Kepler Finds Earth’s Cousin Astro 101 Slide Set: Kepler Finds Earth’s Cousin Developed by the Kepler Team 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 https://www.astrosociety.org/education/resources-for-the-higher-education-audience/

The Discovery In July 2015, NASA announced the discovery of the closest “cousin” to Earth yet discovered—in data collected by the Kepler spacecraft during its four-year mission. The planet, designated Kepler-452b, is 1.6 times the diameter of Earth, orbiting every 385 days in the “habitable zone” of a star about the same size and temperature as the sun and just 20% brighter. The habitable zone of a star is the region around it in which an orbiting planet could experience temperatures suitable for liquid water to exist on the surface. Kepler’s initial data-collection mission lasted four years, from May 2009 to May 2013, when on-board equipment failure compromised the ability to point the spacecraft with sufficient accuracy to continue the original mission. The spacecraft was repurposed for the K2 Mission, in which the spacecraft continues to search for and discover extrasolar planets around stars by aiming at starfields along the ecliptic for periods of about 75 days. In Artist’s conception of Kepler-452b. NASA Ames/JPL-Caltech/T. Pyle.

How was the Discovery Made? Kepler detects possible 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. The time between transits gives the orbital period of the planet. If the star’s size and temperature are known, the dip in the light curve indicates the size of the planet. Mission scientists require at least three transits—three orbits, or planetary “years”—to verify that a planet is present. 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. Credit: NASA Ames/D. Berry. Run your cursor over the upper image and click on the arrow to run the animation. If you have trouble running the animation, you may need to load Quicktime. Note: Kepler can only detect the presence of planets whose orbits are oriented so that the planets pass across the face of their parent stars from the spacecraft’s perspective. Photometric—referring to photometry, the measurement of the intensity of light. A light curve is a graphical representation of the intensity of light over time. Lower light curve of Kepler-452b caption: Phase-folded, detrended flux time series for Kepler-452 versus phase in hours. The flux time series plotted here phase folded at the period of the planet, 384.84 days. The triangle symbol indicates the location of the four transits at zero phase. The full resolution data are represented by black dots. The cyan filled circles represent the results of binning the data in phase with a bin width of 1/5 of the DV-fitted duration of the transit, 10.5 hr

The Big Picture One of the key questions Kepler was created to answer was whether there were other planets like Earth. Most confirmed Earth-size planet discoveries orbit smaller and cooler stars than the sun and have short years—because these planets transit more often and are thus more readily confirmed. M, K, and G stars are spectral classes of stars—categories which sort stars by their spectral types, which essentially sorts them by surface temperature. M and K stars are cooler and G stars are hotter; the sun is a G-type star. Credit: NASA/JPL-CalTech/R. Hurt Kepler-452b—which took just over three Earth-years to make three orbits of its star—is the first discovery of an Earth-size planet orbiting a sun-like star in a period similar to Earth.

What are the Implications? Not only is Kepler-452b a close cousin to Earth in its size, orbit, and parent star; it also orbits within its star’s “habitable zone.” This zone is the region around the star where the temperature range could allow an orbiting planet to have liquid water--essential to life as we know it—on its surface. The planet also has a good chance of being rocky, setting up conditions that could be similar to Earth. The reasoning that Kepler-452b has a good chance of being rocky is based on its size; smaller planets are more likely to be accumulations of rocky materials, while large planets are more likely to have large gaseous envelope Comparison of the known stars with planets orbiting in their habitable zones: Kepler-186, Kepler-452, and our sun. Credit: NASA/JPL-CalTech/R. Hurt So far, Kepler-452b is the best candidate for being a close cousin to Earth, and is a planet that could possibly harbor life. 

Resources Press Release: https://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth Press briefing announcement: http://www.nasa.gov/keplerbriefing0723 (includes more graphics) Paper: Discovery and Validation of Kepler-452b: A 1.6-Re Super Earth Exoplanet in the Habitable Zone of the G2 Star, Jon Jenkins, et al., Astrophysical Journal: Download at http://arxiv.org/abs/1507.06723 Kepler-452b data in the Exoplanet Archive: http://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=Kepler-452+b Kepler Web Site: http://www.nasa.gov/mission_pages/kepler/overview/index.html

Kepler Finds Earth’s Cousin BONUS CONTENT

Habitable Zone a.k.a, the Goldilocks Zone Credit ; NASA Ames/SETI Institute/D. Berry The Kepler Mission uses the functional definition of a habitable zone as the distance from the star where liquid water could be retained on the surface of a planet. This illustrated the habitable zone in green is shown for stars of three sizes. Goldilocks zone: Not too hot, not too cold….just right. In our own solar system, Venus is on the inner edge and Mars is on the outer edge of the habitable zone. Habitable zones—the regions where liquid water could exist on planets orbiting there--vary in size and distance from the parent star depending on the star’s temperature.

Kepler’s Earth-size Worlds Of the Earth-size worlds discovered by Kepler Mission, only Kepler-452b orbits a star like our Sun. The others orbit smaller and cooler stars. Kepler-452b is the first near-Earth-Size planet in the habitable zone of a star very similar to the sun. More: Kepler 20e was the first planet found that was smaller than Earth, orbiting a star slightly cooler and smaller than our sun every six days. But 20e is scorching hot and unable to maintain an atmosphere or a liquid water ocean. Kepler-22b was found to be the first planet in the habitable zone of its star, but is more than twice the size of Earth and therefore unlikely to have a solid surface. Kepler-186f is the first Earth-size planet found in the habitable zone of a small, cool M dwarf, which is about half the size and mass of our sun. Artist’s conception. Credit: NASA Ames/Wendy Stenzl

Is Kepler-452b Habitable? Kepler-452b is both larger and older than Earth. 452b receives 10% more energy than Earth; if it were the same size as Earth, there would be a run-away greenhouse effect. Because is is 60% larger than Earth, its size may allow it to retain an atmosphere and surface water. The star Kepler-452 is six billion years old compared to our sun’s 4.5 billion years, so the planet Kepler-452b has existed 1.5 billion years longer in the habitable zone of its star than Earth. Kepler-452b is receiving 10 percent more energy from its parent star than the Earth is from the Sun. If Kepler-452b had the same mass as Earth it would be almost experiencing a runaway greenhouse effect and the loss of its water inventory. However, since it is 60 percent bigger than Earth, and is approximately five Earth masses, its larger size provides additional protection from any greenhouse effect for another 500 million years. Note: The habitability of Kepler-452b is debated among astronomers. It is not absolutely a habitable world. Credit: NASA Ames/J. Jenkins. If it does, it may have characteristics that could make it habitable to some form of life.  But we cannot yet say without further evidence.

Kepler’s Earth-size Planets Since launch in 2009, several exoplanets in or near the habitable zone have been discovered. Kepler-452b is nearest in size and temperature to Earth. The exoplanets are shown relative to the temperature of their star and with respect to the amount of energy they receive from their star (energy at Earth = 1). Habitable Zone: The dark green area represents an optimistic estimate for the habitable zone, while the light green area represents a more conservative estimate for the habitable zone. The candidates are plotted as a function of the star's surface temperate on the vertical axis and by the amount of energy the planet candidate receives by its host star. Solid vs open circles: The circle size represents the size of the exoplanet relative to Earth. Solid circles are confirmed planets; open circles are exoplanet candidates that await confirmation by other observational methods. Blue vs. yellow planets: The blue planets were discovered and announced in earlier versions of the Kepler Mission catalog. The yellow circles and disk are from the 7th catalog, announced in July 2015. Open circles indicate exoplanet candidates that await confirmation by further observations. The sizes of the blue disks indicate the sizes of these exoplanets relative to one another and to the image of Earth, Venus and Mars, placed on this diagram for reference. Note that all the exoplanets discovered up until now are orbiting stars which are somewhat to significantly cooler and smaller than the Sun. Kepler-452b is the first planet less than twice the size of Earth discovered in the habitable zone of a G-type star. The vast majority of exoplanets discovered by Kepler orbit single stars. Credit: NASA Ames/N. Batalha and W. Stenzel.

Kepler’s Planet Candidates In July 2015, the 7th catalog of Kepler planets was released. Notice the large number of Earth-size planets in the new data release. = July 2015 = Earlier catalogs There are 4,696 planet candidates now known with the release of the seventh Kepler planet candidate catalog - an increase of 521 since the release of the previous catalog in Jan. 2015. The blue dots show planet candidates from previous catalogs, while the yellow dots show new candidates from the seventh catalog. New planet candidates continue to be found at all periods and sizes due to continued improvement in the detection techniques. Notably, several of these new candidates are near-Earth-sized and at long orbital periods, where they have a chance of being rocky with liquid water on their surface. The new data release includes a large number of Earth-size planets compared to earlier releases because it takes longer to confirm smaller planets, which are harder to detect and often require more orbits to confirm. Credit: NASA Ames/W. Stenzel.