Searching for Planets from Your Sofa An extrasolar planet or exoplanet is a planet orbiting a star outside of our own Solar System. When an exoplanet passes or transits in front of its parent star, a small portion of the star's light is blocked out. The star momentarily dims, signaling the existence of another solar system outside our own with a distant world. This dimming of starlight lasts for a few hours or more and repeats once per orbit of the planet. NASA's Kepler mission has spent the past four years staring at a single patch of sky, simultaneously monitoring the same ~160,000 stars for these signatures of transiting exoplanets. A Jupiter-sized planet produces a large 1% drop in light as it transits across a Sun-like star. Rocky planets generate a much smaller dimming, with the Earth only producing a 0.01% drop in our Sun's light! From the ground it is difficult to detect the small transit depths from rocky planets, but above the atmosphere with nearly uninterrupted observing Kepler is uniquely providing a census of both gas giant and terrestrial planets. Understanding the abundances of different types of planets is crucial to understanding planet formation and providing context for our Solar System. Kepler is capable of detecting small rocky planets, enabling the measurement of the frequency of potentially Earth-sized planets in the habitable zone, the Goldilocks region around a star where it is not too hot and not too cold for liquid water to exist on the surface of an orbiting rocky world. The Kepler team developed automated computer algorithms to search the Kepler light curves, the time series of brightness measurements, for the repeated signal of exoplanet transits. To date over 3,000 planet candidates have been discovered, but the Kepler light curves are complex, many exhibiting short-lived brightness variations that are often difficult to characterize. Despite the impressive success of the computers, the star itself may have natural variability, this sometimes makes it difficult for the automated
A family portrait of the PH1 planetary system: The newly discovered planet is depicted in this artist’s rendition transiting the larger of the two eclipsing stars it orbits. Off in the distance, well beyond the planet orbit, resides a second pair of stars bound to the planetary system. ŠHaven Giguere/Yale
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routines to find transits. The human brain excels at pattern recognition and easily recognizes transits that sophisticated automated routines may miss. It is impossible for a single person to review the 4 years of observations for each of Kepler's ~160,000 stars, but with the Internet we can gather the help of hundreds of thousands of people to hunt for exoplanets. Planet Hunters (http://www.planethunters.org) uses the World Wide Web to enlist the general public to identify transits in the pubic Kepler light curves. Visitors to the Planet Hunters' website are asked to draw boxes to mark the locations of visible planet transits. Anyone can help. No training required, all you need is a web browser. To date, 280,000 volunteers worldwide have participated, contributing over 20 million classifications. Planet Hunters is a novel and complementary technique to the Kepler Team’s automated detection algorithms. Planet Hunters is completing an independent assessment of the exoplanet population and identifying planet candidates that have been missed by the automated routines. In the past 3 years, Planet Hunters has discovered PH1 b - one of only 7 known transiting circumbinary planets (where the planet orbits both stars in a stellar binary) and the first confirmed planet residing in a four star system, PH2 b - a confirmed Jupiter-sized planet in the habitable zone of a Sun-like star, and over 40 additional planet candidates previously missed by traditional techniques.
The light curve of a transiting planet with a 9 Earth radii (Jupiter-sized) planet on a ~9 day orbit about its parent star. ŠPlanet Hunters/Zooniverse
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We have just scratched the surface of the Kepler observations. A tiny fraction of the Kepler data to date has been searched by Planet Hunters for additional planets that the automated routines may have missed. There are so many light curves that have yet to be seen by human eyes on Planet Hunters. We need your help! Join in the search for exoplanets today at http://www.planethunters.org (Author/Megan Schwamb)
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