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UBC Theses and Dissertations

Transiting super-Earth exoplanets : search and characterisation Dragomir, Diana


The only way to measure the diameter of a planet outside the Solar System is if it transits its host star. Transit data combined with radial velocity (RV) data give an exoplanet's mass. Mass and volume yield density, so transits give unique insights into the interior structure of an exoplanet. Super-Earths are a class of exoplanet with masses a few times that of Earth, for which no examples exist in our Solar System. The goals of this thesis: to detect and characterise transits of exoplanets (particularly super-Earths) discovered via RV surveys orbiting bright stars. The observational tool: MOST (Microvariability & Oscillations of STars) - a microsatellite housing an optical telescope which can monitor stars with high precision. We searched 12 super-Earth candidate systems, applying special data reduction and analysis techniques. The search gives upper limits on the transit depths of 11 of the planets. For GJ 581e (innermost planet of an M dwarf system), the 1σ upper limit excludes most transiting configurations for a planet with water ice or H/He compositions. MOST data of HD 97658 rejected a claim of a transit detection in this system. The timely MOST rejection cancelled observatory programmes dedicated to follow up on the transit claim, preventing the waste of valuable time on major facilities. In parallel with the MOST survey, we analysed a decade of ground-based photometry to exclude most transiting configurations for the massive exoplanet HD 192263b. We showed that the star's rotation period does not coincide with the planet's orbital period, as was previously reported. We also find evidence for an 8-year activity cycle in the host star. The twelfth star in the MOST survey is 55 Cancri, whose innermost planet (55 Cnc e) has an orbital period of less than a day and transits the star. We present 42 days of new MOST photometry of this system, and derive one of the most precise radius values (1.99 ± 0.08 Earth radii) known for any super-Earth. We explore the possibility of star-exoplanet interaction, and set a limit on the albedo (reflectivity) of the planet, which has implications for its atmospheric composition.

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