Astro 101 Slide Set: Kepler’s Exoplanet Discoveries Exceed 1,000 0 Topic: Exoplanets Concepts: Transit Detection, Exoplanet Statistics Mission: Kepler Coordinated by: the NASA Astrophysics Forum An Instructor’s Guide for using the slide sets is available at the ASP website ducation/resources-for-the- higher-education-audience/ ducation/resources-for-the- higher-education-audience/ Developed by: the Kepler Team DRAFT 6/15

The Discoveries In January 2015, the Kepler team, analyzing data gathered by the Kepler spacecraft during its four-year primary mission ( ), announced its 1,000 th verified extrasolar planet (“exoplanet”) discovery—including three more that are both less than two Earth diameters in size, and orbit within the “habitable zone” of their parent stars (i.e., the regions where the temperature range could allow for liquid water on planetary surfaces)—bringing the current total of these to eight. The team also added 500+ candidates to the roll of 3,000+ potential planets yet to be verified, including six of near- Earth size orbiting in the habitable zones of Sun-like stars. 1 Three of the eight verified near-Earth-size planets orbiting in habitable zones are among the newly-validated. Two of these— Kepler-438b and Kepler-442b--are less than 1.5 times the diameter of Earth and are likely made of rock. They orbit stars smaller and cooler than the sun, 475 and 1,100 light years away, respectively. Credit: NASA Ames/Kepler Mission/W. Stenzel.

How Are the Discoveries Made? Kepler detects planets by taking a photometric measurement of the stars in its field of view every 30 minutes. A planet transit will show as a small periodic dip in the “light curve” of a star over time. If the host star’s diameter and temperature are known, then from the simple dip in the the light curve Kepler can determine the planet’s diameter and orbital period; the planet’s surface temperature can also be estimated. Kepler cannot independently determine the planet’s mass or composition, nor its atmospheric composition. 2 The animation shows how Kepler detects planets. As the planet passes between the host star and the spacecraft, the observed star brightness decreases slightly, signaling the potential detection of a planet. Kepler looked at over 150,000 stars continuously for four years in the constellations Cygnus and Lyra, seeking to record the slight periodic brightness changes in stars that could reveal the presence of planets. Credit: NASA Ames/Kepler Mission/SETI Institute/D. Berry. Kepler field of view. Credit: NASA Ames/Kepler Mission/C. Roberts and W. Stenzel.

The Big Picture 3 Prior to Kepler, the vast majority of known exoplanets were Neptune-size or larger. This was a selection bias due to the difficulty of detecting smaller exoplanets. Kepler can detect, and is detecting, smaller planets not possible by other methods, and is increasing the odds of finding planets that resemble Earth. An important goal in the search for exoplanets is to find that “other Earth”—one of similar size, orbiting in the habitable zone of a sun-like star, possessing an Earth-like atmosphere, possibly harboring life as we know it. Artist’s conception of Kepler-186f. Credit: NASA Ames/SETI Institute/Caltech. Kepler’s 1,000 exoplanet discoveries— and counting—have demonstrated that planets are everywhere, and that small planets are more common than large ones.

What are the Implications? Kepler has shown that small planets are more plentiful than larger, Jupiter- sized worlds, and are abundant in our galaxy. Current models show that planets with a diameter smaller than roughly 1.5 times that of Earth are more likely to have rocky surfaces that could support liquid water and appropriate conditions to foster life as we know it. Earth-size planets are therefore key to discovering life beyond our solar system. As Kepler finds more exoplanets, that ultimate goal comes closer! 4 Credit: NASA Ames/Kepler Mission/W. Stenzel.

Resources 5 The Kepler Misson Websites o o Scientific Paper o Kepler Planet Catalog Paper, “Planetary Candidates Observed by Kepler VI: Planet Sample from Q1-Q16 (47 Months)” Additional Resources o Exoplanet Resource guide nomy-resource-guides/the-search-for- planets-around-other-stars/ nomy-resource-guides/the-search-for- planets-around-other-stars/ o Online Exoplanet database o The online Kepler data Repository NASA Exoplanet Archive: o Image Credit: NASA Ames/Kepler Mission/W. Stenzel.

Exoplanet Discoveries 6 Animation Credit: NASA Ames/Kepler Mission/N. Batalha

Exoplanets Plotted 7 This figure plots exoplanet discoveries on a graph showing the size (radius) versus the orbital period. Kepler transit discoveries are shown as yellow dots. The pink dots represent transit discoveries by other means than Kepler. The light blue dots represent discoveries using the radial velocity method (measuring the toward-and-away wobbles of stars induced by the gravitational tugs of orbiting planets). The other colors account for direct imaging and other methods. Image credit: NASA Ames/Kepler Mission/N. Batalha.

Planets in Habitable Zones This figure shows both currently confirmed and candidate exoplanets orbiting in the habitable zone of their respective stars. Their positions on the graph are determined according to the surface temperature of their parent star versus the energy received by the planet. The green bands represent the habitable zone—the region in which water on a rocky surface could exist in liquid form under the right conditions. The graph shows Venus, Earth and Mars for reference. Recently confirmed planets are labeled. 8 Image credit: NASA Ames/Kepler Mission/N. Batalha/W. Stenzel.