for flight harbor multiple planet candidates. The over- and the associated planet candidate catalogs on December 20, 2001. all reliability of the catalog (80–90%) is are, therefore, referred to by the quarters As Kepler was being designed and built, discussed below. bracketing the data. The most recent planet exoplanet discoveries were growing at an The demographics of the observed pop- candidates were identified in a search of 12 accelerated pace. By the eve of Kepler’s ulation has changed remarkably. Kepler has quarters of data (Q1–Q12) where the first is launch, over 300 discoveries had been re- increased the roster of exoplanets by nearly only slightly longer than one month in du- ported including nearly 70 transiting systems. 400%. More remarkable still is the change in ration (hence the 34.5-mo time span). All non-Kepler discoveries up through April the distribution: 86% of the non-Kepler dis- Previously detected candidates are reex- 2014 are shown in Fig. 1, Left, in a plot of coveries have masses larger than Neptune amined as larger data volumes become avail- mass (or minimum mass for nontransiting whereas 85% of the Kepler discoveries have able. However, this does not occur with every planets) versus orbital period with symbols radii smaller than Neptune. Kepler is filling catalog release. Some of the candidates in the color-coded by the discovery method. (Meth- in an area of parameters space that was not cumulative archive at the NEA were discov- odologies with small numbers of discoveries previously accessible. The increase in sensi- ered with less than 34.5 mo of data and have have been left out for clarity). Collectively, tivity afforded us by Kepler has opened the not yet been reexamined. This nonuniformity there are 697 (non-Kepler)exoplanets(with floodgates to the small planets so difficult to will be resolved as Kepler completes its final a measured orbital period and radius or mass) detect from ground-based surveys. The most search and vetting of the entire 17 quarters associated with 583 unique stars. Approxi- common type of planet known to us is a (48 mo) of data acquired during its primary ’ mately 16% of these host stars are known to population that does not exist in ourown solar mission lifetime. Kepler splanetcandidate harbor multiple planets. system: the super-Earths and mini-Neptunes catalog is also known as the Kepler Object of Fig. 1, Right,showsthesamepopulation between1and4Earthradii. Interest (KOI) Catalog. However, KOIs also together with the Kepler planet candidate include events that are classified as false discoveries in the cumulative table at NEA Status of Kepler’s Discovery Catalogs alarms or astrophysical false positives. Only as of April 2014. Detections are plotted as Catalogs of Kepler’s viable planet candidates those flagged as planet candidates in the planet radius versus orbital period, and the have been released periodically since launch NEA cumulative catalog are shown in Fig.