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Exploring exoplanet populations with NASA’s Kepler Mission

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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 Administration’s long-term goal of finding habitable environments beyond the solar system.

Section headings:
  • NASA’s 10th Discovery Mission
  • Kepler Transforms the Discovery Space
  • Status of Kepler’s Discovery Catalogs
  • Planets in the HZ
  • Catalog Reliability
  • Planet Confirmation and Characterization
  • Requirements for Reliable Planet Occurrence Rates
  • Estimates of Planet Occurrence Rates
  • Summary
  • Acknowledgments
  • Footnotes

Our blinders to small planets have been lifted, and the exoplanet landscape looks dramatically different from what it did before the launch of NASA’s Kepler Mission. A picture is forming in which small planets abound and close-in giants are few, in which the HZs of cool stars are heavily populated with terrestrial planets and the diversity of systems challenges preconceived ideas. The picture will continue to evolve over the next few years as we analyze the remaining data, refine the sample, and quantify the observational biases. Characterization instruments will continue to gain sensitivity ensuring that Kepler’s exoplanet discoveries will be studied for years to come. Although Kepler’s primary data collection has officially ended, the most significant discovery and analysis phase is underway, enabling the long-term goal of exoplanet exploration: the search for habitable environments and life beyond the solar system.

Fig. 1. plot of non-Kepler exoplanet discoveries vs Kepler discoveries
Fig. 1. Non-Kepler exoplanet discoveries (Left) are plotted as mass versus orbital period, colored according to the detection technique. A simplified mass–radius relation is used to transform planetary mass to radius (Right), and the >3,500 Kepler discoveries (yellow) are added for comparison. Eighty-six percent of the non-Kepler discoveries are larger than Neptune, whereas the inverse is true of the Kepler discoveries: 85% are smaller than Neptune.
graph: Stellar effective temperature versus insolation
Fig. 2. Stellar effective temperature versus insolation (stellar flux at the semimajor axis) for Kepler exoplanets larger than 2 R⊕ (plusses) and smaller than 2 R⊕ (circles). Symbols are colored blue if they lie within the HZ and are sized relative to the Earth (represented by a superimposed image) if they represent a planet smaller than 2 R⊕. The confirmed HZ exoplanets (Kepler-22b, Kepler-62 e and f, Kepler-61b, and Kepler-186f) are displayed as the artist’s conceptions.
Fig. 3. The radius distribution (Left) and period distribution (Right) of planet occurrence rates expressed as the average number of planets per star. The distributions have been marginalized over periods between 0.68 and 50 d (radius distribution) and radii between 0.5 and 22.6 R⊕ (period distribution). H12 refers to ref. 88, F13 refers to ref. 47, and D13 refers to ref. 92. The reported one-sigma uncertainties are shown.

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