Discovery and characterization of transiting circumbinary planets from NASA's Kepler mission
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Date
2014-08-21
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Johns Hopkins University
Abstract
Planets with two suns have long fascinated our imagination yet it was only recently that astronomers were able to provide direct evidence of their existence. Several candidates have been proposed since 2003, based on measured timing variations in binary stellar systems, but it was not until 2011 and the launch of NASA’s Kepler mission that circumbinary planets were unambiguously detected through their transits. At the time of writing, the peerless-quality data from Kepler has enabled the confirmation of eight planets orbiting both members of seven gravitationally bound, eclipsing binaries (one of the systems has two circumbinary planets). This thesis presents our contribution to the field in terms of discovery and characterization of three of these transiting circumbinary planetary systems, specifically Kepler-47, Kepler-64, Kepler-413.
As predicted by theoretical models, the planets we discovered are smaller than Jupiter, have orbits close to the limit for dynamical stability, and are nearly co-planar to their host binaries (although the circumbinary system Kepler-413 is sufficiently misaligned that, due to fast orbital precession, the planet does not transit at every inferior conjunction). The results of our work deliver important insight into the nature of this remarkable new class of objects, and provide deeper understanding of a) the type of binary stars that can support circumbinary planets; b) the orbital and physical properties of these fascinating systems (e.g. sizes, masses, orbital eccentricities, inclinations, precession rates); and c) planet formation and evolution in multiple stellar systems. Adding new members to the still small family of circumbinary planets has direct relevance for estimating the occurrence frequency of planets around binary stars in particular, and the Galactic planetary census in general, and for the extension of the concept of habitability to binary stellar systems.
Here we describe the unique observational signatures of transiting circumbinary planets, the detection method and analysis tools we developed to find and characterize these systems, and the theoretical implications of our discoveries. Specifically, we present the custom-built algorithm we invented to search for individual transit signatures in a light curve. We applied the algorithm to the light curves of ~800 eclipsing binaries from Kepler, and discovered the aperiodic planetary transits in the three circumbinary systems mentioned above. To capitalize on these transits, we developed an analytical model that uses their measured depths and durations to constrain the properties of the host binary star. In addition, we present our ground-based spectroscopic and photometric observations that allowed us to measure the radial velocities of the circumbinary host stars and to constrain photometric contamination from unresolved sources. Finally, we discuss the photometric-dynamic model we developed and applied for the complete characterization of the transiting circumbinary systems Kepler-64 and Kepler-413.
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Keywords
exoplanets, binary stars