Person: Berta, Zachory
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Berta
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Zachory
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Berta, Zachory
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Publication Super-Earth and Sub-Neptune Exoplanets: a First Look from the MEarth Project(2013-10-08) Berta, Zachory; Charbonneau, David; Goodman, Alyssa; Latham, David; Protopapas, Pavlos; Sasselov, Dimitar; Mamajek, EricExoplanets that transit nearby M dwarfs allow us to measure the sizes, masses, and atmospheric properties of distant worlds. Between 2008 and 2013, we searched for such planets with the MEarth Project, a photometric survey of the closest and smallest main-sequence stars. This thesis uses the first planet discovered with MEarth, the warm 2.7 Earth radius exoplanet GJ1214b, to explore the possibilities that planets transiting M dwarfs provide.Publication Constraints on Planet Occurrence Around Nearby Mid-to-Late M Dwarfs From the Mearth Project(IOP Publishing, 2013) Berta, Zachory; Irwin, Jonathan; Charbonneau, DavidThe MEarth Project is a ground-based photometric survey intended to find planets transiting the closest and smallest main-sequence stars. In its first four years, MEarth discovered one transiting exoplanet, the 2.7 R⊕ planet GJ1214b. Here, we answer an outstanding question: in light of the bounty of small planets transiting small stars uncovered by the Kepler mission, should MEarth have found more than just one planet so far? We estimate MEarth’s ensemble sensitivity to exoplanets by performing end-to-end simulations of 1.25 × 106 observations of 988 nearby mid-tolate M dwarfs, gathered by MEarth between 2008 October and 2012 June. For 2–4 R⊕ planets, we compare this sensitivity to results from Kepler and find that MEarth should have found planets at a rate of 0.05–0.36 planets yr−1 in its first four years. As part of this analysis, we provide new analytic fits to the Kepler early M dwarf planet occurrence distribution. When extrapolating between Kepler’s early M dwarfs and MEarth’s mid-to-late M dwarfs, we find that assuming the planet occurrence distribution stays fixed with respect to planetary equilibrium temperature provides a good match to our detection of a planet with GJ1214b’s observed properties. For larger planets, we find that the warm (600–700 K), Neptune-sized (4 R⊕) exoplanets that transit early M dwarfs like Gl436 and GJ3470 occur at a rate of <0.15 star−1 (at 95% confidence) around MEarth’s later M dwarf targets. We describe a strategy with which MEarth can increase its expected planet yield by 2.5 × without new telescopes by shifting its sensitivity toward the smaller and cooler exoplanets that Kepler has demonstrated to be abundant.Publication Transit detection in the MEarth survey of nearby M dwarfs: bridging the clean-first search-later divide(IOP Publishing, 2012) Berta, Zachory; Irwin, Jonathan; Charbonneau, David; Burke, Christopher J.; Falco, Emilio E.In the effort to characterize the masses, radii, and atmospheres of potentially habitable exoplanets, there is an urgent need to find examples of such planets transiting nearby M dwarfs. The MEarth Project is an ongoing effort to do so, as a ground-based photometric survey designed to detect exoplanets as small as 2 R ⊕ transiting mid-to-late M dwarfs within 33 pc of the Sun. Unfortunately, identifying transits of such planets in photometric monitoring is complicated both by the intrinsic stellar variability that is common among these stars and by the nocturnal cadence, atmospheric variations, and instrumental systematics that often plague Earth-bound observatories. Here, we summarize the properties of MEarth data gathered so far, emphasizing the challenges they present for transit detection. We address these challenges with a new framework to detect shallow exoplanet transits in wiggly and irregularly spaced light curves. In contrast to previous methods that clean trends from light curves before searching for transits, this framework assesses the significance of individual transits simultaneously while modeling variability, systematics, and the photometric quality of individual nights. Our Method for Including Starspots and Systematics in the Marginalized Probability of a Lone Eclipse (MISS MarPLE) uses a computationally efficient semi-Bayesian approach to explore the vast probability space spanned by the many parameters of this model, naturally incorporating the uncertainties in these parameters into its evaluation of candidate events. We show how to combine individual transits processed by MISS MarPLE into periodic transiting planet candidates and compare our results to the popular box-fitting least-squares method with simulations. By applying MISS MarPLE to observations from the MEarth Project, we demonstrate the utility of this framework for robustly assessing the false alarm probability of transit signals in real data.Publication A Super-Earth Transiting a Nearby Low-Mass Star(Nature Publishing Group, 2009) Charbonneau, David; Berta, Zachory; Irwin, Jonathan; Burke, Christopher J.; Nutzman, Philip; Buchhave, Lars A.; Lovis, Christophe; Bonfils, Xavier; Latham, David; Udry, Stéphane; Murray-Clay, Ruth; Holman, Matthew; Falco, Emilio E.; Winn, Joshua N.; Queloz, Didier; Pepe, Francesco; Mayor, Michel; Delfosse, Xavier; Forveille, ThierryA decade ago, the detection of the first transiting extrasolar planet provided a direct constraint on its composition and opened the door to spectroscopic investigations of extrasolar planetary atmospheres. Because such characterization studies are feasible only for transiting systems that are both nearby and for which the planet-to-star radius ratio is relatively large, nearby small stars have been surveyed intensively. Doppler studies and microlensing have uncovered a population of planets with minimum masses of 1.9–10 times the Earth’s mass (Mcircle plus), called super-Earths. The first constraint on the bulk composition of this novel class of planets was afforded by CoRoT-7b , but the distance and size of its star preclude atmospheric studies in the foreseeable future. Here we report observations of the transiting planet GJ 1214b, which has a mass of 6.55Mcircle plus and a radius 2.68 times Earth’s radius (Rcircle plus), indicating that it is intermediate in stature between Earth and the ice giants of the Solar System. We find that the planetary mass and radius are consistent with a composition of primarily water enshrouded by a hydrogen–helium envelope that is only 0.05% of the mass of the planet. The atmosphere is probably escaping hydrodynamically, indicating that it has undergone significant evolution during its history. The star is small and only 13 parsecs away, so the planetary atmosphere is amenable to study with current observatories.