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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